Plant selection

This section covers some of the plant species most commonly grown in aquaponic systems. Details are provided on the ideal growing conditions, the length of the growing cycle, common pests and diseases, and recommendations for harvesting and storage. Many varieties of vegetables are available from seed houses. While both field and greenhouse varieties can be grown in a greenhouse, it is advantageous to use greenhouse varieties whenever possible, since they have often been bred to yield very heavily under controlled environmental conditions (Resh 2013).

Leafy greens

Lettuce

Lettuce (Lactuca sativa) takes up relatively little space, and has a short growing cycle when it is healthy: 5-6 weeks from transplant, or 9-11 weeks from seed. It can be grown in media bed, NFT and DWC systems with 20-25 heads/m². Many varieties can be grown in aquaponic systems, including iceberg lettuce which is ideal for cooler conditions, Romaine lettuce which is slow to bolt, and loose leaf lettuce which has no head and can be sown directly onto media beds and harvested by picking single leaves without collecting the whole plant. The most common pests and diseases affecting lettuce are aphids, leaf miners, and powdery mildew.

Ideal growing conditions for lettuce:

• Temperature: 15-22⁰C

• pH: 5.8-7.0

The seeds take between 3 and 7 days to germinate at 13-21⁰C. Supplemental fertilization with phosphorus during the second and third week of growth favours good root growth and reduces stress at transplant. Plant hardening, through exposing the seedlings to colder temperatures and direct sunlight for 3-5 days before transplanting, also results in higher survival rates. The seedlings can be transplanted into the hydroponic unit after 3 weeks, when the plants have 2-3 true leaves.

When transplanting lettuce in warm weather, place light sunshade over the plants for 2-3 days to avoid water stress (Somerville et al. 2014c).

For head growth, the air temperature should be 3-12⁰C during the night, with a day temperature of 17-28⁰C. The generative growth is affected by photoperiod and temperature: extended daylight and warm conditions (>18⁰C) at night cause bolting. Water temperatures above 26⁰C may also cause bolting and leaf bitterness. Some varieties are more tolerant of heat than others. When air and water temperatures increase during the season, use bolt-resistant (summer) varieties. If growing in media beds, plant new lettuces where they will be partially shaded by taller plants. To achieve crisp, sweet lettuce, grow plants at a fast rate by maintaining high nitrate levels. The plant has low nutrient demand, though higher calcium concentrations in the water help to prevent tip burn in the summer. While the ideal pH is 5.8-6.2, lettuce still grows well with a pH as high as 7, although some iron deficiencies might appear owing to reduced bio-availability of this nutrient above neutrality (Somerville et al. 2014c).

Figure 1: Hydroponic production of different lettuce cultivars https://www.maxpixel.net/Natural-Lettuce-Fresh-Healthy-Raw-Food-Green-1239155

Harvesting can begin as soon as the heads or leaves are large enough to eat. Lettuce should be harvested early in the morning when the leaves are crisp and full of moisture, and quickly chilled. Gentle harvests and cold, consistent temperatures extend shelf life. Harvesting techniques can affect shelf life if the lettuce is handled roughly, bruised or crushed during the process. This makes the produce much more vulnerable to post-harvest decay and diseases (Storey 2016f).

Lettuce can be harvested quickly as a batch by taking the whole head, using a harvesting knife to cut each head where it meets the surface of the system. Some growers harvest the entire plant, including the roots, which can extend shelf life. With so much transpiration and moisture, lettuce can be difficult to store for more than a few days before it starts to wilt and decay. It can stay fresh for up to three weeks if it is stored at just above 0⁰C , but it should not be allowed to freeze, as this will cause the leaf epidermis to separate from the other tissues, and the leaf will decay rapidly. Lettuce requires humidity to keep it from drying out, but condensation or heavy moisture on the leaves is detrimental. The best thing that producers can do to avoid condensation is to keep temperatures very consistent (Storey 2016f).

Figure 2: Hydroponic production of lettuces using NFT channels https://www.maxpixel.net/Organic-Greenhouse-Farming-Hydroponic-Cucumber-2139526

Processing should be kept to a minimum. The only absolutely necessary task is to trim the leaves that are dried out, diseased, or which affect the aesthetics of the crop. Preferably do not wash the lettuce before delivery, although some growers use a cold water dunk in the belief that it extends shelf life by closing the stomata (Storey 2016f).

Chard

Chard (Beta vulgaris subsp. vulgaris) is easy to grow in media beds, NFT channels and DWC systems. It is a fairly tough crop, occasionally susceptible to aphids and powdery mildew problem, and although high or low temperatures will affect the taste, the crop is overall very tolerant of stressful conditions.

Ideal growing conditions for chard:

• Temperature: 16-24⁰C and frost tolerant

• pH: 6.0-7.5

Chard is a moderate nitrate feeder, and requires lower concentrations of potassium and phosphorus than fruiting vegetables. Owing to its high market value, its fast growth rate and its nutritional content, chard is frequently grown in commercial aquaponic systems. Although traditionally a late winter/spring crop, it also grows well in full sun during mild summer seasons, though a shading net is recommended when temperatures exceed 26⁰C (Somerville et al. 2014c).

