Article 2-1 Build a Strong Foundation

Roots are amazing – they can grow 50 meters (162 feet) deep into the ground, and even a young Rye plant a few months old has 14 million roots – and 14 billion root hairs! Hydroponic gardeners recognize the importance of the root system to the health and yield of their crops. Otherwise, we would just grow in dirt! Let’s look at the jobs that roots do for plants, and ways to help our garden build and maintain strong foundations in the root zone.

Plants rely on their roots to perform several functions: to anchor the plant in place; to supply the plant with water and minerals, and to provide oxygen to the root system. Roots serve another important duty – they manufacture a growth hormone called cytokinin and distribute it to the rest of the plant. As gardeners, we can appreciate how vital these chores are to the growth and productivity of our crops. We want to keep the roots of our plants healthy and happy for maximum yield and problem-free gardening. So what conditions do roots like? Usually, their needs are simple but specific. Roots prefer an even temperature about 21°C (70°F), day and night. They want a mixture of air and water surrounding them – about 50/50 is best. And most plants prefer a neutral or slightly acidic pH. While plants will tolerate some variation, we try to provide consistent conditions for the roots to encourage straight-ahead gardening and best possible results.

It’s good to keep these basic requirements in mind. Sometimes we get so involved in complicated hydroponic systems that we fail to notice waterlogged roots, or a nutrient solution temperature of 45°C (113°F) ! Unfortunately, the roots always notice! No matter if you’re gardening in flower pots and potting soil or the latest in high technology systems, roots still need these same basic conditions.

Let’s consider the three main types of hydroponic grow mediums – soilless potting mix, rockwool, and gravel or stone -to see how the unique qualities of each medium affect our root zone.


These mixes, often called “potting soil”, are usually composed of sphagnum peat, peat moss, perlite, vermiculite and sand, or various combinations of these ingredients. Most good mixes include Dolomite lime to correct the low pH of these (acidic) ingredients. Though the pH of the mix resists change and often will be suitable for the life of the crop without further treatment, gardeners who frequently “flush” their crops with water, or who grow long-term crops (over 3 months), should watch for signs of acidic conditions in their root zones. Simple pH test kits for soil will give you an accurate picture and let you know when to mix Dolomite lime into the topsoil of your containers.

Potting soils have two other characteristics that can affect roots: They hold water and nutrients, and they slowly compact and decompose.

Since potting soils hold a lot of water – usually several times their own weight they act as reservoirs for the roots, and don’t require the almost constant watering of a plant in a gravel or hydro corn system. Of course, we don’t want to let the roots dry out or keep them waterlogged. When to water plants in potting soil? When they need it! In general, small young plants have shallow roots, so we can’t let the containers dry out as much as established plants with deep root systems. On the other hand, large plants with lots of leaves place greater water demands on the roots.

As well, the rate of water consumption by the roots depends on the type of plant, rate of growth, light intensity, length of day, amount of air movement through the top growth, and the temperature of the garden! The point is your garden is unique and ever-changing, requiring a skilled eye and a knowledge of your plant’s watering needs. If the potting soil is dark brown or black, wet to the touch, and the container and soil are heavy when you lift them – don’t water. Commercial growers lift and “heft” their plants to see if they need watering – this is a good technique to see how wet or dry the soil in a container is, since you don’t have to disturb the roots. Be aware of the first signs of water stress from over drying the roots. A loss of sheen or shininess on the surface of the leaves (even hairy leaves have a natural sheen when healthy) followed by curling down of the leaves. If you encounter these signs of drying stress, heft the container and remember the weight, then water the plant immediately! It’s best to water crops before they show signs of water stress.

Because potting soils soak up and hold nutrients we feed these crops differently than plants in other mediums. Usually, we feed crops by watering the root zones well with food-and-water mix at room temperature (210°C = 7000°F). When the potting mix has dried enough to require watering, we use room temperature water, with no fertilizer. By alternating – feed-water-feed-water, we avoid buildup of nutrients (that can damage roots) in the potting mix. When we water the roots, the plain water re-dissolves food held by the potting soil, making it available again to the roots.

Exceptions to this feed-water-feed technique would be periods of hot weather, when there is greater chance of nutrient build-up in the potting soil. During heat spells, switch to a feed-water-water routine to avoid burning roots.

