Gardening with Allen: Irrigation frequency generally linked to depth of root system
By Allen Wilson
Published: May 24, 2018, 6:05am
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I know that different plants need different amounts of irrigation. How often should I irrigate the various plants in my landscape?
In general, the deeper the root system, the less frequently plants need to be watered.
If you have an automatic irrigation system, the shallowest-rooted plants determine how often you need to water. Lawns, ground covers, flowers and vegetables generally have roots that go up to 6 inches deep. An interval of 2 to 3 days will take care of these shallow-rooted plants. Daily irrigation is undesirable because it keeps the top of the soil moist and encourages weed seed sprouting. For those of you who have been watering daily, double the irrigation time and change to every other day. Most modern irrigation systems are set up with a sensor that will delay irrigation for a day if it rains.
It normally requires 1/2 inch of water to reach a depth of 6 inches in the soil. You can calculate how long using irrigation tables. If you don’t have access to a table for your current system, place several shallow cans such as tuna fish cans on the lawn and run your system for a specific time interval such as 15 minutes.
Measure the accumulated water and calculate how long it will take to reach 1/2 inch. Most spray-type heads will require about 20 to 30 minutes. Rotating heads may take twice as long. Drip irrigation is much slower and may require several times as long to reach 1/2 inch.
No sprinkler system?
About 1/3 of homeowners in the Pacific Northwest choose not to irrigate their lawns in the summer and let them go brown. However, other plants do need to be irrigated.
I have a small lawn that I cut at a height of 2 inches. I can keep it green by watering twice a week. I use an oscillating sprinkler which I set twice. Oscillating sprinklers waste about half the water to evaporation, but they are more efficient in the evening and early morning.
Flowers and vegetables need about twice-weekly irrigation once they are established. Once a week is usually enough for shrubs. Shrubs on the east and north sides may go longer, but shrubs on the sunny south and west sides may need more frequent irrigation.
Established trees can get by with monthly irrigation, especially if they get some when you are watering something else. This should be deep irrigation that will allow the water to reach a depth of a foot or more.
Of course, newly planted plants need more frequent irrigation until they get their roots established in the soil.
Allen Wilson is a Vancouver gardening specialist. Email Allen Wilson at allenw98663@yahoo.com
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