Chard is easiest to grow from seed, and germinates within 4-5 days at 25-30⁰C. The seeds produce more than one seedling, so thinning is required as the seedlings begin to grow. The seedlings can be transplanted at 15-20 plants/m². As plants become senescent during the season, the older leaves can be removed to encourage newer growth (Somerville et al. 2014c). Chard can be harvested 4-5 weeks after being transplanted, and yields well. Growers should only harvest partially, leaving 30% of the foliage for the plant to photosynthesize for the next crop. The largest leaves should be clipped as close to the base of the plant as possible. Harvesting in the morning or evening can help keep chard fresh, and will keep for over a week without beginning to wilt if treated correctly. Chard lasts longest when stored without washing in sealed containers or bags at cool temperatures, which dramatically reduce respiration and decay (Storey 2016b).

Kale

Growing kale (Brassica oleracea) in aquaponic systems can be a simple and profitable option. The crop grows relatively quickly with a six-week cycle from transplant to harvest, or can be harvested partially, leaving 30% to regrow for the next crop.

Ideal growing conditions for kale:

• Temperature: 8-29⁰C

• pH: 6.0-7.5

Kale is a cool weather crop, and many growers even apply cooler temperatures (down to 5⁰C) purposefully to draw out a smoother, improved flavour. Fortunately, kale is another crop which, when grown indoors, is targeted by only a few pests such as aphids and some powdery mildew (Storey 2016p).

Pak choi

Pak choi (Brassica chinensis), also known as bok choy or Chinese cabbage, comes in a range of sizes, including large varieties like Joi Choi and smaller varieties like Shanghai Green Pak Choy, which offer more compact, tender heads with a delicate flavour. Tatsoi (Brassica narinosa, also called broadbeaked mustard) has the same thick leaves and light veins as pak choi and can be grown in similar conditions. Napa cabbage (Brassica rapa pekinensis) is another brassica member which, while it looks different to pak choi and tatsoi, shares the same pH and EC range of pak choi, and tastes better when grown at cooler temperatures (Storey 2016i).

Ideal growing conditions for pak choi:

• Temperature: 13-23⁰C

• pH: 6.0-7.5

Although pak choi is typically milder in cool temperatures, it is fairly temperature tolerant, which makes it an easy fit in many hydroponic and aquaponic systems. Deficiencies in pak choi can be difficult to identify, as the more obvious symptoms like interveinal chlorosis, burning, or bronzing are not common. Deficiencies are marked by stunted growth, cupping, and some yellowing. Plant pak choi from seed and transplant as soon as there are true leaves on the plant; this will typically occur in about four weeks. Though the highest yields occur at six weeks from transplant, pak choi may be grown in shorter rotations of four weeks (Storey 2016i).

Figure 3: Pak choi growing in the NFT system at Lufa Farms https://commons.wikimedia.org/w/index.php?curid=27515408

Cabbage

Cabbage (comprising several cultivars of Brassica oleracea) is a fairly hands-off crop to grow. General pest control measures using an IPM plan usually keep pests at bay, and cabbage needs no extra pruning or training. The heads grow large (3.5 kg is not uncommon), so farmers can get a fairly large crop from a small space.

Ideal growing conditions for cabbage:

• Temperature: 15-20⁰C (but frost tolerant)

• pH: 6.0-7.2

Cabbage is vulnerable to common pests such as aphids, as well as bacterial diseases such as blackleg and black rot. The latter are usually due to the crown of the plant being kept moist. Other than pests and diseases, the most common problem with cabbage cultivation is splitting, when the head cracks and splits. This looks unappealing to consumers and can catch dirt and disease. Splitting can be avoided by keeping growing conditions consistent, and harvesting at the right time (Storey 2016k).

Cabbages grow best in media beds because they reach significant dimensions and may be too large and heavy for rafts or grow pipes. As a nutrient-demanding plant, it is not suitable for newly established aquaponics units (less than four months old). Nevertheless, owing to the large space required (4-8 plants m²), cabbage crops take up fewer nutrients per square metre than other leafy vegetables (lettuce, spinach, rocket etc.). Cabbage likes full sun and grows best when the heads mature in cooler temperatures, so they should be harvested before daytime temperatures reach 23- 25⁰C. High concentrations of phosphorus and potassium are essential when the heads begin to grow. Integration with organic fertilizers delivered either on the leaves or substrates may be necessary in order to supply the plants with adequate levels of nutrients (Somerville et al. 2014c).

For best germination rates, seedlings should be kept a little warmer than mature crops (18-29⁰C). Scarification of seeds can also increase germination rate. After being planted, seeds will germinate in 4-7 days, and the seedlings will be ready to transplant 4-6 weeks later when they have 4-6 leaves and a height of 15 cm. It is important to leave enough room for each head to grow to the desired size. In the event of day temperatures higher than 25⁰C, a 20 percent light shading net should be used to prevent the plant from bolting. Depending on the type of cabbage and the size of head desired, the crop will be ready for harvest 45-70 days after transplanting. It should be harvested when the head is firm and big enough for the market, by cutting the head from the stem with a sharp knife, and discarding the outer leaves (Somerville et al. 2014c).

Mustard greens

Mustard greens (Brassica juncea) are another member of the brassica family (a relative of kale and cabbage).

Ideal growing conditions for mustard greens:

• Temperature: 10-23⁰C

• pH: 6.0-7.5

Mustard greens can be managed in a similar way to kale – grown from seeds, which take 4-7 days to germinate, the seedlings will be ready to transplant at 2-3 weeks later (3-4 weeks from the seed planting). After 4-6 weeks growing, the plants should be harvested partially, taking only 30% of the plant and leaving the rest to continue to grow (Storey 2016g).

