Subterranean Tactics: Root Zone Manipulation in Hydroponics
Growers usually tinker with atmospheric conditions above the soil or substrate level, but not as many know they can also manipulate the environment around the root zone and that these subterranean changes can have a huge impact on plant performance.
While most gardeners understand how changing a crop’s aerial environment—temperature, light, CO2 levels, and more—influences plant growth, fewer know that manipulating root zones could also potentially improve plant performance or help overcome difficult growing situations. Commercial hydroponic growers commonly use root zone manipulation techniques to achieve specific goals. Examples include chilling nutrient solution around the root zone to produce cool-season crops under tropical conditions, limiting irrigation to improve fruit compositional quality, and improving root genetics via the use of rootstocks to boost yields.
Deficit irrigation is a widely used practice in horticulture. It’s particularly useful in soil-based systems, but also has application in hydroponic systems for certain crops. Deficit irrigation is the careful restriction of water flow, so plants experience slight moisture stress. While this has several physiological effects on plants, it is largely practiced to enhance the compositional quality of fruits like tomatoes or grapes. With restricted water uptake, more sugars and other compounds concentrate in fruit tissue, thus giving an improved flavor profile. In herbs, this practice can also result in higher levels of volatiles which contribute to their distinctive flavors and aromas.
The issue with deficit irrigation in hydroponics is it requires a high degree of control and grower skill to ensure plants are only mildly stressed and not permanently damaged, which would result in a significant loss in yield. It requires substrate moisture sensors and continual adjustment of the irrigation program as the plants develop. An alternative method of applying mild plant stress is to increase the electrical conductivity (EC) of the nutrient solution. This technique also restricts the uptake and accumulation of water by the plant, but it’s much easier to maintain than a certain level of deficit irrigation. It’s already a well-used tool in tomato production as a high EC during fruit formation maintains crop quality under low winter light conditions and improves the fruits’ brix levels and flavor profiles year-round.
Apart from improvements in fruit quality, deficit irrigation and higher EC are also used to steer crops away from being overly vegetative and towards being more generative (producing more flowers and fruits). To achieve deficit irrigation in this situation, growers could reduce the volume of nutrient solution applied at each irrigation, allow more time between irrigations, or allow the medium to dry slightly overnight by restricting early morning and evening watering, among other options. No matter the method, however, using deficit irrigation and higher EC to force plants into generative growth must be exercised with caution. Too many moisture fluctuations in the root zone can lead to an increase in fruit problems such as blossom end rot and the fruit splitting under certain growing conditions.
Hydroponic crops experience some degree of root volume restriction, while most plants grown in soil do not. (For example, a field-grown tomato may average more than 200 liters of rooting volume per plant thanks to its almost unlimited access to soil depth for foraging water and nutrients.) However, root zone volume control in soilless systems helps increase the productivity and produce quality of some crops in certain situations. Vegetable seedlings grown with some root restriction usually result in shorter, hardier transplants that are better able to survive the stress of planting out and establishment. In fruiting crops like apples and grape vines, root restriction limits vegetative growth while improving the quality of the fruit in terms of soluble sugars and other parameters. There is also evidence that root restriction in the seedling stage of some hydroponic crops helps hold back excessive vegetative growth, leading to earlier flowering, more compact plants, and an advantageous vegetative-reproductive balance. To achieve this kind of root restriction, growers carefully select the size of the seedling rooting container or, as is more common, hold the seedlings in their propagation cubes/containers for a longer period so root restriction begins to occur before planting out.
Other studies show root restriction can improve the nutritional value of hydroponically grown vegetables. Pak choi, edible chrysanthemum, endive, and lettuce cultured in a deep-flow system with restricted root zone tubes had an increased carbon-to-nitrogen ratio and percentage of dry matter, as well as higher ascorbic acid (vitamin C) and anthocyanin levels. These results could be due to a stress response like that found in crops grown under deficit irrigation or with a high EC, but it is most likely thanks to a combination of internal plant processes triggered by compounds produced by the restricted root system.
