Perhaps some of the most versatile and easy to grow plants in hydroponic systems are lettuce and the huge range of salad greens we have available these days. It was not so long ago that the average fresh salad consisted of iceberg lettuce with a few slices of tomato and cucumber. But we now have a huge range of colored gourmet lettuces, fresh herbs and salad greens – many of which can still be remembered as rather annoying weeds not so ago. However, those weedy salad greens we now cultivate as gourmet crops do have a habit of being rapid growers and prolific producers of succulent green leaves which are well suited to grow in artificial light, indoor systems, window box systems and outdoor production.
Why Hydroponic Salads?
Supermarkets these days do tend to stock a range of premixed, prewashed, predressed, salad packs with combinations of salad greens and herbs, ready to eat. But you can not beat the freshness of plucking a few leaves from your own hydroponic system and making your own designer salad. Hydroponic systems such as NFT, Aeroponics and others largely avoid the problem of ‘grit’ contamination. This can be a problem in commercially prepared salads which have used soil grown plants. Picking your own salad also avoids the problems of browning and oxidation of cut leaves and stems that is common in pre-prepared salads, and it also means a grower can ‘formulate’ their own salad mix from the greens they like to grow. Growing salad or specialty greens has also turned into a profitable cash crop for many small hydroponic growers who use their hydroponic system to create clean, high quality salad mixes for local markets.
Growing Hydroponic Salads
Lettuce and salad greens also have the distinct advantage of being relatively low light, cool season plants, making winter production possible in many areas where the average tomato would be turning blue with cold. Optimum temperatures for most salad greens are in the range 16° – 22°C, however most will still grow, albeit slowing, in average temperatures of 5° – 10°C and will survive as low as 2°C for a limited time. Many of the more ‘heat resistant’ greens will in fact grow reasonably well in temperatures of up to 30°C, and some types of lettuce will even grow under tropical conditions.
Lettuce and salad green crops growing under winter conditions do benefit from nutrient solution heating in NFT or Aeroponics and even in media based systems.Bringing the nutrient temperature up to 16°C in winter (a small aquarium immersion heater will do this), speeds up growth yields of most salad green crops, making them a rewarding winter crop when the price of other green vegetables has sky rocketed!
Still the most popular ‘salad green’ plant, the selection of lettuce varieties is endless, although green butterhead and the coral varieties are still most widely grown. The tall upright Cos (romaine) and baby Cos (Little Gem or Diamond Gem) are also widely grown, and the old standard ‘crisphead’ produces well in cool season hydroponic systems. For those who like to harvest lettuce one leaf at a time, the ‘salad bowl’ types such as red and green oak leaf, are ideal as they withstand the continual removal of outer mature leaves and still produce well for weeks or even months without running out of steam. Of the lettuce varieties which perform particularly well in hydroponics and under artificial lighting are:
- Green Ice (a large green loose leaf variety with crisp textured glossy green leaves with fringed leaf margins – slow to bolt and with a long picking season).
- Red Fire (a deep red, loose leaf type) with ruffled margins and is suitable for both cool and warm climates.
- Tango (under cooler conditions only) is grown for its fantastic deeply cut leaves which resemble an endive, has a tangy flavor.
- Royal Oak Leaf (heat tolerant darker green type) seems to be particularly well suited to hydroponic production.
While there are improvements occurring all the time in terms of lettuce growth rate, size, head weight and coloration, the best cultivars to trail in hydroponic systems are those which have increased resistance to ‘tip burn’ and bolting (pre mature flowering), longer standing or holding time, heat tolerance – for summer season or warmer temperature crops and resistance or tolerance to diseases such as mildew and high degree of coloration for winter or cooler season crops.
Other Hydroponic Salad Green Plants
The most commonly produced salad green plants which are easily grown in hydroponics systems alongside lettuce are listed below. Many are grown for their appearance or color while others such as rocket, corn salad, and various types of cress are grown more for their distinctive flavors. Most of these plants are highly productive and withstand harvesting of a few leaves (always harvest the outer leaves from salad green crops) every week while continuing to grow.
- Chicory and Endive: Mediterranean salad plants – The most common type of Italian chicory produced hydroponically is radicchio which has a sharp, tangy flavor and is red and white in coloring. Endive salad greens have tender, finely divided and curled leaves that look great in salad mixes.
- Kale: Red and Green varieties are available – smaller leaves are used in salads, larger leaves as garnishes or decoration.
- Mustard Greens: Small, quick growing varieties are suitable for salad greens, they add a tangy flavour and come in both red and green types.
- Mizuna: Very quick growing in hydroponics, Mizuna is a commonly grown Japanese green. Grows into a large clump of finely dissected leaves which can be harvest from many times.
