This summer we’ve had near record-breaking numbers of successive days of temperatures over 100 degrees. The long periods of high heat with no rainfall have really stressed plants and trees and we’re once again seeing signs of drought stress in mature landscape trees. Even though last winter’s rains helped the central San Joaquin Valley begin to recover from five years of severe drought, leaves are dropping two to three weeks earlier than normal and many topmost branch tips and small limbs are dead.
In order to preserve the health and vigor of shade trees, deep-irrigate them at least monthly during the summer months. If you haven’t been giving supplemental water to your large trees this summer, start now to prevent further dieback and to get the trees into the best shape possible before they enter winter dormancy in November.
Water from overhead or lawn sprinklers only penetrates about 4 inches deep into the soil. Most mature tree roots lie under the top 12 inches of soil and spread to the outer edges of the canopy although the roots of some trees including native drought-tolerant Valley oaks spread much further out if water sources are not nearby. Younger feeder roots that take up water lie just at the outer edge of the tree canopy where rainfall naturally drips down off the leaves (if we had rainfall in summer). Deep irrigation near the tree trunk doesn’t supply water to the feeder roots; slow steady soaking of the soil to wet the top 12 inches must be directed to the edge of the tree canopy.
Soaker hoses, bubbler attachments on a hose, multiple drip emitters, or PVC pipes perforated with holes and large enough in diameter so that a hose can be inserted then driven one foot deep into the soil are all good methods of delivering water to trees’ roots. So are berms or water wells with a dirt ridge at the edge of the canopy, refilled with water several times over a three- to four-hour period.
Slowly soaking the soil evenly around the canopy will take several hours depending on soil type. Water quickly runs through and away from the root zone in sandy soils. Clay soils hold water well but become nearly impermeable when dried out. After soaking the soil, poke a screwdriver or a metal pole into various spots to determine just how deeply the water has penetrated. The screwdriver or pole will not easily penetrate past the limit of wet soil.
Fruit and nut trees will also need deep irrigation after harvest until mid-November. Citrus should be deeply irrigated at least weekly during the hot months – every three to four days when temperatures are really high. Lack of sufficient water is the major cause of crop failure.
Send Elinor Teague plant questions at firstname.lastname@example.org.
Concerts in the garden
The Clovis Botanical Garden is once again putting on “Twilight Thursdays Concert Series” on Thursdays beginning Sept. 14 through Oct. 5.
Time: 7 p.m.
Place: Northeast corner of Clovis and Alluvial avenues
Amenities: Admission and parking are free. Cold drinks and dessert are available for purchase. Bring your picnic dinner and chairs and enjoy great music in the lovely garden.
Details: Check the garden’s website, clovisbotanicalgarden.org, for a list of performers.