Nasturtium

Nasturtium (Tropaeolaceae tropaeolum) is a tender plant native to South America. Unlike many crop plants, both the leaves and flowers are edible and have a sharp peppery taste similar to mustard or watercress. Nasturtiums are easy to grow in hydroponic systems for their leaves. However, if growers are optimizing for flower production they may need to adjust nutrient ratios and light to cue flowering. It may also be necessary to control the ratio of nitrogen to potassium to cue the vegetative and fruiting stage, and to switch the system from a greens mix to a strawberry mix when they are about half of their mature size in order to start flowers. This gives the crop a chance to establish roots and photosynthesizing tissue, so that when they flower they are able to produce more. Nasturtium suffers from typical pests like aphids and spider mites. It can be sourced as two different varieties: a vining variety and a bush variety (Storey 2017b).

Ideal growing conditions for nasturtium:

• Temperature: 13-23⁰C

• pH: 6.1-7.8

Nasturtiums are light-lovers but do best with low heat stress. Seeds can be germinated at 13-18⁰C, and adult plants do best at about 21⁰C. The flowering crop does well in low EC systems like those optimized for leafy greens or strawberries. Nasturtium seeds take 7-10 days to germinate in the right conditions and are ready to transplant as soon as true leaves appear, which is usually 2-3 weeks from germination. Plants will produce flowers 5-6 weeks later, but if the grower is only interested in the leaves, these can be harvested earlier. Some growers prefer to grow nasturtiums at a high density and harvest the leaves while they are still very young (Storey 2017b).

Herbs

Herbs are usually more profitable than leafy greens. Different herbs have different needs, and lack of understanding of this can reduce shelf life or even ruin produce before it can be used. Tips to keep herbs fresh after harvest include (Storey 2016o):

• Keep it cool, but not too cool

Respiration rates slow down when produce is kept cool, as the stomata close and gas exchange decreases. Harvesting during a cool part of the day will also help. Some herbs, such as basil, are sensitive to chilling and can become damaged. Basil should not be kept below 13⁰C, for example, but can attain a shelf life of 12 days at 15⁰C.

• Be consistent

Temperature and moisture fluctuations are largely responsible for disease and decay issues. These can be avoided by reducing the number of times that produce is moved from one place to another, and by keeping the the temperature of coolers and transport vehicles steady.

• Decrease plant damage

The production of ethylene is increased by wounds, and accelerates the rate of deterioration. The use of clippers when harvesting herbs, rather than tearing, will help to avoid this.

• One size does not fit all

Harvesting and packaging practices should be specific to the herb and its age, since needs vary widely. Most of the herbs commonly used differ in their origin, needs, and life cycles. This means that each herb should be treated differently to increase shelf life.

• Packaging should balance water loss with decay

Tender herbs such as basil or chives lose less water when packaged in plastic bags, but condensation increases decay rates.

• Control light exposure

Whether stored under light or in the dark may influence the decay rate, depending on the herb.

Coriander

While coriander (Coriantrum sativum) is an easy crop for soil gardeners, indoor and hydroponic growers may not get the highest space use efficiency from this crop, as it has a comparatively long growing cycle and limited yield. On the other hand, it is low-maintenance, and if growers are sure that they can get a good price, then coriander can still be a good crop. Since it is small-statured, coriander can be grown in almost any hydroponic system, so long as pH and EC ranges are appropriate (Storey 2017a).

Ideal growing conditions for coriander:

• Temperature: 5-23⁰C

• pH: 6.5-6.7

Coriander can be a tricky crop to grow since it bolts very easily, especially in hot conditions. It prefers cooler temperatures (5-23⁰C) and low salts. The preference for cool temperatures extends to germination as well; temperatures of 15-20⁰C will result in the best germination rates. If bolting is triggered, which makes the flavour of the herb more bitter, the bolts should be trimmed and the environmental conditions adjusted. Growers can purchase slow bolting seeds to minimize the potential for crop failure. Two of the most common diseases of coriander in hydroponics are bacterial leaf spot and powdery mildew. Coriander is also vulnerable to Pythium, which can become problematic in systems with inadequate aeration around the roots (Storey 2017a).

Coriander seeds germinate in 7-10 days, with leaves ready to harvest 40-48 days later. From seed to harvest, coriander takes 50-55 days. Coriander can be harvested fully or partially, requiring very little maintenance like trimming. If using a partial harvest, the first harvest will take place at about 5 weeks after transplant and the second at about 8 weeks after transplant. The second harvest will be lower than the first. Coriander may be packaged in various ways depending on the farmer and, even more importantly, market preference (Storey 2017a).

Mint

There are dozens of types of mint, but the main varieties are spearmint (Mentha spicata), peppermint (Mentha x piperita), and pennyroyal mint (Mentha pulegium); some of the other mints like lemon mint (Monarda citriodora) are actually not mint at all. Mint is one of the the easiest crops to grow. It is easy to plant, grows quickly, and easy to harvest.

Ideal growing conditions for mint:

• Temperature: 19-21⁰C

• pH: 6.5-7.0

Mint is tolerant of low EC and some temperature variation, although it doesn’t do well when heat spikes above 26⁰C. It struggles less with pests than many of the herbs, although verticillium wilt and powdery mildew can become problematic. Mint can be grown from seed, but using cuttings or rootstock is much quicker, especially on a commercial scale. Stem cuttings can be made by removing healthy green sprigs and setting them in water. Roots will form and the plants will grow to maturity within a few weeks. Mint can be harvested by cutting about 5 centimetres from the surface of the system. A second harvest will be ready in only 2-3 weeks, once it has grown out to about 20 centimetres (Storey 2016m).