- Cresses: Water cress, land cress, winter cress and common cress are very easy to grow and produce at a rapid rate in hydroponic systems. Cresses have a distinctive peppery flavor and can be added to many salad types or used on their own as a vegetable. Watercress is a semi aquatic plant and can be grown from seed or cuttings and is particularly well suited to NFT, raft, float and similar systems.
- Rocket (Arugula): Rocket, sometimes called Arugula, roquette or rucola, is an ancient medicinal plant which is now becoming a very popular salad green crop. Both annual and perennial forms can be grown in hydroponics. The leaves have a distinctive nutty, peppery flavor that some say is reminiscent of peanuts. The flavor can be strong in older plants and those which are flowering (rocket flowers are also edible and can be used in salads).
- Chard: Baby leaves of Swiss chard (red, orange yellow, white and green) are commonly grown and used in salad mixes as they add a distinctive splash of color. Harvest young from hydroponic systems before any strong flavors develop.
- Spinach: Baby spinach leaves used raw in salads should be used as older plants can become bitter. Orache or mountain spinach (Artiplex hortensis) is also a useful plant as it comes in green, red and yellow varieties and is more easily grown in hydroponics than common spinach.
- Corn salad (Lambs lettuce): A plant that certainly looks like a small, hairy weed – but corn salad has a distinctive flavor (nutty, sharp and a little corn like) which blends in well in salad mixes.
- Miner’s Lettuce: Grown more for its unusual leaf shape and crisp texture, Miner’s Lettuce is an incredibly fast grower in hydroponics. Miners lettuce isn’t actually a lettuce, its a member of the Portulacaceae family and is also called winter purslane. It formed a valuable part of the diet of miners (hence its common name) during the gold rush of the 1850s, helping prevent scurvy in times when fresh vegetables were scare. This is an annual plant, grown from seed.
- Amaranth: Several members of the Amaranthus are used as leafy salad plants. Both red and green amaranth are heat tolerant and good for summer conditions. The flavor is similar to spinach and most need to be harvested while still quite young for use in salad mixes.
- These are just a few of the wide range of salad green plants which thrive in hydroponic systems. Keeping a regular check on seed catalogues is a good idea to see what the latest and greatest new salad green plant is!
Physiology of Salad Green Plants
Lettuce, as well as most of the salad greens, is a cool season crop; it has a lower temperature optimum than many other hydroponic crops such as tomatoes and other fruiting plants. Lettuce is like pretty much all other crops under protected cultivation: It is more the micro climates the grower creates and the genetics of the particular variety which can override whatever undesirable conditions exist outside. This is where the physiology of the lettuce plant in particular needs to be taken into account.
Bolting (Pre Mature Flowering)
Being a cooler season plant, lettuce crops can do a number of annoying things when they think conditions are a bit warmer than they would like. This is probably one of the most limiting factors a grower will come across with greenhouse production or growing under lights.
Firstly, lettuce plants have a tendency to bolt or go to seed prematurely before they have reached a harvestable size. Bolting (the plant going to seed and producing a long tall flower stalk) is a physiological response to a number of factors. Plant age is the usual one, when the plant is old enough it begins to flower. However with lettuce, this process is speeded up by any type of plant stress. Warm temperature can contribute to bolting while long day length can speed up the bolting process. Stress such as lack of moisture, high EC, high light and root death will all act to speed up the bolting as well.
Sometimes in the early stages of bolting a grower may not realize what is happening. Inside the centre of the plant, the stem starts to gradually elongate and spiral upwards. Sometimes if you pull the leaves of a mature, harvested lettuce you can see the stem, on which the leaves are attached, has started to stretch upwards. Many salad green plants and herb crops also have the same tendency to bolt. Some can even start to produce a flower stalk while they are only just seedlings if conditions are practically bad.
One of the best ways to avoid this problem, apart from good environmental control, is to select cultivars which are “slow bolting”, have “high bolting resistance” or are listed as “long standing before harvest” since the tendency to flower early in the plant’s life can be influenced by genetics.
The other physiological problem that a cool season plant can face in warmer climates and in greenhouses or grow rooms in particular is tip burn. Some commercial growers have said they can lose up to 50% of their crop to tip burn at certain times of the year.
What is actually worse is the tip burn which can occur inside the head of those varieties which form a tight heart of leaves; while the outside of the plant may look perfect, when its cut open all of the inner leaves can be severely affected by tip burn which tends to form a blackened rotting mass as pathogens start to decay the dead tissue.
The majority of tip burn is actually a lack of calcium in the tissue on the very edges of the leaves. When tissue is deficient in calcium, the cell walls begin to disintegrate and breakdown resulting in dead leaves which then dry up or rot depending on how high the humidity levels are.
