Chapter 12

Greenhouse Irrigation Systems

Water Recycling in Greenhouses

Water recycling is done by some greenhouses as a way of reusing tailwater created by excess irrigation or purposeful leaching from containers. It has the potential to be collected and reused, though treatment may be necessary. It is synonymous in this use with irrigation runoff. Water recycling systems here include sub-irrigation systems such as ebb and flow and capillary mat systems as well as other kinds of irrigation systems that use water recycled from the operation. Issues for recycled water are similar no matter how the recycled water is re-distributed.

Advantages/Disadvantages

The main advantages of recycling tailwater is reducing water waste and preventing off-site pollution. When water is recycled, nutrients and other water additives are recycled, so reduced amounts fertilizers and other chemicals are another advantage of recycle systems.

Design of Water Recycling System

For sub-irrigation systems, the recycled water is stored in or just outside of the greenhouse and requires only enough volume for one cycle of flow. As this volume is used, it is replaced with fresh water to keep the salts and nutrient concentrations at appropriate levels. Small operations with limited land space may find closed storage tanks sufficient for recycled water. Larger operations will most likely need open ponds to store the recycled flows. Runoff can be collected by gravity flow into a pond or pumped into a storage tank.

Rainwater Capture Systems

Capturing rainwater from greenhouse roofs is often a part of a recycling system. Rainwater at any given site may be more than the storage ponds or tanks can handle, consequently the system must have overflow capability.

Disease Management

While water recycling affects all the management components of a greenhouse operation, disease management becomes a critical part of recycling management. Diseased plants are a source of pathogen that can spread to other plants. Pathogens released into drainage water can end up in the captured runoff, so make removing diseased plants as soon as they are spotted a high priority. Since waterborne pathogens can be dispersed through the runoff, infections can result when this water is reused to irrigate susceptible plants. However, many factors must be present for the infections to occur. Plant susceptibility and age, pathogen concentrations, cultural practices and environmental conditions all contribute to whether disease will take hold at any given time.

Managing Salinity

The key to managing high salinity in recycled water is having a source of low salinity water to mix with recycled water. In some greenhouses where water is recycled, salinity is not a problem because of the availability of high quality water and because runoff, including rainwater, is collected. As a guide, an electrical conductivity (EC) of above 1 dS/m indicates a salinity problem in unused water.

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