How to Grow Aquatic Crops Hydroponically
Aquatic plants are easy to grow in modified hydroponic systems and provide both great indoor displays and exotic food crops to tempt the taste buds. Better yet, aquatic crops thrive when their roots are continually submerged, making them easy to grow. Try growing exotic species such as Chinese water chestnuts, lotus root and water spinach.
Exotic aquatic crops are perhaps some of the most ideally suited plants for solution-culture hydroponics. Adapted to warm, wet, flooded environments, they thrive when their roots are continually submerged and generally have robust constitutions.
You are probably already familiar with aquatic food crops like watercress and rice, at least from the grocery store, but there are tons of other species worthy of your attention. Chinese water chestnuts, lotus root and water spinach grow and produce extremely well in small hydroponic systems.
Unlike many aquatic food crops from subtropical areas, watercress grows and produces well in cool conditions similar to those that are ideal for lettuce. It is grown mainly for use as a fresh salad crop and has a peppery, tangy flavor, but it can also be cooked.
As one of the oldest known leaf vegetables consumed, watercress is widely grown in hydroponic systems because the clean growing environment is preferred over crops harvested from the wild.
Watercress can be propagated from seeds, or from cuttings that form aerial roots at the nodes of mature plants. A cut stem placed in water will develop a new root system within days that is ready to be transplanted into a hydroponic system. Using this method, plants can be started from bunches of watercress stems purchased fresh from the grocery store.
Read More: Algae Growth in Your Hydroponic System - Friend or Foe?
While commercially grown hydroponic watercress is typically produced in either a nutrient film technique or gravel-bed system, it also grows well in raft or float systems and alongside many other aquatic plants in pond systems.
A small number of nutrients can assist with growth, but high nutrient-solution temperatures should be avoided as they promote early flowering on the stems. Watercress is a quick and easy plant to produce. The stems can be cut for use at a young stage, leaving the plant to regenerate more foliage for further cuts.
Most watercress plants can be continually harvested over many months and only need to be removed when the root systems become too large for the hydroponic system. At this stage, stems are simply cut from the plants and slotted into new planting holes to start new crops. The main pests when it comes to watercress are caterpillars, which infest the leaves at certain times of the year.
Chinese Water Chestnuts
The Chinese water chestnut plant, which is an aquatic sedge, is among the most fascinating and exciting plants to grow hydroponically. Water chestnut corms that form on the root system are eaten fresh and are described as slightly nutty with an almost sweet coconut flavor, and are considerably better than the canned version used in most Asian cuisine.
Their flesh is white and crisp, and forms under a brown, papery skin. In China, water chestnuts are grown in the same way as rice, in flooded paddies constructed with a clay pan base in a warm and humid environment.
In hydroponic systems, the plants require temperatures around 77-86°F for rapid growth, making indoor gardens or greenhouses perfect for year-round production.
Water chestnuts are propagated from last season’s mature corms, which can be kept moist in the fridge until planting. When conditions are sufficiently warm (above 73°F), corms sprout new shoots and roots, which can be planted into hydroponic aquatic systems.
In cooler areas, the corms can be sprouted in stonewool propagation cubes on a heating pad until sufficient foliage and roots have developed.
Water spinach, also called kangkong, ong choy or water convolvulus, has a mild flavor more like lettuce than traditional spinach, and is eaten raw as well as cooked in many Asian dishes. Being a subtropical plant, it prefers warmer conditions above 77°F, and growth is extremely rapid, particularly in hydroponics.
Kangkong produces succulent, almost swollen, stems and large leaves after about three weeks from planting out into aquatic hydroponic systems, and also produces well in nutrient film technique, deep flow technique and raft or float systems.
When grown from seed, kangkong is easily and rapidly propagated, and there are a couple of different types to choose from, including bamboo- and broad-leaved strains.
While small-scale indoor rice plants might not yield much in the way of grains, the plants are attractive and interesting to grow due to ornamental strains such as black rice, which has an attractive, dark purple foliage.
Rice is grown from seed and is a tropical crop requiring a long growing season and temperatures of at least 75°F. A mini rice paddy can be made from a child’s plastic paddling pool lined with gravel and sand to support the young plants in the early stages.
Hydroponic gardeners who have experimented with rice often have problems with growth after the initial first few weeks, which is likely due to the unique nutritional requirements of rice plants—rice needs high levels of silicon (80-100 ppm) in the nutrient solution.
Read More: Perfecting the pH of Your Hydroponic Nutrient Solution
Lotus plants have been cultivated in China for centuries for both their striking flowers and their edible rhizomes. Lotus rhizomes, or roots, are either pickled in vinegar or cooked with other vegetables. The young leaves can also be eaten raw or cooked. In hydroponic systems, young lotus rhizomes are planted in tubs of substrate, such as coarse sand or gravel and filled to a depth of at least 1 ft. within a large growing container or pond. The temperature of the nutrient solution should be maintained at between 73 and 82°F for maximum growth, although plants can withstand cooler conditions.