Increasing root restriction, however, also inhibits growth. So, growers need to establish a compromise in the garden. Roots should be restricted to the point where a crop still produces sufficient foliage while seeing an improved nutritional value.
In many hydroponic systems, plants often share a growing bed, container, or slab, allowing root systems to intermingle. Some studies show that plants sharing a root zone with a neighbor can produce more root mass in comparison to those growing alone. It is believed sharing space allows plants to enhance their competitive ability. These findings, however, may be species specific; it appears that the roots of some plants can sense a neighbor’s roots and respond to them accordingly, while others cannot. Further studies may also eventually help us determine if and how plants grown side-by-side in hydroponic systems influence each other’s growth.
Root System Grafting
Genetics control many functions of a plant’s root system. Thus, the ultimate form of root zone manipulation is to graft a plant onto different, more desirable rootstock. Grafting involves growing two separate seedlings: one for the rootstock and one for the scion (the plant top that gets graphed onto new roots). The scion plant is bred specifically for its fruiting characteristics and other traits, while the rootstock seedling is bred specifically for its desirable root zone. This root zone has distinct advantages over the scion’s natural roots, including disease resistance, improved nutrient uptake, and plant/root vigor. These traits allow the graphed plant to better overcome unfavorable growing conditions, including heat and cold stresses.
Root zone grafting is most commonly carried out in tomatoes, peppers, cucumbers, aubergines, and melons (other hydroponic crops may also be suitable). Studies show that in hydroponic tomato production, grafting can result in fruit weight, number of fruit, and overall yield increasing up to 27 per cent. Grafted tomato plants also often produce fruit with higher soluble solids (sugars) and vitamin C than non-grafted plants. Grafting as a form of root zone manipulation is particularly successful with heirloom tomatoes, which naturally have lower vigor and yields than commercial greenhouse hybrid plants. Grafting heirloom scions onto a good commercial rootstock can significantly increase disease resistance and boost yields, making a crop much more economically viable.
While its possible for growers to graft their own plants, they can also buy grafted seedlings for evaluation in hydroponic systems. Plant breeders continually develop new and improved rootstocks for many common hydroponic crops, and these root zone genetic advantages could soon be commonplace in many indoor hydroponic gardens.
Chilling Root Zones
Root zone temperature plays as much of a role in plant growth and development as the surrounding air. This is because the root tissue sends numerous non-hydraulic messages to the shoots, and these influence the way the shoots respond to the environment.
Nutrient solution temperature can build up surprisingly fast and become excessive under hot lights and in systems with limited root zones. Research shows that even a short duration of root zone heat buildup can have harsh effects that cannot be compensated by having a low daily temperature average. Just a few minutes a day of root zone temperatures above 86˚F can retard the growth of some heat-sensitive crops like lettuce and parsley.
While regular checks ensure roots don’t get cooked, growers can more actively manipulate root zone temperatures to fool many plants into handling higher-than-optimal air temperatures. Root zone chilling is a well-known technique used by many commercial growers in warm or tropical climates for cool-season crops like butter head lettuce, herbs, and other vegetables. Research shows chilling the nutrient solution down to as low as 61-64˚F allows these crops to grow and produce well at ambient air temperatures of 82-97˚F, which are above optimal. Other researchers report that chilling the nutrient solution of lettuce crops reduces the occurrence of the fungal root disease Pythium aphanidermatum. Without chilling, the root zone could warm up to the level of the air, creating numerous problems like slow growth, lack of heart formation, bolting, tip burn, and low marketable yields. However, trials show that root zone chilling must be applied soon after early crop establishment and maintained for the life of the crop for maximum effect.
There are many different approaches to manipulating root zones to influence plant physiology. And while techniques like increasing the EC to boost tomato flavor and controlling the temperature of the root zone to prevent bolting in lettuce are well-known, researchers are continually developing new methods for wider range of indoor hydroponic crops.