It has been about one month since we had our first taste of autumn rainfall here in Napa. It was enough to create a cascade of water coming off the roof in front of my kitchen window.
I got around to to cleaning the rain gutters last weekend and found them clogged with dry leaves. Bone dry. It struck me as an indication of moisture conditions in tree and garden root zones: The effects of that bit of early rain have dissipated and the forecast shows dry weather for the near future.
This is the time of year when many people want to turn off their automatic irrigation systems, but fall and winter dryness can be damaging. We need to make well-informed decisions to prevent drought stress, even in winter.
Keeping it simple as possible, consider:
- The tree or plants of concern
- Detectable moisture in the root zone.
Mature California native oaks are supremely adapted to the mid-California pattern of ample winter rainfall followed by dry conditions the rest of the year. And many of the common non-native landscape trees tolerate those conditions too. But they may become more susceptible to stress when subjected to multiple years of drought, altered or limited root zones, disease outbreaks and other sources of stress.
For example, native oaks and some species of non-native oaks were subjected this year to a terrible outbreak of foliar diseases: powdery mildew and anthracnose. These diseases might not be of much concern as a passing, one-year, heavy infection, but they add to stress especially if there are consecutive years of infection and the trees are stressed by drought.
Large trees in root zones limited by pavement, retaining walls or other obstructions may have less access to reserves of soil moisture. Similarly, new transplants have all of their roots in a very limited volume of soil.
And many of our favorite, non-native garden trees, like dogwoods and Japanese maples, require fairy consistent moisture.
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In some cases, we can “read” the signs of drought stress, such as reduced shoot growth, folded or wilting leaves, or premature fall coloration and shedding.
Careful irrigation can provide some drought relief, but when is the right time, where should the water be applied, how should it be applied, and how much is enough?
A lot of this has been worked out for farming and landscapes using various technical means like evapotranspiration measurements, soil moisture sensors, plant species classifications by their water-use characteristics, and other methods.
For practical purposes in the home landscape, sometimes we need to make decisions as best we can with limited information at hand.
Here is a simple, low-tech method anyone can use: Feel the soil. Getting a feel for root zone moisture means getting your hands dirty.
A small sample of soil pulled from about 12 inches deep can be a good indicator of moisture content. If the sample feels moist and holds together in a ball when squeezed together, it is probably moist enough for roots to access the water. If the sample feels dry and does not hold together, it is probably too dry.
You don’t need a specially designed soil probe to do this. Use a shovel, a spading fork, or even a long screwdriver and check samples at a few locations.
Although a large native oak might have roots 30 feet deep, it will absorb a great deal of moisture through fine roots near the surface. Applying a modest amount of water at the soil surface can make a big difference for a drought-stressed tree.
Regarding where the water should go, the most common mistake is to irrigate only at the base of an established tree. Root systems spread wide, so spread out the water using soaker hoses, drip irrigation or sprinklers.
After watering, sample again to be sure the watering was adequate. Then check again in a week or two and repeat if dry conditions persist.