Doing a little digging in the garden the other day, I was surprised at how dry it was in an area I thought was getting a fair amount of irrigation.

That area has only a star jasmine vine, one of the reliable survivors in local landscapes. Then an e-mail about some birches with dying branches reminded me this is a crucial period for mesic (water-needy) trees. The “dog days” of summer coincide with reliably dry weather here in mid-California.

I dropped by to look at the trees and saw that they are cutleaf weeping birches, a fairly common variety of European white birch (Betula pendula). Although the leaves and growth habit are somewhat different, the cultural requirements are the same: plenty of irrigation coupled with good drainage. When water requirements are not met, their susceptibility to the same problem increases: borer infestation, namely bronze birch borer, Agrilus anxius, a small beetle. That’s not the only borer that attacks birches and it’s not the only species of Agrilus that attacks local trees. The principle of drought stress contributing to pest susceptibility also applies to White alder trees (Alnus rhombifolia). For them, the devastating pest is a similar borer, Agrilus burkei, also known as flatheaded borer.

In birches, the problem is, typically first noticed when branches in the upper canopy produce fewer and smaller leaves. Upper branches die back while lower branches may remain green and healthy looking. Typically, within a few years, the whole tree dies. A close look at the bark usually reveals swellings on the trunk and the distinctive 1/8” exit holes shaped like a capital letter “D” made by the young adult beetles when they emerge from their tunnels under the bark. In white alder, the closely related fatheaded borer makes the same shaped exit hole.

One reference states that “Bronze birch larvae cannot survive in healthy trees.” But that was from G.W. Barters’ studies on bronze birch borer in New Brunswick. Napa Valley is a far cry from Canada!

 Our climate and latitude make it all the more difficult for birch trees to reach their full potential, and easier for opportunistic pests to have their way with the trees. And now, the pest has built up a large local population. Birches are adapted to life in northern forests, where they have a much better chance at growing to their full potential. When I visited northern Germany in 2004, I was shocked to see what white birches can become in their natural climate. Somewhere, I have a picture of myself standing next to a European white birch with a trunk about 30 inches in diameter. That’s nearly two times the trunk diameter of the biggest birches I’ve seen in California. Even in their ideal climate, with the best growing conditions, birches are not long-lived trees. A half-century is a pretty long life for a birch tree.

Apart from intensive pesticide applications, or avoiding planting them, the best we can do here for white birches and other mesic trees is to understand where the roots grow and put water there on a regular basis.

 In general, most of the fine, water-absorbing roots grow near the surface, and they spread far beyond the drip line — the edge of the canopy — if they have the opportunity. A couple of drip irrigation emitters placed at the base of the tree is fine for a small, new transplant, but the irrigation pattern needs to grow with the tree.

This is probably why birches, alders and maples often grow and look better in well-irrigated lawns: irrigation is distributed uniformly over a large area. But it’s not safe to assume that your lawn irrigation is doing the job for your trees. Frequent, brief irrigation cycles might not thoroughly moisten the upper foot of soil. And on some sites, it might keep it too wet, preventing normal oxygen diffusion in the soil pores. Roots need air as much as they need water and soil. 

Alternatively, root zones can be covered with a 3 to 4 inch deep layer of mulch and irrigated with a water-efficient drip tubing, like Techline, which has internal emitters spaced evenly along its length. The key is to use enough of it to broadly distribute water across and down into the root zone, and then let it thoroughly drain and partially dry before repeating.  

So, how do we judge soil moisture content and properly schedule irrigation? In a recent column I discussed various devices that can be used for this. After some practice with two kinds of meters and lots of practice with an auger-style probe, I’m still not ready to entirely trust the meters. No irrigation system, and no monitoring system is perfect. The best we can do is to feel the soil for moisture content in the upper foot or so and with some practice, get a “feel” for the appropriate frequency and duration for irrigation cycles at a given site.

It’s possible for bronze birch borers to emerge from infested trees that are taken down, so it’s best to have your tree service chip the wood or take it to the yard waste recycling center to be chipped and composted. If it is to be saved for firewood (hardly worth the effort) the woodpile should be tightly tarped and “solarized” to prevent borers from emerging.

Pesticide applications are now done fairly commonly to attempt to prolong the lives of birches. They typically include protective (“prophylactic”) sprays with permethrin (Astro or Dragnet) and/or curative applications of imidichloprid, a systemic insecticide (Merit and Bayer Advanced). The costs and benefits should be weighed for each situation.

If you are thinking of planting birches, I’m sorry to say I can no longer recommend European white birch for our area. Only one species of birch, the river birch (Betula nigra) is reported to be very resistant to bronze birch borer. With tan, peeling bark, larger leaves and a broader growth habit, they are nice in their own way. Sadly, there is no perfect substitute for European white birch.

Bill Pramuk is a registered consulting arborist. Visit his Web site, www.billpramuk.com, or e-mail info@billpramuk.com or call 226-2884.