Basil

Owing to the higher nitrogen uptake, basil (Ocimum basilicum) is an ideal plant for aquaponics, and it can be grown in media beds, NFT and DWC systems. However, if mint is one of the easiest herbs to grow, then woody herbs like basil are at the other end of the scale. Although basil isn’t needy in terms of water and pH, it does require pruning (see below) to achieve full yields, and grows best in high temperatures which can be tough to match with other crops, so it may be best to grow it as a monocrop. Many cultivars of basil have been tried and tested in aquaponic systems, including Genovese basil (sweet basil), lemon basil, and purple passion basil.

Ideal growing conditions for basil:

• Temperature: 18-30⁰C, optimal 20-25⁰C

• pH: 5.5-6.5

Basil seeds need a reasonably high and stable temperature to initiate germination (20-25⁰C), and should germinate within 6 to 7 days. The seedlings should be transplanted to the aquaponic system when they have 4-5 true leaves. Once transplanted, basil grows best in warm to very warm conditions, with full exposure to the sun. However, better quality leaves are obtained through using slight shading. If temperatures exceed 27⁰C the plants will need to be ventilated or covered with shading nets (20%) to prevent tip burn. Basil can be affected by various fungal diseases, including Fusarium wilt, grey mould, and black spot, particularly under suboptimal temperatures and high humidity conditions. Air ventilation and water temperatures higher than 21⁰C help to reduce plant stress and incidence of diseases (Somerville et al. 2014c).

The shape of basil leaves causes them to catch water and hold it, so controlling condensation is very important. Humidity in the greenhouse should be kept between 40-60%. Basil is very sensitive, so it requires good air flow but not a draught. It grows well with 10-12 hours of light, but supplementing light will increase yield. Dying leaves should be removed, as they tend to stick to the other leaves and damage them, or grow fungus. Plants that are end- or top-heavy should be pruned using sharp shears rather than pinching, as this risks damaging or pulling off a whole stem. If the growth on the end of the stem is too heavy, it will split from the main root base and become bitter. The bitterness in basil can be eliminated by harvesting before bolting to flower, throwing out any old/tough growth, and removing broken stems (Storey 2016e).

Figure 4: Basil growing in an NFT system https://www.goodfreephotos.com/public-domain-images/plants-in-the-green-house.jpg.php

Basil has been bred to be a single-stemmed plant growing upward (apical growth). For most growers, a bushier plant is better. A pruned plant looks better, yields more, and can be easier to transport depending on the growing method. To change the way that basil grows, growers can trigger a secondary type of growth that moves outward and up instead of straight up (lateral growth). A young basil plant (12-25 centimetres tall) has lateral buds on the side of the stem that will only grow if the main stalk gets badly damaged or removed. This means that if growers clip the stem right above those lateral buds (1 centimetre or so), the buds will be triggered to grow out. By pruning basil this way, growers can increase the production of that branch and control the shape of the plant. The plant should be cut above the second pair of buds so that the growth fans out and doesn’t stop airflow or light penetration. Correct pruning will result in increased yield in each of the first three harvests (around weeks 5, 8, and 11) (Storey 2016e).

The harvest of leaves starts when plants reach 15 cm in height and continues for 30-50 days. Basil needs to be handledl gently, as bruising can increase the rate of deterioration. It should not be stored in a chiller, where the temperature is usually kept at 5-7⁰C, as it is a warm weather crop and does not have the cellular machinery to deal with those temperatures, and will decay rapidly. To extend its shelf life, it should be stored above 13⁰C (preferably at a temperature of 16⁰C). At this temperature, it can attain a shelf life of 12 days. If growers package basil in bags or cartons that reduce moisture loss (plastic with little or no air exchange), the storage temperature will need to be kept steady to avoid condensation (Storey 2016e).

Chives

Chives (Allium schoenoprasum) are a tough crop that will survive a wide range of temperatures and can even go without water for a while without it impacting quality. Chives are also fairly pest- resistant, rarely infected with diseases and rarely being targeted by insect pests. The most common issues in hydroponic systems are viruses and fungus gnats (Storey 2016n).

Ideal growing conditions for chives:

• Temperature: 18-26⁰C

• pH: 6.1 to 6.8

Chives propagate rapidly from roots, and can be planted by division. Rarely will growers need to use seeds to grow chive seedlings, unless mature chive plants are nowhere to be found. If chives are grown from seed, seedlings will be ready to transplant about 4 weeks later, and ready to harvest 3-4 weeks later. When planted from root, chives will be established within 2-3 weeks and will grow thicker with every harvest. Chives should be harvested every two to three weeks by trimming back to about 2.5-5 centimetres above the crown (Storey 2016n).

Parsley

Parsley (Petroselinum crispum) grows well in media beds, NFT and DWC systems, and is common in commercial aquaponics units due to its high market value. Large leaf varieties like Italian flat leaf (P. crispum var. neapolitanum) grow particularly well. Pests on parsley are rare, but growers might see aphids or thrips.

Ideal growing conditions for parsley:

• Temperature: 15-25⁰C; very cold hardy

• pH: 6.0-7.0

Parsley is a biennial herb that is traditionally grown as an annual. Most varieties will grow over a two-year period if the winter season is mild with minimal to moderate frost. In the first year the plants produce leaves while in the second they will send up flower stalks for seed production. Parsley enjoys full sun for up to eight hours a day. Partial shading is required when temperatures exceed 25⁰C (Somerville et al. 2014c).

