Trees clean and cool the air. Most of us have trees in our yards that were planted before we moved in. Many of these trees were planted shortly after the house was built. Often we take these trees for granted. That can be good for them as once a tree is well established there is seldom need to fertilize or water it. In fact fertilizer and overwatering can do harm. Yet in these times of drought we may need to take additional measures to ensure we don’t lose the trees that shade our yards, cradle our homes, and clean the air.
In the case of trees, what not to do in the garden is more important than what to do. Here are the basics.
- If the tree is healthy, leave it alone. Trees hate change.
- Always use a certified arborist for any tree care. Trees should be laced out, not topped.
- Mature trees do not need supplemental water except in times of drought or fertilizer unless to correct a deficiency and administered by a certified arborist.
- Know the tree’s growing season.
- Check with a soil probe during times of drought, if the soil is bone dry, irrigate with a soaker hose to the depth of at least 12” during the tree’s growing season.
- The root crown (base of trunk) is a vulnerable area. Never leave dirt, debris or mulch piled against the trunk and do not allow water to collect on or near the root crown.
- Do not till soil or plant anything other than ground cover under the drip line (the outermost circumference of a tree’s canopy).
- Keep lawn and groundcover well away from the trunk.
- Tree roots are in the top 12-24” of soil and extend well beyond the drip line (the outermost area of the tree canopy (foliage).
- Protect your trees during construction following the advice of a certified arborist. Construction anywhere in the root area can cause harm. The harm comes from soil compaction due to heavy equipment; mechanical injury of roots, trunks or branches; change of grade either exposing roots or suffocating them by raising soil level.
If a tree shows stress such as change in color or the dying of some branches call a certified arborist for diagnosis. Unfortunately by the time that stress is visible it is often too late. Damage during construction or during drought may not show up for 2-3 years and by then it may be impossible to reverse.
How does one know when to water? The only sure way is to check the soil to a depth of about 15” with a soil probe. With a soil probe or sampler you take a core of soil out and examine it. Most often you will find wet gummy soil, a sign to cut back on irrigation. But in times of low rainfall, especially after 2 or more years of drought, you could find dry, dusty dirt.
The best way to water trees is with soaker hoses wrapped around the tree at the drip line and left on for several hours. After a session with the soaker hose, wait one day then test the soil again to make sure the water penetrated to 12-16”. Do not water trees when they are dormant. Most trees are winter dormant but California natives are summer dormant.
Judy M Horton is a horticulturist, garden designer, and Hollywood Hills resident. The LA Times featured her last year in an LA Times feature article: Gardening with Window Views in Mind. Her firm Judy Horton Garden Design can be found online.