Master gardener: Deep watering under the canopy

By Harriet Phelps

As in all things, 2020 this has been an unforgettable year for high heat and drought. The “non”soon did not produce any significant rainfall, leaving us in a deficit for moisture. Notably all plants are desperate for moisture.

Sadly, we witnessed the felling of trees from high wind and the saguaro collapsing from lack of moisture. The National Weather Service is predicting a La Niña pattern winter, bringing higher than normal temperatures and lower averages for moisture. The snowbirds will love it, but we natives know we need to protect our trees.

In the low desert, most yards have irrigation systems. If you have reduced or turned off your water to plants, you may want to reconsider for several reasons. Trees, shrubs, and vines are woody plants that can reach a height of 15 feet or more with or without self-supporting upright stems. Roots go outward from the root ball 1.5 to 3 times as wide as the canopy and within one foot of the soil surface or have a tap root growing down several feet.

Water penetration must reach at least one to two feet deep; most is absorbed outside the canopy drip line. Watering is governed by how fast the water is absorbed by the soil so the application must reflect the absorption rate. If water is running off, then slow down the application rate.

Deep watering promotes deep rooting of trees and shrubs and means survival. If not, tree roots may grow on the soil surface and risk a shortened life span.

Pushing a soil probe, a smooth rod about 1/4 to 3/8 inch in diameter, into the ground will determine penetration depth. When wet the probe will easily slide through the soil until it becomes difficult to penetrate, reaching the hard, dry soil.

Typical root zone for mature shrubs is 12-24 inches and for trees 18-36 inches. A good rule to remember is to water when your soil probe will not penetrate deeper than 3-4 inches. Remember to water deeply a couple times a year – three times as deeply as normal to leach salts out of the root zone to promote a healthy plant.

The canopy of a tree gives nature and humans several benefits. The function of trees and shrubs provide us with shade, screening, windbreak, noise reduction, ornamentation, wildlife shelter, edible fruit and nuts.

The canopy is so very important other than its beauty. Low desert characteristics we require are relief from the sun and the protection of shade. Our ecology, which includes us, requires the function of trees for air purification and food sustainability.

Taking wisdom from the rainforest and each level of importance up to the canopy, our trees provide us with solar protection, ecology for wildlife in our area and food production.

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This column appears in the November issue of InMaricopa magazine.