Marin Master Gardeners
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Drought isn’t only problem impacting conifers

May 14, 2016
Wendy Irving

Do browning, dropping leaves mean your redwood is in trouble?

The prolonged drought has us thinking about our precious trees. At a time when millions of California’s trees are dying because of drought and drought-related disease and pests, we need to pay close attention to those on our property.

The UC Marin Master Gardener’s help desk has had many inquiries about conifers. These trees make up the majority of California’s forests, and are popular in our urban and suburban landscapes. Conifers are cone-bearing seed plants and include pine, fir, cypress, spruce, juniper, cedar, larch, hemlock and yew, not to mention our beloved redwood trees.

Almost all conifers are evergreen, so is it the drought that is causing some to start browning and dropping leaves? Maybe. But all evergreen conifers go through a natural process when old leaves or needles fall as the tree’s energy moves towards producing new growth. Shedding old leaves is cyclical, and occasionally, leaves will brown and drop, mainly from the interior of your conifer, when no longer useful.

But if your conifer is yellowing or browning from the top down, or the outside in, you may have a problem — a disease, insect or chemical contamination, perhaps drought-related. Some of our bay and oceanside conifers sensitive to salt suffer when rainfall is not plentiful enough to keep brackish water flushed away. If you think your conifer is in trouble, but don’t know what it is, call the help desk at 415-473-4204.

Conifers are diverse in their cultural needs. Some are drought and salt tolerant, others, not so much, like Marin’s redwood trees. Redwood trees often get all the water they need if they are living where they like it most, the coastal fog belt, but if planted in sunny San Rafael or Novato, they may need some help. Since drought-stress symptoms take a few years to show themselves in sensitive conifers and a conifer is not as likely as a deciduous tree to recover. It might be a good idea to check the soil condition around your redwood tree. Check on your other conifers, too, and give us a call with questions about their particular water/soil preferences.

Despite nice rainfall totals, experts say we’ll still be under restrictions this year. So decisions will still need to be made as to how to best use available water in our gardens. If you have mature evergreen trees on your property that need watering over the course of the summer, you might consider sacrificing something else, like a more easily-replaced lawn, in order to keep your valuable trees alive.

Young conifers require watering more frequently so the roots do not dry out. But for a larger tree, it’s a little trickier. Check the soil for moisture now and make it a habit to do so throughout the summer. At the drip line of the tree (the outside edge of the canopy), take a spade and push it into the soil as deep as you can (try for 12 inches), pulling the soil apart. Slide in your hand and feel the soil. If it’s dry, it’s time to water. Surprisingly, the majority of a tree’s roots are within 12 to 18 inches of the surface and it is important to slowly soak to that depth. You can use a soaker hose or a drip system laid out just beyond the canopy’s edge. Some people use 5-gallon buckets with ¼-inch holes in the bottom placed evenly around the tree just beyond the drip line. Although the temptation may be great, avoid using graywater on your redwoods, although it may be OK to use on other more salt-tolerant conifers.

How much water to use? A general guideline is approximately 10 gallons per inch of tree trunk width applied, perhaps just once a month, but don’t rely on that. Since soils and conditions may differ, your fingers are the most reliable judge. Make sure your tree is well-mulched away from the root flare. And leave those naturally-shedding brown leaves alone that may have had you worried — they keep the moisture in and provide valuable nutrients over the course of time.

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