Leguminous plants—which include peas, beans, lentils, alfalfa, chickpeas, carob, tamarind, soybean, and peanuts—are staple food crops worldwide and are consumed almost daily in a range of different forms.
While commodity crops such as pulses (which are part of the legume family, though “pulse” refers only to the dry seed) are cheap and readily available, the tender and gourmet fresh versions of legumes are well worth a place in a hydroponic garden. In fact, they are often grown as commercial greenhouse crops.
A legume is a plant in the Fabaceae (or Leguminosae) family, which produces seed pods and have the important characteristic of forming a symbiotic relationship with nitrogen-fixing bacteria contained in nodules on the plant’s root system. Thus, in soil production, legumes can play an important role in crop rotation and nutrition and this benefit is not just limited to outdoor farming systems.
Legumes and Nitrogen Fixation
Nitrogen fixation by legume plants occurs when symbiotic bacteria known as Rhizobia colonize the root system and take up residence within visible root nodules. These bacteria can absorb nitrogen gas (N2) from the air and convert it into ammonia (NH3) to be used by the plant. In return, the bacteria receive a carbohydrate food supply from the plant.
The bacteria’s contribution is particularly useful for crops where nitrogen may be limiting in the soil or growing medium as it provides a source of free nitrogen fertilizer. In a well-fed hydroponic crop where we can supply plentiful nitrogen in a highly soluble and immediately available form, the use of nitrogen fixation may not seem to be that beneficial.
However, with the increasing interest in organic nutrients and hydro-organic systems, nitrogen-fixing crops do have a major advantage. Organic nutrients and systems often experience issues with nitrogen nutrition as plants have a high requirement for this element and can deplete it from organic solutions and systems very rapidly under rapid growth conditions.
Nitrogen availability is dependant on the rate of microbial conversion of organic materials to plant-available forms of nitrogen, and this may not keep pace with high-density, vigorous plant growth.
Even though there is no soil present to act as a plentiful source of inoculum in hydroponics, Rhizobia symbiosis does often occur naturally. To speed up the process, plants can be inoculated with the correct species of nitrogen-fixing bacteria. These are widely available as microbial products or incorporated as seed coatings.
Also, keep in mind that the root nodules that form on hydroponic legume crops are visible and appear as knobbly, rounded growths that can reach the size of a pea, firmly attached to the root system. These have sometimes been mistaken for root pathogens or other diseases by inexperienced growers, but they are a perfectly natural occurrence.
The legume crops commonly grown in hydroponic systems tend to exclude commodity crops such as pulses and instead focus on the higher-value fresh vegetable, garnish, and gourmet baby produce types.
The development of new, dwarf, or “container” varieties of peas and beans are particularly advantageous to indoor gardens and small hydroponic systems, as they produce high yields on relatively small and compact plants that don’t require trellises or supports like the tall climbing types do.
Both peas and beans are also used as microgreens and sprouts, taking even less space than mature plants and requiring only a few days from seeding to harvest.
Hydroponic systems suitable for growing peas and beans include substrate and solution culture, but large, deep growing containers are recommended for taller varieties and longer-term crops such as climbing pole beans, fava beans, and soybeans due to the extensive nature of the root system. Bush/dwarf beans, dwarf pea varieties, and snow peas can be grown in larger-sized nutrient flow technique (NFT) channels.
Pea seeds grown to produce pea shoots and mung beans grown as sprouts can be produced on plastic trays, on mats or pads of rockwool or other substrates, and misted with dilute nutrient solution once the first seedling leaves have expanded.
While mung bean sprouts can be grown in the dark to produce the white/yellow pale shoots, pea shoots require a moderate level of light to form the green leaves and tendrils these are harvested with. With shoot, sprout, or micro green production, it is essential that strict hygiene measures are followed.
This includes the use of clean, untreated seed, sterilized trays, and high-quality water. For some seed, surface sterilization in a diluted bleach solution followed by rinsing with water helps prevent fungal pathogens and reduces food safety risks during sprout or micro green production.
Fresh beans are a popular vegetable and commonly grown in commercial hydroponic greenhouses year-round. While the green pole or runner bean are the most commonly grown, other beans grown hydroponically include fava (also known as broad bean), lima, and the increasingly popular soybean.
Fava beans have the advantage of being more tolerant of cold than most other beans and thus can be grown in unheated conditions in many climates; however, the plants are large and tall at maturity and more suited to roomier hydroponic systems.
