Water & Drought

How much can we safely reduce watering in yards where large trees grow?

Q: We have large old sycamore trees lining our street. I have been told these trees do not have “tap roots” and depend on getting large amounts of water from roots in the top few feet of soil. My tree’s roots spread out under my entire front lawn. I am guessing these kinds of trees used to grow near rivers because they got extra water from intermittent flooding, in addition to the annual rainfall. Don’t these trees depend on significant amounts of supplemental landscape irrigation, particularly during droughts, to stay alive?

If people replace their lawns with cement, gravel and semi-desert plants to reduce the need for landscape irrigation, do they also risk killing the urban forest that Sacramento depends on to cool our homes and streets, clean our air, prevent urban “heat islands,” etc.?

It makes obvious sense to irrigate at night, when less water is lost to evaporation, and to irrigate carefully, so water is not wasted running into streets and storm drains. But how much can we safely reduce watering in yards where large trees grow?

– Robert Meagher, Sacramento

A: Sycamores are one example of many trees that need a helping hand during drought. More than 7 million trees are part of the Sacramento region’s urban forest and very few grew totally on their own. Historically, the Sacramento region had few native trees except for valley oaks and trees that grew along waterways.

Lawn trees face a combination of challenges. Fed a steady stream of sprinkler water, they’ve grown accustomed to shallow irrigation. They have no need to stretch their roots down or out in search of moisture. But even in abundance, they may not be getting all the water they need.

According to EcoLandscape executive director Cheryl Buckwalter, trees need individual watering separate from the lawn. This encourages deep, healthy roots.

“They’re always competing with lawn for water,” Buckwalter said. “They’re not getting deep watering from sprinklers. Instead, water them slowly with a hose. Let it soak in. Then, take a (soil) probe and see how far the water reaches.”

Ideally, that water should reach down a foot or more. Many of the roots are at about 18 inches deep. Move the hose around, including away from the trunk. The roots extend about one foot beyond the dripline, the farthest reach of the tree’s foliage.

Sycamores are tougher than a lot of lawn trees. Their roots run 4 to 5 feet deep. While native to river banks, they can get by on twice-monthly deep irrigation. They learned to cope when rivers ran low. So even if the lawn is brown, the sycamores should survive – if they get a little individual attention.

Coastal redwoods and birches, both common in Sacramento, are particularly vulnerable to drought stress. They prefer weekly deep irrigation.

Mulching around trees also helps them retain that precious moisture. Spread mulch (preferably bark, wood chips or chopped leaves) 4 to 6 inches thick in a 4-foot circle around the tree. Leave 6 inches between the mulch and trunk to avoid crown rot.

For more on how to help trees during drought, check tips from the Sacramento Tree Foundation at www.sactree.com/drought.