Gardening with Allen Wilson: Plan before installing irrigation

I see irrigation running at the same places early every morning when I go for my run. Some places are not covering very uniformly. Is it necessary to water every day? I’m having an irrigation system installed and I want it to be as efficient as possible. I only water now twice a week.

You are wise to ask questions before you install your irrigation system. The best advice I can give you in installing an irrigation system is to have 100 percent overlap. That is the best way to get uniform coverage because sprinkler heads put out the most water near the head, less and less as the spray gets farther from the head. One head’s reduced output is matched to the next head’s maximum output. Separate shrub and flower beds from lawn irrigation because they need less water.

Where possible, put the beds on drip irrigation. It is more efficient because of much lower evaporation loss.

Watering the lawn every day tends to keep the top of the soil moist which is just what weed seeds need to germinate. Letting the top of the soil dry out between irrigations reduces weed sprouting and growth.

Daily irrigation schedules typically apply water for only 5 to 10 minutes. Few sprinkler systems apply enough water in 5 to 10 minutes to reach more than an inch or two deep.

If the only place where the soil is moist is the top inch or two, then plant roots will grow only in that area. With shallow roots, plants need frequent irrigation. It normally requires a half inch of water to wet the soil to a depth of 6 inches which is the ideal situation for most plant roots.

A simple method for determining how long it takes to apply ½ inch of water is to set shallow cans, like those for tuna, near each different type of sprinkler nozzle. Run the sprinklers for a specific time interval such as 10 minutes and then measure the amount of water in the can with a ruler. Most sprinkler nozzles require a minimum of 15 to 20 minutes to apply ½ inch.

During hot weather you may need to irrigate lawns every two days. If maximum temperatures are in the 70s, every three days should be adequate. Flowers and vegetables should have a frequency similar to lawns. Trees and shrubs need less frequent and deeper irrigation.

Early morning is the most efficient time to irrigate because of reduced evaporation loss. However, sprinkler systems should be run through a two minute cycle during daylight at least monthly to check for plugged and misdirected nozzles and other coverage problems.

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