Several years ago, a client was very concerned about his hedgerows of Emerald Green arborvitaes. They were no longer green. My assignment was to find out why and make recommendations to get them back to normal.
American Arborvitae is an evergreen commonly planted in local gardens as a vertical accent or screen. It goes by the botanical name Thuja occidentalis and comes in many varieties. Emerald Green seems to be the local favorite because of its neat, dense narrow shape and its soft green foliage. The species is native to the eastern U.S. and, according to theSunset Western Garden Book,” it “needs moist air to look its best.”
Some of the best local specimens I have seen have been planted adjacent to well irrigated lawns.
After a little digging — for information and in the soil — I discovered the irrigation system had been turned off for the winter and there had been zero rainfall for the whole month of January.
Going with the simplest explanation first, I recommended improvements in irrigation and mulch to help retain moisture. Over the next few months, the plants slowly showed improvement.
Key factors in the problem included:
— The species is native to regions where there is more abundant moisture and greater humidity than we have here in mid-California.
— The plants were not mature and well established.
— There was no source of moisture nearby except for the drip irrigation system that watered only a small area near the base of the trunks.
— The soil surface was bare.
As to our situation now, looking back at Napa rainfall history (www.wunderground.com) I found this:
January 2015: 0 inches
January 2016: 1.30 inches
January 2017: 2.22 inches
January 2018: 2.04 inches
That looks pretty good for January this year but now we are into February with virtually zero chance of rainfall through Feb. 13 and highs in the 70s and upper 60s.
Anticipating possible winter drought stress it is best to make informed decisions before running the irrigation.
First, mature and healthy, well-established plants and trees tend to be more drought tolerant than are young transplants.
California natives and plants adapted to a “Mediterranean” climate are susceptible to drought stress until they are fully rooted in the native garden soil. Conversely, mature, established moisture-needy species such as camellias may be drought-tolerant if they are firmly established.
Irrigation systems should be revised periodically to grow with the plant. New transplants need irrigation directly into the planted root ball until new roots find their way into the loosened soil in the planting hole and beyond. When I examine trees and shrubs that have been growing in landscapes for several or more years, I almost always find that the drip irrigation systems have never been revised to water an expanding root system.
Most often, I find that garden irrigation scheduling is relegated to guesswork. The gardener guesses that the plants are OK because it’s January or February and everything is dormant.
Take out at least some of the guesswork by using a soil probe to check moisture content. For a new transplant, that means probing directly into the transplanted root ball as well as the soil under and beside it. If a small sample of soil forms a ball when squeezed together and easily breaks apart it is probably moist enough.
Lastly, apply and maintain a layer of mulch over root zones but not against the trunks, wherever practical. Surface mulch is a huge help in conserving root zone moisture as well as improving soil structure and health and suppressing weeds. Generally, a 4-inch thick layer of coarse wood chip mulch is adequate.
Mulch settles and breaks down over time, so remember to check it periodically and add more as needed to maintain the desired depth.