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 JJohn W. Bartok Jr.
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Q: Sub-irrigation: a good way to provide containment
There are several areas of concern covered under runoff regulations, including water from greenhouse roofs, runoff from driveways and parking areas, excess interior irrigation water and leaching from exterior growing areas. Subirrigation is one method that contains irrigation water. Instead of letting leachate drip from benches or ground beds into the soil, the water is collected, stored and recirculated.
Containment advantages
Besides the benefit of reducing runoff, there are several other advantages:
- Less labor. Most of the time needed for the setup and operation of an irrigation system is eliminated.
- Uniform plant growth. Every plant gets the amount of water it needs.
- Less water is needed. A savings of 50 percent or more of water can be realized.
- Less fertilizer is needed. Without leaching, fertilizer rates can be reduced by 25–50 percent.
- Lower humidity. Because the foliage remains dry, there is less evaporation.
- Less disease. As there is little water movement between containers, spread of disease is limited.
- Increased space efficiency. With the use of bench and floor systems containers can be spaced more efficiently.
Containment system concerns
Challenges to ebb-and-flood irrigation include:
- Greater system cost. Depending on the system installed, cost can be $10-$20 per square foot of growing area.
- Learning curve. Production techniques are different than conventional production with overhead irrigation.
- Greater monitoring of irrigation solution. Electrical conductivity and pH have to be monitored on a regular basis.
System components
In addition to troughs, benches or flood floors, the basic containment system consists of tanks to collect and hold the water or nutrient solution, pumps and piping to move the water and a control system that regulates when the system is to operate and for how long.
Concrete or polyethylene tanks are sized at ½- to 1-gallon capacity per square foot of growing area. Two or more tanks are usually installed for adequate water and nutrient storage. PVC pipe and fittings give good service for handling the water or solution.
Stainless steel centrifugal pumps are used to handle the nutrient solution and filters screen out materials such as medium debris and leaves. Timers or a controller are needed to adjust the length of time the pumps operate.
Small budget solution
Trough system. This system allows the use of existing benches reducing cost. PVC or aluminum troughs, wide enough for the pots to set in are placed on the benches. Benches are adjusted to provide a slight slope so the water drains from the supply to the discharge end.
Water is pumped from the reservoir tank under the benches to the high end of the troughs. A cross gutter at the discharge end collects the water and piping carries it back to a holding tank. Water from the holding tank is pumped to the reservoir tank, thus the closed system. Trough systems can also be suspended from overhead trusses to gain additional growing space.
Ebb-and-flood on existing benches. In this system, the benches remain level and plastic liners are inserted to contain the water. Water from a reservoir is pumped through a two-way valve to the bench. When the level of water reaches ½ to 1 inch and the growing medium in the containers has absorbed as much as needed, the valve reverses and the remaining water is siphoned out.
Research at Geremia Greenhouse in Wallingford, Conn., has shown that partial saturation by controlling the time that water is left on the bench can restrict uptake of water resulting in higher quality plants. It can also affect the rate of plant growth reducing the need for growth regulator applications. Partial saturation is accomplished by adjusting the time that the supply pump operates using an environmental controller.
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