Parsley comes as an affordable seed and germinates within 8-10 days with good moisture and a temperature of 20-25⁰C. If the seeds are not fresh, germination can take as long as 5 weeks. To accelerate germination, seeds can be soaked in warm water (20-23⁰C) for 24-48 hours to soften the seed husks. Emerging seedlings will have the appearance of grass, with two narrow seed leaves opposite each other. Seedlings are ready to transplant after 5-6 weeks when they display their true leaves. They can be planted at 10-15 plants/m². The first harvest typically happens 20-30 days after transplant, once the individual stalks of the plants are at least 15 cm long. Harvest the outer stems first as this will encourage growth throughout the season (Somerville et al. 2014c). Alternatively, parsley can be harvested multiple times, by using shears or a harvesting knife to cut the crop down to 5 centimetres from the surface of the system. Another harvest may be taken about 3 weeks later. A new cycle should be started after the second harvest (Storey 2016a).

Fennel

Fennel (Foeniculum vulgare) rarely struggles with pests if it is kept healthy, although aphid infestations could affect the crop.

Ideal growing conditions for fennel:

• Temperature: 16-21⁰C

• pH: 6.4-6.8

Fennel prefers a lower EC and moderate pH. Although it often proves to be both, heat, and cold tolerant, it is not frost tolerant. Fennel has a wider range of germination rates, from about 60% to 90%. Seeds take 1-2 weeks to germinate and are typically ready to transplant 3-5 weeks later. From transplanting it takes about 6-8 weeks to reach harvesting size. The bulbs can be harvested as soon as the grower wants, but 250 g to 500 g bulbs are standard at most markets. Fennel can be harvested twice (once just for the greens, once for the bulb and greens together) if there is a market for the greens. As with chard and kale, only 70% of the greens should be removed in the first harvest (Storey 2016d).

Fruiting crops

Pruning is important for fruiting crops grown in aquaponic systems. Without regular pruning, excessive growth can occur, which is very hard to manage. The root systems of aquaponic plants are not as strong as plants growing in soil because the roots do not have to spread out in search of nutrients, and plants in aquaponic systems are not able to support heavy loads due to poor anchorage of the roots. Pruning is also important for greenhouse production because, due to higher cost per square foot, growers need to use the area very efficiently. Therefore, pruning allows high density planting and better quality products.

Tomatoes

Tomatoes (Solanum lycopersicum) typically grow in one of two patterns, depending on the variety. Bush varieties (determinate – seasonal production) are especially common in heirlooms and can be more difficult to manage. Bush tomatoes tend to sprawl along a greenhouse floor, making trellising difficult or even impossible. As a result, growers can have trouble reaching the fruit, pruning plants, and navigating the greenhouse. Vining varieties (indeterminate – continuous production of floral branches) are preferable to most growers since the plants can be pruned to a single ‘leader’ and trellised. This makes plants more accessible and much faster to harvest and prune. A typical Bato bucket and tomato setup (see 9.2.4) includes two plants per bucket, with buckets 60-90 centimetres apart. If grown as single plants (such as in a slab system), tomatoes can be pruned to two leaders per plant. Tomatoes are prone to many pests and diseases, the most common being Verticillium wilt, Fusarium, nematodes, spider mites, aphids, damping off, and mosaic virus. When purchasing tomatoes or seeds, look for the ‘VFN’ label which indicates resistance to Verticillium, Fusarium, and nematodes (Storey 2017c).

Ideal growing conditions for tomatoes:

• Temperature: 13-26⁰C

• pH: 5.5-6.5

Tomatoes, as a fruiting crop, are nutrient greedy (see Table 1). They like heat, and will grow well in the same environment as crops like okra or basil. A downside of tomatoes is that their taste is particularly influenced by the medium in which they grow. It is therefore necessary to ensure that the growing medium is at a properly maintained ratio. Because tomatoes are such a commonly grown crop, there is an abundance of data on troubleshooting and deficiencies. Common deficiencies for tomato plants are phosphorus and magnesium (Storey 2017c).

Table 1: Recommended nutrient solution compositions matched to the growth phase of tomatoes in soilless culture (from Raviv & Lieth 2007)

Seeds will germinate in 4-6 days at 20-30⁰C. Stakes or plant supports should be set before transplanting to prevent root damage. Seedlings can be transplanted to the aquaponics system 3-6 weeks after germination when the seedlings are 10-15 cm high and when night time temperatures are constantly above 10⁰C. Tomatoes can be grown in media beds, avoiding waterlogged conditions around the plant collar to reduce any risks of diseases. Given their high nutrient demand, especially for potassium, the number of plants per unit should be planned according to the fish biomass in order to avoid nutrient deficiencies. Tomatoes prefer warm temperatures, with full sun exposure. The optimal daytime temperature is 22-26⁰C, while night time temperatures of 13-16⁰C encourage fruit set (Somerville et al. 2014c).

¹ Anthesis is the flowering period of a plant, from the opening of the flower bud

Pruning is crucial for tomato production, as it ensures proper utilization of energy for the growth of fruits and the main stem. Once the tomato plants are around 60 cm tall, the growing method (bush or single stem) can be determined by pruning the unnecessary upper branches. Bush varieties can be left to grow as bushes by leaving 3-4 main branches and removing all the auxiliary suckers in order to divert nutrient to the fruits. Vining tomatoes can grow up to a height of 4 metres, while 2 metres is a normal height. Pruning is required for vining tomatoes, as 50 percent of tomato yield is reduced without pruning and trellising. Both bush and vining varieties should be grown with a single stem (double in case of high plant vigour) by removing all the auxiliary suckers. Hand removal of suckers 2 to 2.5 mm in length once a week is the best method. At this size, the suckers can be easily broken off without injuring the main stem. In bush varieties, the apical tip of the single stem has to be cut as soon as the plant reaches 7-8 floral branches in order to favour fruiting. Tomatoes rely on supports that can either be made of stakes (bush varieties) or bound to vertical plastic/nylon strings that are attached to iron wires pulled horizontally above the plant units (vining varieties). It is also important to remove the leaves from the lower 30 cm of the main stem to favour a better air circulation and reduce fungal infection. The best way to remove them is to bend them upward first and then pull down in order to prevent peeling of the skin on the stem. Remove the leaves covering each fruit branch soon before ripening to favour nutrition flow to the fruits and to accelerate maturation (Singh & Dunn 2017; Somerville et al. 2014c).

