As this summer progresses, I am seeing more and more trees suffering from drought stress. This is largely due to our irrigating less to save water, comply with governmental regulations and save money. While these are very important reasons to water less, I am finding that many people do not realize that in the majority of cases, the trees in their landscapes require irrigation to thrive.

Even relatively mild drought stress will slow or stop the growth of trees. As drought stress severity increases, leaves wilt, yellow, burn and drop. Twigs and branches die back and debilitating insects or diseases often infest stressed trees. Eventually the tree will die, over months or years.

How much water a particular tree will require will depend upon its age, drought tolerance, soil conditions, rooting depth, climate and tree exposure. Recently planted trees will require more frequent irrigation until they become established. Species which have evolved in dry climates can survive with less water than those which are native to areas with more frequent rainfall or ground water. Trees with deep root systems will have access to water longer into the summer than those with shallow root systems. Watering needs increase with greater tree exposure to sun and wind.

When planning for the irrigation needs of your trees, remember that the roots of trees are generally shallow and wide spreading. It is also important to water the majority of the root system of the tree. Therefore, leaving the hose on at a trickle or using a couple of drip emitters at the base of a mature tree will result in only a small percentage of the root system being watered. It is much more effective to spread the water out under and beyond the canopy of the tree. This can be accomplished by using 1) existing in-ground or hose-end sprinklers, 2) soaker hoses with lines two feet on center or 3) drip systems with emitters spaced every couple of feet.

At each irrigation, the water should be left on long enough to wet the soil to a depth of at least 18 inches. This could take up to several hours depending upon the application rate of the irrigation system. Start with your best guess as to how long this will take and then either dig or use a sharpened steel rod (tile probe) to determine whether or not the length of time is adequate to moisten the soil to a depth of at least 18 inches.

How often irrigation is required will depend upon the tree’s age, drought tolerance, rooting depth and soil conditions, climate and tree exposure as mentioned above as well as existing sources of water. As a starting point, for most urban situations, I would recommend watering mature trees that are not drought tolerant every 7-10 days and drought tolerant trees every 2-3 weeks.

Proper irrigation is the most important tree management concern in Northern California. Call Tree Associates Inc., at (530)231-5586 and I will prescribe an effective and efficient irrigation strategy based on your landscape conditions and tree needs.

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Figure 1. Callery pear and London plane suffering drought stress symptoms in early August. Many tree owners have stopped watering lawns and the trees are suffering as they are not getting the water they need .
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Figure 2. Drought stress symptoms of wilting, early fall color.
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Figure 3. Leaf burn from drought stress on an ornamental cherry.
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Figure 4. These unirrigated Chinese hackberry on Russell Boulevard in Davis have lost some to all of their leaves and are under extreme stress.
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Figure 5. Coast Redwood trees at the Davis Post Office. These trees have suffered from a lack of irrigation and a fungal disease (redwood canker) has caused significant dieback from which the trees are unlikely to recover.

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