Tuesday, March 20, 2007

Evergreen issues in San Francisco, California

I have a garden with a large american elm, which faces two different evergreens, one a spruce and one a fir. On the branches that face the elm I have experienced lots of dropped needles. is there something about the roots or other element of the tree, like pheromones or some sense of closeness to other trees, which may be playing a role? this has been happening gradually over the last few years, and the evergreens in question are about 25 years old, with the very nearby elm pre-existent by at least as many years. but the pattern is striking, and would
suggest that to me. any advice would be appreciated.


Well, sure enough, a picture is worth a 1000 words, but I hope my advice doesn't go on quite that long! First, I want to clarify the two evergreens. The tall, more cylindrical tree (images 2385 and 2387) is neither a spruce nor a fir. It clearly is a scale-leaf evergreen which means it is most likely an Arbovitae or possibly a kind of Chamaecyparis. Arbovitae are a popular screening tree for that kind of narrow location where everyone is trying to achieve more privacy and screening. The catch is that Arbovitae need full sun to be their happiest. Therefore the holes/patches of defoliated branches are clearly signs to me of too much shade. The other tree (2386 and 2392) you are right, is either a kind of spruce (Picea) or fir(Abies). Here is a quick way of figuring out which one it is. Grab a piece of branch that has both needles and bare stem, meaning a piece closer to the center of the tree. Next, pull off a few needles one at a time. Spruce, when you pull a needle, leaves a little bump, or "peg" where the needle emerged. Hence the catchy phrase, "Picea have pegs". If you pull the needles and they take a tiny piece of the bark with them, leaving a clearly "circular leaf scar" where the needle emerged, then that is a fir tree. Generally people say that spruces are more hard and prickly and firs are softer and more fragrant, but typically that is not enough for a concrete plant ID. However, either way, both spruces and firs, yup, like sun also. Sounds to me, Aaron, like it's not an issue of a disease at all. And yes the space is limited, and for the roots as well, but honestly I think the problem is much simpler. The elm is slowly shading out the other two trees. It grows as fast as the other two, especially if the other two don't get enough sun, its canopy casts more shade, and it makes sense that you have been watching this gradually happen over the last five years. The Arbovitae is going to continue to lose needles on the side facing the elm because that's the shadiest. The fir/spruce/? will also continue to drop needles from the base upwards. You have new growth on the tips of both evergreens so they are still alive, but just getting out competed for the necessary sunlight. At a cost arborists may be able to come in and thin out the canopy of the elm to allow a little more light through, but it might be worth thinking about what you want to do in the long run. Certainly you can cut the dead branches off the evergreens because they will not leaf out again from those spots.

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