Systems for Aquatic Plants
Unlike the majority of plants grown hydroponically, aquatic crops are adapted to a flooded or semi-flooded root zone. Watercress and water spinach will also thrive in traditional hydroponic systems such as nutrient film technique, while others such as lotus, rice and water chestnuts prefer to be semi-submerged for much of their life cycle.
Building or modifying a suitable hydroponic system for growing a range of aquatic crops is relatively simple. All that is required is a sufficiently large, water-tight planting trough, pool or container, and a way to fill and drain it of its nutrient solution once a season.
Simple systems for smaller aquatic plants can be made from buckets, storage bins, glass/plastic aquariums, polystyrene boxes or chiller bins, old nutrient reservoirs or even plastic paddling pools, provided at least 6 inches of nutrient solution can be maintained.
These types of mini aquatic systems are ideal for a few water chestnut or rice plants, while deep flow technique, float or raft systems give support to watercress plants and keep their creeping foliage above the water.
Substrates – Water chestnut corms and rice plants need a substrate in the base of the aquatic system to anchor the plants. It’s also where the corms, in the case of water chestnuts, will form at the end of long stolons produced by the root system.
A non-floating substrate such as a mixture of fine and coarse gravel and washed river or propagation sand, or aquarium gravel is ideal and should be maintained at a depth of 2-3 in. under the surface of the nutrient solution.
Lotus plants are the largest of the aquatic crops and require at least 1 ft. of solution. An old bathtub can be used for one plant, while water tanks, swimming pools and garden ponds make ideal growing systems.
Oxygenation – These basic aquatic hydroponic set-ups can be run as a non-flowing or stagnant system, with occasional replacement of the nutrient solution, or as flowing systems, which assist with oxygenation of the root zone.
Kangkong and watercress benefit from some oxygenation of the nutrient solution, particularly under warm growing conditions, while rice, lotus and water chestnuts are well-adapted to more stagnant systems.
Algae blooms in aquatic systems are common and generally don’t harm the plants in any way. In fact, they are best left uncontrolled, as algaecide compounds can damage young plants.
Read More: Hydro for Health - Understanding Bioactive Compounds
Algae tends to bloom and die over the course of the aquatic crop cycle and equilibrium is often reached where algae growth is minimal. In float or raft systems, which may be used for water spinach and watercress, there is no light reaching the nutrient solution, which prevents algae problems.
Nutrient Solution – Nutrient solutions for aquatic crops are the same as those used for other hydroponic vegetables. A low-EC, general-purpose vegetative formulation is suitable for most aquatic plants, apart from rice, which requires silicon.
Rice plants grown without silicon do not develop normally and tend to produce stunted shoots and restricted growth. Nutrients and additives containing beneficial elements such as silicon need to be used when rice is grown hydroponically as there is no guarantee that any particular water supply will have sufficient levels of this element for rice growth.
It’s likely that all aquatic crops could benefit from supplemented silicon, although it is only an essential element for rice.
Running Aquatic Systems
Running an aquatic crop system is a little different than other hydroponic set-ups. In non-flowing or pond systems, the nutrient solution the plants are growing in—usually semi-submerged—still requires monitoring of EC levels.
Over time, water evaporates from the system and is lost from the plants via transpiration, and under warm growing conditions, the EC can climb in the solution. To maintain the correct EC level (around 1-1.4 for most aquatic crops), simply add more water. If the EC drops over time, nutrient concentrates can be added to bring it back up.
Generally, the system’s pH does not need to be controlled for most aquatic crops as they are tolerant of a wide range of pH conditions, unless the water supply is particularly alkaline to start with. Raft, float, deep flow and nutrient film technique systems used for watercress and kangkong benefit from solution flow rates that provide dissolved oxygen to the root systems and help maintain temperatures.
In the case of float or raft systems, a gentle circulation of the nutrients, or the use of aerators in the solution, can be beneficial. Some pond-based aquatic systems growing water chestnuts, lotus and rice use aerators such as small fountains to help maintain a healthy solution environment by creating oxygenation. This is particularly beneficial under high light and temperature growing conditions.
Both ornamental and food-producing aquatic plants can be grown in modified hydroponic systems, providing both great indoor displays and exotic food crops to tempt the taste buds. Being highly adapted to semi-submerged growing conditions, aquatic plants are generally easy to grow and respond well to the balanced nutrition provided in hydroponics.
Systems as small as a plastic bucket can be used for growing watercress, kangkong and water chestnuts, so experimenting with these fascinating crops is possible in even the most restrictive indoor gardens.
Read Next: Maximizing Crop Flavor and Aroma in Hydroponics
Written by Lynette Morgan | Author, Partner at SUNTEC International Hydroponic Consultants
Dr. Lynette Morgan holds a B. Hort. Tech. degree and a PhD in hydroponic greenhouse production from Massey University, New Zealand. A partner with SUNTEC International Hydroponic Consultants, Lynette is involved in remote and on-site consultancy services for new and existing commercial greenhouse growers worldwide as well as research trials and product development for manufacturers of hydroponic products. Lynette has authored five hydroponic technical books and is working on her sixth.