Tomatoes are normally wind pollinated or pollinated by bees when grown outside. In greenhouses, however, air movement is insufficient for flowers to pollinate themselves. Pollination can either be carried out manually, or by using bumble bees (Bombus sp.). It is important to maintain the correct population levels of bumble bees, as overpopulation may result in the bees overworking the tomato flowers. For manual pollination, vibration of the tomato flower clusters is essential. This can be done by tapping the flowers with a stick, fingers, or an electric vibrator such as an electric toothbrush. Pollination must be done while the flowers are in a receptive state, which is indicated by their petals curling back. Plants should be pollinated at least every other day, since blossoms remain receptive for about 2 days. Pollinating should be done between 11:00 am and 3:00 pm under sunny conditions for best results. If pollination has been done correctly, small beadlike fruit will develop within a week or so. This is called fruit set. When young plants produce their first trusses, pollinate each day until fruit set is visible. It is important to get the fruit set on these first trusses, as it throws the plant into a reproductive state, which favours greater flower and fruit production as the plant ages. After the first few trusses have set, pollinating can be done every other day. Research has shown that a relative humidity of 70% is optimum for pollination, fruit set, and fruit development (Resh 2013).

The growth time is 50-70 days until the first harvest, and fruiting continues for 90-120 days in bush varieties and up to 8-10 months for vining varieties. For best flavour, harvest tomatoes when they are firm and fully coloured. Fruits will continue to ripen if picked half ripe and brought indoors. Fruits can be easily maintained for 2-4 weeks at 5-7⁰C under 85-90 percent relative humidity (Somerville et al. 2014c).

Bell peppers

Bell peppers (Capsicum anuum) prefer warm conditions and full sun exposure. As with other fruiting plants, nitrate supports the original vegetative growth (optimum range 20-120 mg/litre), but higher concentrations of potassium and phosphorus are needed for flowering and fruiting (Somerville et al. 2014c).

Ideal growing conditions for bell peppers:

• Temperature: 19-23⁰C

• pH: 5.5-6.5

Table 2: Recommended nutrient solution compositions matched to the growth phase of bell peppers in soilless culture (from Raviv & Lieth 2007)

Seeds will germinate in 8-12 days at 22-30⁰C. The seedlings can be transplanted as soon as the night time temperature settles above 10⁰C, and when they have 6-8 true leaves. Bushy, heavy yielding plants need to be supported with stakes or vertical strings hanging from iron wires pulled horizontally above the buckets. The first few flowers that appear on the plant should be picked in order to encourage further plant growth, and the number of flowers should be reduced in the event of excessive fruit setting to favour the growing of fruits to reach adequate size (Somerville et al. 2014c).

Because of a pepper’s unique growth patterns, pruning is essential in ensuring a successful crop. Pruning will reduce production cost, increase yield and reduce disease susceptibility. Sweet pepper pruning is different from tomato pruning because peppers do not produce side shoots like tomatoes. After pinching (removal of the plant tip), the top two nodes start to grow. The main objective of sweet pepper pruning is to develop a strong vegetative frame for supporting fruit growth and weight during production. Here are the steps for sweet pepper pruning (Singh & Dunn 2017):

1. Remove the growth point or stem tip after the first 40 centimetres

2. Treat each of the two stems as an individual and alternate between removing the inner and outer side shoot from each main stem

3. Remove the side shoot when it is 50 mm long

4. On each individual stem, remove alternate flower clusters. Heavy fruit load on a plant may lead to lower fruit quality and may cause physiological disorders like blossom end rot

5. Completely remove any yellow leaves from the greenhouse

The growth time is 60-95 days. Like tomatoes, peppers also need to be pollinated either manually or by introducing a bumble bee hive to the greenhouse. For sweet red peppers, the green fruits should be left on the plant until they ripen and turn red. Harvesting should begin when the peppers reach marketable size, and continue throughout the season to favour blossoming, fruit setting and growth. Peppers can be easily stored fresh for 10 days at 10⁰C with 90-95 percent humidity (Somerville et al. 2014c).

Cucumbers

Cucumber (Cucumis sativus) comes in three sexual breeds: a half-and-half mix of male and female flowers (monoecious); a seventy-thirty mix of female to male flowers (gynoecious); and entirely female flowering plants (parthenocarpic). Planting only female flowering plants will ensure a flowering fruit with each plant, and therefore a crop that can fruit without pollination. However, the pollen transmitted by bees and other pollinators can corrupt parthenocarpic plants, so it will be necessary to keep potential pollinators out of the greenhouse (Valdez 2017a). Cucumbers can be grown in media bed units as they have a large root surface, and on DWC floating rafts, although in grow pipes there could be a risk of clogging owing to excessive root growth (Somerville et al. 2014c).

