6.4 PLANT SELECTION

To date, more than 150 different vegetables, herbs, flowers and small trees have been grown successfully in aquaponic systems, including research, domestic and commercial units.

Appendix 1 provides a technical summary of, and detailed growing instructions for, the 12 most popular herbs and vegetables. In general, leafy green plants do extremely well in aquaponics along with some of the most popular fruiting vegetables, including tomatoes, cucumbers and peppers. Fruiting vegetables have higher nutrient demands and are more appropriate for established systems with adequate fish stocks. However, some root crops and some sensitive plants do not grow well in aquaponics. Root crops require special attention, and they can only be grown successfully in deep media beds, or a version of wicking beds discussed in more detail in Section 9.3.
Vegetables vary regarding their overall nutrient demand. There are two general categories of aquaponic plants based on this demand. Low-nutrient-demand plants include the leafy greens and herbs, such as lettuce, chard, salad rocket, basil, mint, parsley, coriander, chives, pak choi and watercress. Many of the legumes such as peas and beans also have low-nutrient demands. At the other end of the spectrum are plants with high-nutrient demand, sometimes referred to as nutrient hungry. These include the botanical fruits, such as tomatoes, eggplants, cucumbers, zucchini, strawberries and peppers. Other plants with medium nutrient demands are: cabbages, such as kale, cauliflower, broccoli and kohlrab. Bulbing plants such as beets, taro, onions and carrots have medium to high requierements, while radish requires less nutrients.

The style of grow bed influences the choice of plants. In media bed units, it is common practice to grow a polyculture of leafy greens, herbs and fruiting vegetables at the same time (Figure 6.7). Provided media bed units are the right depth (at least 30 cm), it is possible to grow all the vegetables mentioned in the categories above. Polyculture on small surfaces can also take advantage of companion planting (see Appendix 2) 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 in fact occur almost immediately if the flow is disrupted, with devastating effects on 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 cultured in media beds because NFT and DWC units do not provide a good growing environment and adequate support to the plants
It is important to consider the effect of harvesting the plants on the entire ecosystem. If all of the plants were to be harvested at once, the result would be an unbalanced system

FIGURE 6.7
High nutrient demand vegetables growing in media beds, including eggplants (Solanum sp.) (a) and tomatoes (Solanum sp.) and cauliflower (Brassica sp.) (b)
without enough plants to clean the water, resulting in nutrient spikes. Some farmers use this technique, but it must correspond with a large fish harvest or a reduction of the feed ration. However, the recommendation here is to use a staggered harvesting and replanting cycle. The presence of too many plants growing synchronously would result in the systems being deficient in some nutrients towards the harvest period, when the uptake is at a maximum. By having plants at different life stages, i.e. some seedlings and some mature, the overall nutrient demand is always the same. This ensures more stable water chemistry, and also provides a more regular production both for the home table and the market. Staggered planting schemes are discussed in more detail in Chapter 8.