4.5 Plants

Stocking and harvesting strategies can also be implemented in the hydroponic portion of the system. The three most common strategies are staggered cropping, batch cropping, and intercropping (Rackocy et al. 2006). Their implementation and success depend on geographic location (tropical or temperate regions), crop variety (leafy vs. fruiting crops), and market demand.

Aquaponic producers typically grow leafy green crops, which have a lower value per unit value and high yield. Lettuce, Swiss chard, kale, basil, and other herbs are typically ready for harvest between 3-5 weeks from transplanting (6-8 weeks from seed), resulting in a steady income stream. Fruiting plants like tomatoes, cucumbers, and peppers take 10-16 weeks to harvest, resulting in longer growing periods and lower yields, but they have a higher individual value. Producers often grow a variety of crops to diversify their markets and reach a number of consumer groups.

It is critical to invest time in a production strategy that realistically evaluates inputs and product output. Market demands vary among countries, regions, and even among neighboring cities. Producers should calculate the real-estate value of their system, often in price per square foot. To illustrate this, a comparison between two types of lettuce can be used. Figure 12 shows two different types of lettuce grown at the University of Virgin Islands in St. Croix. Although Parris Island romaine has a higher individual value ($/ head) than Boston bibb, when the planting density and growth period are considered, Boston bibb brings a higher value per square meter of growing area per week than the Parris Island romaine. The main takeaway here is that high density and frequent harvests may an increased value, even when individual value of the crop is low. Information presented here is just an example and calculations should be tailored to a specific crops, farm, market, and regional costs for production.

To understand if the crop is profitable, the cost of labor from seed to harvest, price of seed, propagation supplies, and retail packaging will need to be subtracted from the price/m2/week. If the selling price is below that of its “real-estate value,” the hydroponic portion may be operating at a loss. In addition, producers may have multiple harvests from the same crop. Kale and Swiss chard are crops that can sustain multiple harvests without a decrease in quality of the produce, therefore increasing the value of that real estate. The strategies included here are not a comprehensive list but can be developed and adapted for individual plants.

Staggered Crops: Staggered cropping is growing multiple stages of crops in the same system and typically allows a consistent and regular harvest to be maintained (Somerville et al. 2014) (Figure 13).

For example, if a head of lettuce takes three weeks to reach maturity, three stages are cultivated at the same time, resulting in a weekly harvest. This method is used with crops that are ready for harvest in a short time, usually leafy greens or herbs. This method maintains a constant nutrient uptake by the plants, resulting in better control of the system and water quality parameters, making system management and outputs more predictable.

Batch Crops: Batch cropping is commonly used when a longer growing period is required, such as with tomatoes and cucumbers. Produce is collected in batches as it ripens or becomes available.

Intercropping: Some producers will intercrop their plants, meaning crops with a short time to harvest are planted along with larger, fruiting ones (Figure 14). For example, if a producer is growing lettuce and tomatoes together, the lettuce crop can be harvested before the canopy of the tomatoes grows tall enough to shade it out.

Source: Janelle Hager, Leigh Ann Bright, Josh Dusci, James Tidwell. 2021. Kentucky State University. Aquaponics Production Manual: A Practical Handbook for Growers.