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Saving drought stressed trees

September 18, 2015
Barbara Robertson

Brown lawns are badges of honor for Marin’s water conserving householders. But are these good citizens throwing out their “babies” with their nonexistent bathwater?

The “babies” I’m concerned about are our lovely mature landscape trees. Everywhere I walk I see signs of seriously drought-stressed trees in formerly well-watered gardens. But, if any plant deserved water this summer it was a tree. Replacing a 40-year old, full-grown tree would take, well, 40 years.

The most important time to water trees was June or July when days were long and often hot and trees were still growing, according to Steven Swain, environmental horticulture advisor for UCCE in Marin and Sonoma Counties, Now, even though it may be hot, days are shorter and trees are considering closing up shop for winter.

Does that mean it’s too late to water now? Maybe.

If a tree has donned a fall wardrobe of reds and golds throughout its canopy, it has survived and is readying itself for winter. You do not need to water that tree and doing so might confuse it into thinking it needs to grow.

On the other hand, if the tree has green leaves with brown edges, bare branches at the top, is dropping leaves, has tiny new leaves, and possibly new growth on branches near the trunk, that tree could use a long drink. It’s probably struggling to survive.

Evergreen trees with brown needles only on bottom branches and near the trunk are working hard, but acting normally. By contrast, if the needles all along the branches have brown tips, the trees are seriously stressed.

These visual checks are subjective, though. The most accurate way to know if a tree needs water is by checking the soil. Soil at the tree’s drip line should be evenly moist at least 12 inches deep. The drip line is the imaginary line on the ground where water would drip from leaves at the edge of the canopy. On most trees, that’s several feet out from the trunk.

You can test the soil with a moisture meter, or, better, dig in with a shovel or soil-sampling probe. These inexpensive tools are available online and in home-improvement and garden centers.

If the soil is clay, push a fistful into a ribbon using your thumb and forefinger. If clay soil is sufficiently moist, you can create an unbroken ribbon several inches long. With loam soil, you should be able to squeeze a fistful into a moist dumpling that holds its shape. If the soil is too dry, water until it’s moist 12 inches down.

Why only 12 inches?

Trees absorb water from feeder roots that reach out from the trunk as far as the canopy is wide. Most feeder roots live in the top 12 to 15 inches of soil. They, not deep taproots, supply trees with water and nutrients.

Thus, an effective way to water a deciduous tree is to string a soaker hose around the tree starting a foot or so from the trunk and circle it out to the drip line. For evergreens, extend 2 to 4 feet beyond the drip line. Never water onto or close to the trunk.

How much water will it take? That depends. A rule of thumb is approximately 10 gallons per inch of trunk diameter. To really know, though, use those tools, get your hands dirty, dig into the soil.

Lastly, to help conserve water and aid all trees, spread a 4-inch thick doughnut of organic mulch around the trunk. Keep it about 6 inches away from the trunk. Do not fertilize. Prune dead, broken, insect-infested or diseased branches, and only those. Pray for rain. And give your trees a big hug. They worked hard this year.

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