The easiest and most successful plants to grow aquaponically are green leafy vegetables and herbs. They generally have lower and less tedious nutrient requirements. Lettuce is a very common crop that meets this description. It lends itself well to an aquaponic scenario and is in high demand, it is an ingredient in most salads and comes in many varieties.
Leafy varieties, such as loose-leaf lettuces and romaine are preferable as opposed to crisp head / iceberg varieties. They mature more rapidly, an average of 40 days as opposed to 90 days for the iceberg styles and are more nutrient rich.
Crisphead – iceberg lettuce has tight heads, crispy leaves and higher nutrient requirements.
Butterhead lettuce leaves are loosely bunched on one another and are suitable to harvesting one leaf at a time.
Romaine has semi-tight heads , is slow to bolt.
Loose leaf lettuce, has no head and can be easily harvested leaf by leaf without picking the entire plant
Starting from seedlings that have already been germinated is best, they should be transplanted into the aquaponic setup after 2 but no more than three weeks, at this point they should have at least 2 to 3 true leaves.
Hardening your seedlings is advisable for increased survival rates. Hardening is simply exposing the seedlings to full [sun]light, wind [if planting outside] and cooler temperatures for several days in order to acclimate them.
Phosphorous added to the seedlings BEFORE transplanting into the aquaponic setup will help accommodate healthy root growth and curtail plant stress from transplanting.
Wash off any soil residue before placing them in the aquaponic setup, and I do mean ALL soil residue not just most of it.
Introducing soil borne pathogens and micro organisms from soil clinging to the root system of transplants to any aquaponic garden can be detrimental not only to the plants but the fish as well. Washing off the soil also helps ensure adequate aeration.
If transplanting lettuce in warmer weather it is advisable to provide partial shade if outdoors and if possible use a heat tolerant, bolt resistant variety. Great Lakes crisphead lettuce, Oakleaf leaf lettuce, White Boston butterhead lettuce, Ermosa and Adrianna are a few – there are many others.
Under warm conditions with high lighting, lettuce can be force grown faster with high nitrogen as is provided naturally in an aquaponic setup. You should by no means ever add additional nitrogen to an aquaponic grow as the fish waste should be supplying more than enough.
Spacing of your transplants is important, you’ll want to leave adequate space not only to ensure your plants mature properly but also to minimize the risk of brown rot and assorted diseases which lettuce is prone to. Spacing varies a tad from species to species.
Direct seeding lettuce in an aquaponic set up is feasible but has more drawbacks than using transplants. For starters, in certain scenarios you will lose a higher percentage of your seed as it gets washed down gullies and conduits, as the bed floods and drains the seeds go down further with each cycle. Seed tape is one option so long as it does not disintegrate after repeated exposure to water cycles. Other options include using a fine media or even gravel such as that used in many fish tanks at the base of the grow bed, this however will slow down germination time and success ratio.
Lettuce, with rare exceptions grows best in air temperatures above 60o and below 75o F. The water temperature is also vital to producing abundant healthy harvests of lettuce, water temps should be maintained in the ballpark of 70o F. Higher temperatures will cause the plant to bolt to seed and produce a bitter tasting leaf.
With aquaponics there is the added variable of the fish, you must choose a variety of fish compatible to these temperatures. Tropical fish are for the most part not compatible with a recirculating aquaponic system as they require warmer water.
Tilapia, trout, bass, sunfish, koi, carp all fall within the parameters in relation to temperature compatibility. Fish that require a warmer water temperature can still be grown along with lettuce but it would require a decoupled system, one in which the water temps can be adjusted before transferring from one stage to another.
The optimal pH is 6.0 to 6.2. Lettuce can still grow in a pH as high as 7.0 but a higher pH will sometimes contribute to Iron deficiencies.
Ideally, in aquaponics you should not need to add nutrients as is done in hydroponics. Unfortunately not everything is sugar coated and simplified. Lettuce at times needs additional nutrients not supplied by the fish. Calcium is the most common, Calcium chloride foliar applications are the best way to supplement calcium, it is applied directly to the leaves and has virtually no adverse effects on the fish. Before adding calcium be sure you need it See: Hydroponic calcium
Lettuce also needs high amounts of potassium to grow well, once again a foliar spray is advisable.
Nitrogen excess is sometimes an issue with aquaponic lettuce due to overstocking of the fish. The fish waste contains a high amount of Nitrates and nitrites which are essential to the lettuce plants – but not in extreme excess. Excessive nitrogen leads to poorly formed leaves and wilting even though the leaves grow rapidly. At times a bitter taste also develops.
For best results Lettuce should be grown rapidly. It requires high amounts of potassium to grow well. Some lettuce varieties are nitrogen sensitive and easily get leaf tip burn when nitrogen levels are too high.
Under lower light levels, such as fluorescent lighting, potassium and nitrogen levels should be reduced. Related Article – Hydroponic Nutrients
Basic Aquaponics Hydroponic Lettuce