Hydroponics 101 - Soilless Salad
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by Dr. Chris Kline
Gourmet salad greens are becoming increasingly popular for hydroponics growers, and for good reason. The nutritional value and flavor of common iceberg lettuce pales when compared to other salad greens, a fact realized by more and more higher end grocers, restaurants and gourmet food consumers. For
those willing to pay higher prices, the selection of commercially available designer greens is increasing, but it pales in comparison to the variety available to those smart enough to grow their own.
Lettuce and other greens are some of the most versatile, easy to grow, and well suited plants for hydroponics growing for the following reasons:
Nitrogen Supply – Unlike most of the fruiting vegetables, lettuce and greens need a high nitrogen supply for rapid growth and prolific production of succulent green leaves. What better growing method to easily provide a continuous supply of nitrogen and other nutrients to support this rapid foliage growth than hydroponics.
Cool Environment – Even the so-called heat tolerant varieties of lettuce do best when kept in a cool moist environment. There is an evaporative cooling effect when growing plants in hydroponics that directly benefits lettuce production. Hydroponics units offer a space saving growing environment that is well suited to indoor and greenhouse gardens which can be climate controlled.
Pest and Disease Control – Lettuce and other greens are particularly susceptible to soil born problems caused by various forms of fungus like powdery mildew and pests like aphids and slugs. Most pest and disease problems are greatly reduced or eliminated by growing lettuce in hydroponics systems.
Easy to provide for light Requirements – Most lettuce is recommended for full sun, but it does not need a full day of it. In fact with six hours or so of morning sun or partial shade as part of a 16 hour light day, lettuce will do just fine.
No Grit Contamination – With hydroponically grown salad, there is not worry of the grit or contamination that can come with soil grown plants.
Improvements are occurring all the time in terms of lettuce growth rate, size, head weight and coloration, and each year’s seed catalogs are filled with new varieties. When selecting cultivars for hydroponics, look for descriptions which contain some or all of the following wording: increased resistance to tip burn and bolting (pre mature flowering), longer standing or holding time, heat tolerant, and resistance or tolerance to diseases such as mildew.
Pelleted seeds are also something to consider. They may have advantages for hydroponics starts. Pelleted seeds are easier to handle, particularly for plants like lettuce and greens which have very small seeds. The clay coating retains moisture and therefore reduces any potential desiccation should an irrigation cycle be missed. They also have been pre-germinated through a process of whetting/drying before being pelleted which means that germination time will be shorter than with non-pelleted seeds.
Lettuce seed loses viability quickly with age, so make sure that the seed you buy is dated. The seed is best stored in a refrigerator providing a cold, dry atmosphere. Even under these conditions it may be best to only purchase enough seed for six to twelve months. Pelleted seed does not keep as long, so purchases should be kept to no more than can be used within 6 months.
Other Greens to Consider
Not all plants that work great in salads are lettuce. Many of these greens are used for cooked vegetables when grown to maturity, but are great in salads as baby greens. Some of these greens add color to the salad bowl, while others are known for their distinctive flavors. As a general rule, they can become bitterer, just like salad greens, if left on the plant too long before harvesting. Distinctive flavor is a good thing if used in moderation, but some greens can be overpowering if too large a percentage is added to a salad. It takes a bit of experimenting with flavors to know just how much of each particular green to include in a salad mix. The following are some good choices to grow along side your hydroponic lettuce.
Endive – This green has become very popular in salad mixes and is easily spotted by its finely divided and curled leaves.
Arugula – Also called Rocket is quite common in gourmet salad mixes. Be careful to harvest early as the peppery, nutty flavor can get overpowering in older plants.
Radicchio – This is distinguished in the salad bowl by red and white coloring. It is a form of Chicory with a sharp, tangy flavor.
Kale, Mustard Greens and Chard – These are great producers that need to be continuously harvested when the leaves are still small for salads. Assorted colors are available.
Spinach – Spinach makes a great salad on its own or added to the mix. You guessed it! Harvest baby leaves for the best flavor and tenderness.
Water Cress and other Cresses – These are a natural for hydroponics being semi aquatic plants. They will produce rapidly and have a distinctive peppery flavor.
Miner’s Lettuce – Not a true lettuce, has unusual shaped leaves and is a prolific producer in hydroponic gardens.
Chives, Sorrel, Chicory, Good King Henry, and Salad Burnet also work well.
Most lettuce growing systems do not use a substrate apart from that which the seeds are started in to grow the plants. For this reason it is best to use inert material that can simply be placed in the system with the sprouts when ready. Common choices are rockwool, Oasis cubes or Sure to Grow medium. They all should be soaked with a dilute nutrient solution of 0.5mS EC prior to sowing. If using rockwool, it is also necessary to soak in water with a pH of 5.2-5.4 to lower the pH of the rockwool that initially is 7.5 or greater. Use a half-strength nutrient solution until the seedlings are transplanted into the system.
Common Hydroponic Systems for Lettuce
Lettuce is particularly suited to water culture systems because it is not in the system long enough to suffer from oxygen deficit that can occur with other long term crops like tomatoes, cucumbers and peppers. Most lettuce and greens will grow well in various hydroponic systems using high nitrogen “grow” nutrient solution at moderate EC levels. Lettuce can even do quite well in aquaponic systems that often lack sufficiently complete nutrients to support other fruiting vegetables. Nutrient film technique (NFT) is the most popular culture for growing lettuce.
Temperature and Air Circulation
Lettuce and most other greens like it cold (60°F-70°F) so it can be more economical to grow them in the winter in cold frames and greenhouses than other vegetable crops. While growing under winter conditions salad greens will benefit from nutrient solution heating to 60°F which can be provided with a small inexpensive aquarium immersion heater. Conversely, there is some evidence to support that by cooling nutrient solution to 60°F-70°F during summer months it may be possible to discourage bolting even at higher than optimal air temperatures.
Tip burn and many other problems can be prevented by maintaining good temperature levels (below 78°F) combined with air movement. When growing lettuce indoors, small air circulation fans should be used in the growing area to gently move air across the tops of the plants. This will not only enhance photosynthesis, but also transpiration, meaning that more water and calcium can be moved into the leaf tips. Calcium not reaching the leaf tips is the main cause of tip burn.
Lettuce and many of the salad greens have a much lower light requirement to reach saturation levels than many other vegetables such as peppers and tomatoes, but if they don’t receive enough light, growth and plant quality will be restricted. One advantage indoor growers have is being able to supply higher light levels during the colder months where it may be cloudy and dark outside.
Fortunately with hydroponic lettuce crops grown as a single layer in a greenhouse, there is generally no problem with adequate light except in winter in some areas of the world. Care must be taken when using tiered systems to insure sufficient light still reaches the interior sections.
Lettuce needs 16 hours of daylight. During winter months in the northern latitudes supplementary artificial lighting can be used to shorten the cropping period. Remember with lettuce that the name of the game is to encourage quick growth and harvesting. If artificial lighting is used, metal halide (MH) lighting is best for lettuce and other leafy crops.
Nutrients and pH
The optimum pH of the nutrient solution is between 5.5 and 6.0 and the EC should be from 1.0 to 2.3 mS. If optimal light levels can be maintained, plants can be forced into optimal growth by use of higher nitrogen levels. If optimal light levels are not able to be maintained for a period of time, the EC should be reduced accordingly.
Lettuce and other greens are some of the most well suited plants for hydroponics. They are also one of the most useful food crops to grow fresh right in your own home. Even a small space by a sunny window is good to get started, and the amount of salad production from a small hydroponics garden is a pleasant surprise.