Ideal growing conditions for cucumbers:

• Temperature: 24-27⁰C

• pH: 5.5-6.5

Cucumbers require large quantities of nitrogen and potassium, so the decision on the number of plants to grow should take into account the nutrients available in the water and the fish stocking biomass. They grow best with long, hot and humid days, with ample sunshine and warm nights. Optimal growth temperatures are 24-27⁰C during the day, with 70-90 percent relative humidity, and a night time temperature of 18-20⁰C. They are highly susceptible to frost. Full sunlight and a temperature of the substrate of about 21⁰C are also optimal for production. A higher potassium concentration will favour higher fruit settings and yield (Somerville et al. 2014c).

Seeds will germinate after 3 to 7 days at a temperature of 20-30⁰C. The seedlings can be transplanted at 2-3 weeks when they have developed 4-5 leaves. Once transplanted, cucumbers can start producing fruit after 2-3 weeks. In optimal conditions, plants can be harvested 10-15 times. Harvesting every few days will prevent the fruits from becoming overly large, and favour the growth of the following ones. Cucumber plants grow very quickly and it is good practice to limit their vegetative vigour and divert nutrients to the fruits by cutting their apical tips when the stem is two metres long; removing the lateral branches also favours ventilation. Further plant elongation can be achieved by leaving only the two furthest buds coming out from the main stem. Plants are encouraged to further production by regular harvesting of fruits of marketable size. Cucumber plants need support for their growth, which will also provide them with adequate aeration to prevent foliar diseases like powdery mildew and grey mould. Owing to the high incidence of pests in cucumber plants, it is important to implement appropriate IPM strategies (see Chapter 8) and to intercrop the plant units that are less affected by the treatments used (Somerville et al. 2014c).

Aubergine

Aubergine (Solanum melongena) is a greedy crop, thriving at high temperatures and requiring a lot of space between each plant. It may be difficult to regulate temperatures to keep the aubergines happy while growing other crops in the same environment, so they are best grown as a monocrop to avoid juggling climate control (Valdez 2017a).

Ideal growing conditions for aubergine:

• Temperature: 22-26⁰C

• pH: 5.5-7.0

Aubergine has high nitrogen and potassium requirements, so careful management choices are required regarding the number of plants to grow in order to avoid nutrient imbalances. It enjoys warm temperatures with full sun exposure, and a relative humidity of 60-70 percent. Ideal night time temperatures are 15-18⁰C. Aubergine plants are highly susceptible to frost (Somerville et al. 2014c).

The seeds will germinate in 8-10 days in warm temperatures (26-30⁰C) and the seedlings can be transplanted in springtime, when temperatures are rising, when they have 4-5 leaves. Towards the end of summer, new blossoms should be pinched off to favour the ripening of existing fruit. At the end of the season, plants can be drastically pruned at 20-30 cm by leaving just three branches. This method interrupts the crop without removing the plants during the winter, and lets the plant restart production afterwards. Plants can be grown without pruning, and management of the branches can be facilitated with stakes or vertical strings. The growth time is 90-120 days. Like tomatoes and peppers, aubergines also need to be pollinated either manually or by introducing a bumble bee hive to the greenhouse. Harvesting should begin when the fruits are 10-15 cm long, using a sharp knife to cut the fruit from the plant, leaving at least 3 cm of stem attached to the fruit. The skin should be shiny; dull and yellow skin is a sign that the fruit is overripe. Delayed harvest makes the fruits unmarketable owing to the presence of seeds inside. Plants can produce 10-15 fruits for a total yield of 3-7 kilos (Somerville et al. 2014c).

Strawberries

The garden strawberry (or simply strawberry; Fragaria × ananassa) is a widely grown hybrid species of the genus Fragaria, collectively known as strawberries. Strawberries are different from other crops. They live for a long time, but they are also susceptible to many diseases. Crown or heart rot is a fungal disease that is especially common for strawberries. The crown of the plant is the region where the roots become the stem, so is important to make sure that the crown is kept out of the wet zone. Mites can also be a problem. Different varieties have different environmental preferences and different bearing timelines: one variety may take a month to start bearing fruit after planting, while another may need several months. Some varieties also only bear fruit for part of the year, even indoors. Ever-bearing or day-neutral varieties are best for indoor growers (Storey 2016l).

Ideal growing conditions for strawberries:

• Temperature: 18-20⁰C

• pH: 5.5 to 6.0

Table 3: Recommended nutrient solution compositions matched to the growth phase of strawberries in soilless culture (from Raviv & Lieth 2007)

Grow strawberries from rootstock rather than seed. Vegetative growth (runners) tends to be much faster that sexual reproduction (seeds), so you can cut the time from planting to production by months or years by using rootstock. In a healthy system, strawberry rootstock will have new growth sprouting up in less than a week, with the first flowers at about two weeks, but it is important to pinch back the buds for 4-6 weeks to keep the plant’s resources directed towards vegetative growth, which will give the plant the ability for higher yields later on. If flowers are allowed to develop, fruit forms and ripens in about 2 weeks, though this will vary depending on the variety and growing environment. Outdoors, producers can rely on natural pollinators such bees, flies, and birds to spread pollen from the male parts to the female parts of the strawberry plants. Indoors, growers will either have to host a hive, or hand pollinate. Hand pollination can be done with a paintbrush. By lightly disturbing the centre of the flowers, one after the other, this will spread pollen from flower to flower. Hand pollination can take 10-30 seconds per plant, which can be time consuming on a large scale, so it may be more economical to use bees instead (Storey 2016l).

