Mature landscape trees are some of the Valley’s greatest assets. In our hot, dry climate with higher levels of dust and pollution in our air, the leaves, needles and fronds on large healthy trees create shade that makes the summer heat more tolerable and lowers cooling costs; they filter out pollutants and carbon dioxide and release oxygen into the air we breath; tree roots absorb and filter water and prevent pollutant runoff into our drinking water sources.
The most current weather forecasts for this winter predict a weak La Niña with an increased chance of continued drought. Although we received more than the average rainfall last season, the root systems of many large trees in our valley have not gotten a deep, thorough soaking for six years.
As landscape trees, both deciduous and evergreen, enter winter dormancy during the next few weeks, they will need supplemental irrigation to provide extra water to roots parched by our summer’s intense heat. Our trees have been severely stressed by the continuing drought and without supplemental irrigation now, we risk further dieback and continued tree loss.
Here’s a reminder of how and when to properly deep irrigate our trees.
Deep, slow irrigation applied directly to the root zone underneath the tree canopy is best. Automatic sprinkler water seldom penetrates more than 4 inches into the soil. The majority of a large tree’s root system lies within the top 12 inches. Small, more absorbent feeder roots grow outward as the tree canopy increases in diameter and are found right at the outer edge of the canopy where rainfall hits the leaves as it falls to the ground. Supplemental irrigation should be placed under the outer edge of the canopy, not next to the trunk. (Some trees do have a tap root for the first year or two, but tap roots disappear as the tree matures and are replaced by spreading root systems).
Redesign your existing irrigation system now to add more drip emitters to cover the entire drip line of your landscape trees; or wind a double length of soaker hoses at the outer edge of the canopy; or use a bubbler attachment or small oscillating sprinkler on a hose and soak the soil in overlapping spots for at least three to four hours every week this fall.
The top inch of dried (nonirrigated) soils will have crusted over during the summer, hindering water penetration. It may take an overnight really slow soaking to break through crusted soil and begin the irrigation process. Check soil moisture levels and continue to deep irrigate until steady rains have soaked the soil. Most of our rainfall arrives in early spring – if we have another drought year, we’ll need to deep irrigate landscape trees regularly throughout the year.
Deciduous trees are pruned in winter, after leaf fall. Limb and branch dieback on large trees poses a serious hazard. Trimming dead branches is a job for expert certified arborists who can make cuts that will balance the tree’s structure and preserve its health. Certified arborists can also evaluate a drought-stressed tree’s overall health and determine whether it can be saved or should be removed.
Send Elinor Teague plant questions at firstname.lastname@example.org or email@example.com (“plants” in the subject line).