Soybeans grown for edamame (in which young immature beans are eaten fresh, often lightly steamed) produce well under hydroponic conditions and are perfect to eat immediately after picking. There are many varieties of soybean, so be sure to select a suitable breed if growing for edamame; for example, Envy, which grows to two feet and requires temperatures of around 82˚F for optimum germination.
For smaller systems, bush or dwarf snap beans, also known as French beans, are easily grown and take up little space. These are self-supporting and highly productive, and come in a range of pod types and colors from green to yellow, and even dark purple. Ranging in height from one to two feet, bush beans generally self-pollinate in indoor environments; however, temperatures over 90˚F can restrict flowering and pollination.
Peas are an extremely versatile crop for hydroponic systems. Not only are there varieties with edible pods, but the flowers, shoots, and tendrils can all be grown as either gourmet fresh vegetables or as attractive garnishes and additions to salads. Fresh shell peas can be produced in hydroponic systems; however, they take up considerable space for the yield of young peas produced. Instead, snow peas, snap peas, and pea shoots are typical specialty hydroponic crops.
Snow peas, be it the yellow, purple, or traditional green varieties, are often a highly priced fresh produce item and thus are grown in hydroponic greenhouses year-round.
These are easy to grow but must be harvested at the correct stage before the young seeds inside the pod begin to develop. For edible pea flowers, there are the common white flowered types, as well as red flowered varieties. Afila type pea varieties, which produce vines with many tendrils but few leaves, are useful pea shoot/tendril cultivation, but they can be used to harvest pods as well.
For hydroponic systems, either tall or shorter varieties can be grown. Taller vines need some form of support, while shorter cultivars are up to two feet in height and are often self-supporting. Indeterminate, shorter varieties of peas are the preferred choice for those with limited space, as these can be planted in double rows or groups so that plants support each other without the need for trellising.
Choosing pea varieties with powdery mildew resistance is recommended and many of the new and improved cultivars are worth trialing in hydroponic systems.
Peas prefer lower overall growing temperatures than beans, with optimums in the 55-65˚F range for most varieties. Temperatures above 85˚F can lead to minimal vegetative growth and poor flower development and fruit set. Peas also benefit from good air flow around the base of the plants and avoidance of overcrowding, which can promote the development of fungal diseases.
Peanuts are another legume crop that performs well in hydroponics and makes an attractive, interesting, and unique addition to an indoor garden. The peanut plant produces its pods on the ends of pegs, long stems that develop from the aerial portion of the plant after flowering and then burrow down under the growing media to develop the young fruit (peanut seeds inside brown pods).
For this reason, hydroponic systems for peanut plant production need to have substrate that is soft, friable, and light, and with the surface under the plant exposed for the pegs to grow down into. Finer grades of sterilized coconut fiber or blended coconut fiber and perlite make ideal substrates for this crop.
A well-grown peanut plant can have up to 30-40 pods, so a growing container or bed that holds at least 2.6 gallons of media is required when growing hydroponically. Also, peanut plants need a warm growing environment with reasonably high light levels for maximum growth, and good air flow up and under the crop, as damp conditions favor fungal pathogen development.
Crop timing under ideal growing conditions is usually around 140 days, although is somewhat variety dependant. Outdoors peanuts are harvested when the foliage begins to turn yellow and die back; however, in a hydroponic system, it is possible to dig back some of the growing substrate, observe the development of the young pods, and determine when the peanuts are large enough to harvest some or all from the plant.
Harvested peanuts then need to be dried inside the pods in a warm place for about four weeks before they can be stored.
Hydroponic peanut crops can easily be started from seed. There are a number of varieties, but the large Virginia types seem to perform particularly well in hydroponics, followed by the smaller Spanish peanut types. To obtain peanut planting stock, purchase raw—that is, non-heat treated—seed that is still contained in its outer husk or pod, as this prevents drying out. (You’ll shell them just before planting out.)
It’s best to get large, plump peanuts free of any signs of deformity or rot. Before planting, germinate the peanuts on a heat pad or at 72-84˚F. The essential step with growing peanuts is to surface sterilize any nuts used as seed, as these are prone to fungal attack during the germination stage and damping off disease as young seedlings.
Dusting peanut seeds with a fungicide before planting is also highly effective with getting the crop started with minimal problems. Peanut plants self-pollinate once the yellow flowers have formed and the peg outgrowth is visible within a few days.
Whether it’s trying your hand at a few peanut plants, producing some beautiful pea flower and tendril garnishes, or fresh edamame, legumes have a huge potential as hydroponic crops. Attention to the important aspects of these crops such as cultivar selection, tall or dwarf growth habit, disease resistance, and intended purpose is worth investing some time and research into to obtain those perfect fresh pods.