Weekly deep irrigation can help trees

In the column that appeared June 25, I discussed several common summertime tree problems and gave basic information on deep watering citrus and redwood trees. That column elicited a big response from readers who needed more information on when and how to irrigate their landscape and shade trees.

According to Mark Standriff, director of Communications and Public Affairs for the city of Fresno, landscape trees must be irrigated on the mandated watering days, during the hours between 6 p.m. and 9 a.m., on the same schedule as other landscape irrigation.

During hot weather, most trees should be able to survive with weekly really deep irrigation that reaches well down into the root system, to a depth of at least one foot, more deeply for bigger trees. (Most trees have fairly shallow root systems). Sprinkler water from overhead-type sprinklers seldom penetrates more than three or four inches into the soil — deep enough to soak lawn grass roots, but not deep enough to soak tree roots.

It takes time for water to penetrate deeply — about three to four hours to reach the one-foot depth. Slow soaking is the only way to get water into the roots. After four years of severe drought, the soil in many gardens is bone dry and the soil surface has crusted over so that water runs off the crusted surface. Use a spading fork or a shovel to break up the top inch or two of crusted soil to help prevent water runoff.

Trees usually extend their root systems just to the edge of what is called the drip line (where rainwater naturally drips off the leaves onto the soil). The roots under the drip line absorb and carry water most efficiently; roots near the trunk absorb little moisture.

Irrigation methods (soaker hoses, microsprinklers, bubbler attachments on hoses, PVC pipes with holes pounded upright into the soil so that a hose can be inserted, drip emitters) should be placed at the drip line.

Using multiple drip emitters, several microsprinklers, or soaker hoses (a double length) to irrigate along the drip line will save a lot of effort and time spent moving hoses. Newer soaker hose designs are less likely to clog.

Berms or water wells built from dirt under trees can hold irrigation water and slowly release it into the soil. But it’s a little too late in this season to try to scrape baked soil into a berm and there is a risk of exposing surface roots to the sun and heat.

Remove any underplantings underneath trees and any grass before laying down a three to four-inch layer of mulch that extends just outside the drip line. The mulch helps retain soil moisture, slows evaporation and acts as an insulating blanket to keep the soil cooler. Keep mulches at least four inches away from the trunk to prevent wet mulch from rotting the trunk tissue.

Gov. Brown mandated earlier this year that California cities turn off irrigation systems in medians that have turf areas, including those with trees. Many city water and park divisions are struggling to find ways to continue to irrigate and save city trees. In my neighborhood, volunteers are organizing to schedule weekly hand irrigation of street and median trees.

Perhaps we’ll see a pajama party throughout our cities of dedicated volunteers out at night, hand watering our precious shade trees.

This story was originally published July 14, 2015 at 10:06 AM with the headline "Weekly deep irrigation can help trees."