Monday, July 13, 2020
July 13, 2020

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Gardening with Allen Wilson: Design irrigation system for efficiency

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I would like to keep my lawn green this summer. I have arranged to have an automatic irrigation system installed so I can water with less effort. Could you give me some pointers in designing and operating the system for maximum efficiency and minimum waste?

The two factors that determine the amount and frequency of irrigation are plant type and sun exposure. Grass, flowers and vegetables have the shortest roots so they require the greatest frequency. Trees and shrubs have deeper roots and require less frequent irrigation. Plants growing in full sun require the most irrigation while those in partial or full shade require a lot less.

If trees and shrubs are watered on different cycles than grass and flowers, they can be watered less frequently. South and west facing beds dry faster than north and east facing beds, so it is best if they are also on different cycles.

Drip irrigation systems are more efficient than sprinklers because less water is lost to evaporation. Shrubs, flowers and vegetables are often irrigated with drip systems.

The best way to operate an irrigation system is to apply enough water to reach 6-inches deep and then let the soil dry in between irrigations. When plants are watered every day the soil stays wet on top which stimulates weed seed germination. This usually means a space between irrigations of 2 to 4 days depending on plant type and weather.

It requires a half to 1 inch of water to reach a depth of 6 inches, depending upon soil type. Clay soils hold more water. Different kinds of sprinkler and drip heads apply water at different rates. Spray heads may apply a half inch of water in 10 or 15 minutes, whereas rotating heads may require two to three times as long to apply the same amount of water. Drip heads may take an hour or longer to apply the same amount of water.

The length of watering time can be calculated from the specifications of the heads and water pressure. One easy way to calculate the amount of time for an established system is to place small, straight-sided cans (like a tuna can) at intervals for each type of head. Turn the system on manually for a specific amount of time and then measure the amount of water accumulated. Then calculate how long it would require to deliver a half inch to one inch.

Larger and less frequent amounts are normally applied to shrubs. Sunny areas should be watered longer than shady areas.

As temperatures rise most systems can be bumped up to water a for a bit longer. As temperatures cool in the fall, irrigation times can be lowered.

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