Gardeners who grow in potting mixes should be aware of the physical characteristics of organic components of the soil. The peat and sphagnum mosses will gradually break down, decompose and compact, reducing the spaces available for air and water in the mix. If you notice a “crust” on the top of the mix or if the potting soil has sunk down into the container – often forming a bowl-shaped surface – loosen the top of the mix gently with an old fork or wooden dibble, without disturbing the roots. This will allow air and water to move freely through the soil to the roots.

When plants in potting soil become root-bound, the soilless mix tends to dry out very quickly. Gardeners may notice a space between the soil and the container as the root bound plant’s potting soil shrinks away from the inside of the container. This affects the roots of the plant in two ways : Because the roots have grown to the outside of the potting mix, they are exposed to the air and light when the soil shrinks away from the container. This can dry them out and damage the actively growing roots. When the plant is fertilized, the nutrients run down the space between container and potting soil, hitting the dry exposed roots directly and burning the fine root hairs. Transplant your crop into larger containers as needed, to avoid setting back the plants.


Rockwool has a unique quality that affects our gardening practices : Not only does it hold a lot of water, it gives water up to the roots very easily – even when it is almost dry! Potting soils tend to hold water more and more tightly as they dry out, forcing roots to work harder to draw water from the soil. Because rockwool gives up even the last of its water easily, we can “run’em dry” – we can garden with more air around the roots than in potting soils, which must be kept evenly moist. The advantage of rockwool is that, with more air in the root zone, roots can take up water and nutrients faster and better – a useful quality for crop production. We can garden using stronger nutrient strengths in rockwool (if the crop and growing conditions justify potent fertilizer strengths) since rockwool usually won’t build-up nutrients like potting soils. However, since rockwool doesn’t have the buffering capabilities of potting soils, we must watch the pH and temperature of the nutrient solution more closely.

Gardeners in rockwool must also exercise care when treating roots for pests and disease, since rockwool offers less protection to the roots. For example, gardeners can use a 5% liquid Diazinon pesticide in an outdoor garden bed to handle insects in the ground. This product has damaged roots in rockwool severely – a 2% granular Diazinon is safe and effective in treating pest problems in rockwool-grown roots. Always consult qualified staff in your local hydroponics store for recommended pesticides and fungicides, and explain how you grow your plants and the medium you use. Outdoor “garden center” staff may unwittingly sell you a product that will kill your crops before the bugs can do the job!


These materials hold almost no water, so irrigation systems are required to keep roots from drying out. Usually a single drip-line will provide adequate moisture to plants in 2-gallon containers (or smaller). But in large containers, two driplines are needed to properly water all the stones in the pot and develop a root system through the entire container. To avoid root disease problems it is wise to gradually add a “drying out” period during the dark cycle of your crop as the plants develop a mature, deep root system. For example, many gardeners run their irrigation systems for 24 hours a day for young plants with small, shallow root systems. Once these plants are established and roots have grown deeper, shut down the system for an hour during the dark period, then resume watering. It’s possible to gradually extend the irrigation shut-off period until the plants are only watered during the light cycle hours – very useful for control of humidity levels during the dark cycle, as well as helpful in avoiding plant diseases, which could spread rapidly through the garden in the nutrient solution, once established.

Recall that rockwool holds lots of water and that hydro corn or stone holds almost none. Realizing the difference between these two mediums will help you avoid a common error in hydroponic gardening. Many gardeners like to start seeds or cuttings in rockwool cubes, transplanting the young plants to stone systems when they are established. Problems arise, however, if the rockwool cube fills a large portion of the net container, with a relatively small layer of stones surrounding the rockwool cubes. Here’s the difficulty – how do you water the roots properly? Roots growing in stone need almost constant watering which keeps the rockwool cube almost waterlogged. If you shut off the irrigation system to let the rockwool drain, the roots in the stone will dry out! The solution is to use small cube in a larger basket or net pot filled with hydro corn and water the stones as required, keeping water flow away from the cube so it’s not too wet. If you use a small net pot, try to remove some of the bulk of the rockwool before transplanting into the stone. The best way is to peel away sections of the cube from the top down, taking care not to damage the roots, then transplant into the net pot or basket.

pH, temperature, and disease treatment precautions for rockwool growers apply to stone gardens too!