This lack of calcium at the leaf edges usually isn’t due to deficiency of calcium in the soil or nutrient solution, but actually a calcium transport problem within the plant itself. Calcium, when taken up by plants is carried in the transpiration steam of water, through the xylem vessels right to the tip of the leaf. It’s the transpiration of water from the leaf surface which drives the flow of water through the xylem from the roots to the leaf, and this water carries calcium with it. Under certain conditions, the plant may not transpire well resulting in not enough water and calcium being driven to the leaf tips. This is common when it is warm and humid; transpiration is reduced and not enough water moves in the transpiration steam out the ends of the leaves. Because of this, not enough calcium is deposited in the tissue and the cells begin to break down.
On the other hand when it is overly dry, sometimes the plant just cannot take up and transport water fast enough to get it out to the leaf tips causing them to dry off as well. Then there is another complication; some studies have shown that the xylem vessels themselves which are hollow tubes inside the plant which carry the water and calcium can be come blocked up (just like our arteries have a tendency to do). It appears that this blockage might be plant cell debris, which is sometimes crystalline in nature, but it acts to restrict the flow of water and that all important calcium resulting in tip burn.
Tip burn is best prevented by maintaining good temperature levels (below 25°C) combined with air movement. Using small air circulation fans in the growing area which gently move air across the tops of the plants will enhance not only photosynthesis but also transpiration, meaning that more water and calcium can be moved into the leaf tips. Keeping EC levels down below a level of 1.0 when tip burn becomes present on the plants also helps with the uptake of calcium and transport out to the leaf tips.
Color and flavor
When we look at the physiology of crops such as lettuces, greens and herbs some of the factors which are vitally important are flavor and color. With lettuce in particular, the taste would really be non-existent, the leaves need to be succulent with a high moisture content but there should not be any bitter flavors.
Some of the salad green such as rocket, corn salad, French sorrel, and cress do have distinctive flavors which should be characteristic of the plant and not overly strong or bitter. What causes bitter compounds to develop in salad crops is usually one of two causes: Either the plant has begun to form a flower stalk and bolt, which increases the levels of bitter components in the leaves, or the plant has been slow growing and under stress. A lack of moisture, high EC, incorrect nutrient and/or poor environmental control will all cause a build up of bitter flavours and needs to be avoided. A healthy, rapidly grown plant will not in general have a strong flavour.
Regarding colour, perhaps one of the most common problems with the production of salad greens is with the “reds.” Growers generally prefer a dark deep intense red colour on lettuce leaves which looks good when combined as part of a salad mix. There is a huge range of red varieties to chose from, so selecting those which have the darkest or best colour potential is a good idea. However, what can happen under certain growing conditions, particularly when the plants are growing quickly, is that the intense red colour can become diluted and end up looking like a muddy brown.
Deep red pigmentation is promoted by several factors in lettuce. The colour in the leaves is determined by the concentration and types of pigments, so that plants grown in shaded conditions will have more green and less red pigment than those grown in full light. Red pigments are enhanced by high light levels and cool conditions. The best red coloration usually develops during late winter and early spring, particularly on outdoor grown lettuce crops. Indoor growers have the advantage of being able to supply higher light levels when required for greater colour development, even though it may be dark and dull outdoors. In warmer weather when the plants are growing faster, the pigments become diluted. Other factors such as good nutrition, a higher EC level of the nutrient solution and genetics will help promote more pigment formation and a deeper red colour.
Light and Systems
Light is important for the development of colour in lettuce, but it also affects the physiology of salad green crops as well. Plants need sufficient light to carry out photosynthesis for optimum growth and for coloration, but they also need light for overall plant quality. Lettuce and many of the salad greens have a much lower light requirement to reach saturation levels than a crop of tall, fully-grown tomatoes, but if they don’t receive enough light not only will growth be restricted, but plant quality as well.
Lettuces grown with insufficient light are often pale. The leaves are thin, the plant is light in weight and also more prone to attack by disease pathogens. Overall, a lack of light results in weak plants. Usually with hydroponic lettuce crops grown as a single layer there is no problem where light (except inwinter in some areas of the world) and provided heating is kept up to optimal levels, growth is not overly restricted. However, once there is more than a single layer and tiered systems are used, natural light can become limited on the lower levels. Tiered systems are good in that they make maximum use of growing space but they need to be designed to allow sufficient light penetration on the inner gullies once the plants have reached full size.
There is a large and diverse range of plants in the lettuce and salad greens category, however most will happily grow in a mixed system on a standard “grow or vegetative nutrient solution” at moderate EC and light levels. There will constantly be new plant species introduced into the salad green and herb range so its a good idea to keep an eye out for anything that might be worth trailing and tasting. Just a couple of square meters of mixed hydroponic salad green plants, mean that harvests of tasty, succulent leaves can be turned into an instant salad any time of year.
Some seed suppliers of lettuce and gourmet salad greens:
Johnny seeds: www.johnnyseeds.com
Territorial seeds: www.territorial-seeds.com