Subirrigation is the watering of plants by supplying water from below. The method for supplying water is quite different for container plants in greenhouses and field-grown plants.
Container plants
Plants in greenhouses and homes are sometimes watered by subirrigation. This might be as simple as filling their saucers with water and letting the pot soak it up, or as complex as an automated system of watering an entire greenhouse.
African violets and related plants are susceptible to leaf-spotting from drops of cold water touching their leaves. For this reason, they are often watered from below. The simplest method is to fill their saucer with water, let them sit for an hour, then drain the saucer. However, either of the two methods described below will also work.
Capillary matting — This method of watering is often used in home greenhouses. It saves water and keeps plant leaves dry, avoiding some leaf diseases. The method can be automated by using a simple clock, the type used for automating drip irrigation systems.
The plants must be on a water-tight, completely level surface. Spread a piece of capillary matting on this surface, letting the far end hang over the edge into a trough. Capillary matting may be purchased from a greenhouse supply company or any absorbent, rot-proof material may be used, such as an acrylic blanket.
Plants may be set anywhere on the matting. The pots must not have feet, and their holes must be in the bottom; it’s important that the planting medium contact the matting to draw up water by capillary action.
Apply water at one edge of the matting, or through a network of emitters evenly spaced along it. Ordinary drip system emitters or a soaker hose may be used to apply water.
Apply enough water that some excess drips from the trailing edge of the matting into the trough, where it can be collected for re-use if desired. Keep applying water until most of the soil in the pots has been wetted. This may take a few hours in large pots or with coarse planting medium. Because the matting will grow algae where it is exposed to light, a layer of black plastic is sometimes laid over it. Holes are cut in the plastic wherever pot are to be placed.
Ebb and Flow — Ebb and flow watering is similar to the capillary matting method, but no matting is used. Watertight trays or troughs are flooded with water an inch or so deep. After enough water is absorbed into the pots, the tray is drained.
An automated version of this system is available commercially from hydroponic suppliers. Ebb and flow hydroponic systems pump nutrient solution (water and fertilizer) into a tray to soak the plants, then pump it out again automatically.
Field Plants
Subirrigation is sometimes used to water playing fields and farm crops, also. In both cases, the system is a combination of a drain line and irrigation system. It is usually used where an impervious layer exists under the soil, necessitating a drain system. In the case of sports fields, where the soil is carefully constructed, a plastic layer is placed under the soil, with the drain/irrigation system being laid directly on it and soil placed on top.
To drain the system, the bottom end is opened. To use it as an irrigation system, the drains are blocked, and water is applied into the top end. When the soil is irrigated enough, the drains are opened.
Home Gardens
Subirrigation in home gardens is seldom worth the effort involved. Salts are harder to manage because the system can not be used to leach excess salts from the soil. Since the water flow is upward to the surface, excess salts are deposited there when the water evaporates.
However, this method probably uses less water than any other, and is very efficient when used with high-quality water and careful fertilizer management. Because the surface remains dry, weed seeds don’t germinate during dry periods.