We have a row of Italian cypress trees growing along our fence line. They are now about 20 feet tall and very slender, only about 2 1/2 feet wide. They have grown well for about 10 years but now they are showing some problems: scattered little dead tips, an open, sparse look near ground level where they are shaded by the house, and a few larger dead branches high up. Can you tell me what these problems are and what can be done about them?
Italian cypress is an old favorite around Napa Valley. The species, Cupressus sempervirens, is subdivided into several cultivated varieties that are valued for being evergreen, tall and narrow, and easy to grow. The pencil-thin structure of the tree lends itself to planting where few other tree species would fit. There are quite a few mature, healthy plantings in our region, but I’ve been noticing some showing the problems you describe.
First, the “scattered little dead tips” are caused, most likely, by cypress tip miners, the larvae of a tiny, silvery tan moth: Argyresthia cupressella. The females lay eggs on the green tips. The eggs hatch into larvae that feed on the twig tips.
If the foliage starts dying inside and working outward to the tips, the problem would be entirely different: a fungal infection called Cercospora blight.
For tip miners, U.C IPM Online recommends providing proper cultural care to keep plants vigorous, pruning to remove infested tips and, where infestation is heavy, applying a “broad-spectrum, persistent insecticide such as acephate when the adult moths are active, March to May.
The open, sparse look, resulting from too much shade and probably made worse by the tip miners cannot be corrected by pruning. Unlike many broadleaf plants and a few evergreens, like yews, bare cypress stems cannot sprout new foliage. Where the sparseness has become unsightly it might look better to simply prune off the dead and dying stems.
The larger dead branches could have any of several causes: Broken by squirrels, infested by Western cedar borers (a beetle — Trachykele blondeli) or infected with Cypress canker, a disease caused by the fungus Seiridium cardinale.
If you can get a close look at the dead branches you might be able to the cause. The worst case is cypress canker. It causes resinous lesions that kill stems. At the onset, infected stems start to fade in color before dying.
So, close examination needs to be done to help determine an appropriate course of action.
In any event, the trees should be provided with the best possible cultural care: full sunlight, good drainage, moderately fertile soil, and enough moisture to prevent summer drought stress to help them resist pests and diseases.
Bill Pramuk is a registered consulting arborist. Visit his website, billpramuk.com, email questions to email@example.com or call him at 707-226-2884.