Strawberry pruning consists of leaf, flower, and crown pruning, and runner removal. Leaf pruning involves the removal of old leaves that start turning yellow. These leaves also prevent air circulation and light interception into the canopy, thereby enhancing the chance of disease development. Growth of runners during the production period is unnecessary and a waste of carbohydrates, which can be used for flower production. Therefore, runner pruning is also important for good quality fruit production. Flower pruning in strawberry is done to promote vegetative growth or to promote production of large fruits. When plants are started from runners, plants need to establish a large crown. For crown development, the flowers developed during early growth are removed, so that sugars produced by photosynthesis are allocated to vegetative growth. The size of fruit is inversely proportional to the number of flowers. If there are a large number of small flowers produced, small fruit production is likely, so flower pruning is necessary for good quality fruit production. Crown pruning is also important for flower bud induction in strawberry when plants are overly vegetative. During winter production, crown pruning is necessary for maintaining proper crown density in greenhouse strawberry production (Singh & Dunn 2017).

Figure 5: Strawberries growing in NFT channels https://www.maxpixel.net/Produce-Strawberries-Hydroponic-Farming-Growing-621914

Crop selection for different systems

The style of grow bed influences the choice of plants. In media-based units, provided that they are of the right depth (at least 30 cm), it is common practice to grow a polyculture of leafy greens, herbs and fruiting vegetables at the same time. Polyculture on small surfaces can also take advantage of companion planting for pest and disease control, and better space management, because shade- tolerant species can grow underneath taller plants. Monoculture practices are more prevalent in commercial NFT and DWC units, because the grower is restricted by the number of holes in the pipes and rafts in which to plant vegetables. Using NFT units, it might be possible to grow the larger fruiting vegetables, such as tomatoes, but these plants need to have access to copious amounts of water to secure sufficient supply of nutrients and to avoid water stress. Wilting in fruiting plants can occur almost immediately if the flow is disrupted, with devastating effects for the whole crop. Fruiting plants also need to be planted in larger grow pipes, ideally with flat bottoms, and be positioned over a larger distance than leafy vegetables. This is because fruiting plants grow larger and need more light to ripen their fruits, and also because there is limited root space in the pipes. On the other hand, large bulb and/or root crops, such as kohlrabi, carrots and turnips, are more likely to be grown in media beds because DWC and NFT units do not provide a good growing environment and adequate support for the plants (Somerville et al. 2014a).

Selecting plants for deep water cultivation (DWC) or raft systems requires consideration of a number of important factors (Valdez 2017b):

1. Weight – Rafts are usually quite durable and affordable, but they can only support so much weight. The best crops for deep water cultivation are small and lightweight. Lettuce, for example, is a popular DWC crop and the perfect size to fit on rafts. Larger crops like tomatoes grow top-heavy. Without the root anchoring provided by a dense media, top-heavy plants can fall over or break at the stems.

2. Footprint (volume) – DWC systems function on a single horizontal plane since they are typically too heavy to stack. This means that there is a 1:1 volume to growing area ratio, so it is necessary to fill the horizontal plane efficiently by choosing plants that can be grown at higher planting densities (ie. leafy greens).

3. Water-friendly – Drought loving plants and herbs like oregano and rosemary that prefer ‘dry feet’ don’t do well in DWC systems. On the other hand, thirsty plants like lettuce will thrive in deep water cultivation systems.

Bato buckets (or ‘Dutch’ buckets) are a variation of the media bed technique that uses a series of small media beds in buckets. A Bato bucket system is typically set up with buckets staggered on a bench or on the floor, with the feed line running water to the buckets from above, and the drain line (or return line) running water away from below. The three most common media used in Bato bucket systems are perlite, expanded clay, and coconut coir**.** These can be used by themselves or together in different ratios (Valdez 2017a).

The most popular crops for Bato buckets are large and/or vining crops like tomatoes, cucumbers, peppers, and aubergine. Vining crops grow in ‘leaders’ that vine upward or outward depending on the trellising. Many of these crops can therefore be trellised and trained upward, creating rows of tall towering plants which are easy to access and monitor. Selecting crops for a Bato system requires the following considerations (Valdez 2017a):

1. Disease resistance – Bato buckets can save a lot of space but cluster together crops, creating a vulnerability to disease. Tougher plants mean less risk and disappointment.

2. Footprint and plant style – The plants chosen for growing in Bato buckets will have an influence space, maintenance, and harvest strategies. Since Bato buckets are set up on horizontal planes, on benches or set on the floor, it is important for growers to take advantage of the volume of space above the buckets as much as possible. Vining crops allow growers to do that.

Figure 6: Bato buckets (on the right) being used to grow strawberries at the University of District of Columbia urban farm in Beltsville (https://w)ww.flickr.com/photos/usdagov/32245870463))

The best plants for Bato buckets are:

• Tomatoes – allow 60-90 centimetres between buckets. Two plants per Bato bucket will give maximum produce for material invested. The vining crop can grow to six or even twelve metres tall in a greenhouse setting.

• Bell peppers – allow 30-50 centimetres between buckets

• Cucumbers – allow 60-80 centimetres between buckets

• Aubergine – allow 20-40 centimetres between buckets

Copyright © Partners of the Aqu@teach Project. Aqu@teach is an Erasmus+ Strategic Partnership in Higher Education (2017-2020) led by the University of Greenwich, in collaboration with the Zurich University of Applied Sciences (Switzerland), the Technical University of Madrid (Spain), the University of Ljubljana and the Biotechnical Centre Naklo (Slovenia).