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Plant Answer Line Question

Keywords: Cupressocyparis, Insect pests--Control, Trees--Diseases and pests--Washington

My Leyland Cypress is browning and has Cypress tip moth signs. I'm worried about the brown spots, and wonder if it can survive this attack? How can I control the pests, if it might survive?


From what I can determine, your Leyland cypress (x Cupressocyparis leylandii) trees are probably going to survive this attack unless they are weakened in some other way. x Cupressocyparis leylandii in California survive the Cypress tip moth, though they can be unsightly. Since California is a bit too dry for this tree, the conditions are not identical, but Natural Resources Canada does not indicate that infestations are fatal. Because you said you found evidence of tip moth (Cypress tip moth = Argyresthia cupressella), I will assume that is what the problem is, but a bit of browning, even in conifers, is not unusual right after trees are planted. Be sure that you are not overwatering, as one effect of that is the same as underwatering (i.e., tip die-back or yellowing) because too much water prevents the plant from taking water and oxygen into the roots.

The Organic Gardener's Handbook of Natural Insect and Disease Control (ed. by Ellis and Bradley, 1996, p.183) says about pine tip moth (Rhyacionia frustrana):

Handpicking works if only a few caterpillars are present. Pruning off and destroying infested tips in winter is a very effective control.

I would recommend a prune-and-wait-and-see approach. April is a bit late to prune (and puts root establishment in competition with shoot regrowth), but you may be able to slow the infestation down, so go ahead and do it. Watch the trees this season and then prune again in the winter next year. Be sure to destroy (burn or bag and put in the garbage) the debris so you don't reinfect your tree.

A good gardening resource is the UBC Botanical Garden Forum. Personal testimony/experience is valuable, especially if it's regional. (You might find it useful in the future.) Several people commenting about x Cupressocyparis leylandii note that it is not a very desirable tree; one of its parents, the Cupressus nootkatensis, also called Callitropsis nootkatensis or Chamaecyparis nootkatensis, or, for that matter, plain Nootka cypress) is better. One person recommended planting small trees in the beginning, since they grow very fast. This might save you some money, should you have to replace your trees. The site does not need a password; just click on "Search" in the upper right corner.

Below is some additional information from Oregon State University about cypress tip moth. This site recommends pesticides, but from everything I read, they are not effective without multiple treatments. Since this pest is generally not fatal to the trees, it is probably not worth it to use chemicals which would be dangerous and time-consuming to apply. If you know something about the life cycle of the pest, your observations will yield more information and any manual control attempts are more likely to be effective.

Date 2017-01-11
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Plant Answer Line Question

Keywords: Cupressocyparis, Pruning trees

The previous owners of our home planted Leyland Cypress at the property line. The trees have grown very high. The neighbors have asked us to trim these trees in a "hedge-like" fashion, which means that we would need to cut the tops of the trees. One neighbor, who is a landscaper, insists this will not damage the trees. But several arborists have advised not to "top" the trees. We are willing to have the trees topped as long as this will not compromise the health of the trees.


Pruning Leyland Cypress (x Cupressocyparis leylandii) to look like a hedge can be a challenge.

Topping is not a recommended method for controlling trees, because it often causes them to grow faster (unless they are topped mortally) and thus must be done repeatedly and expensively, and also because it weakens the tree, which may cause it to drop limbs, rot, or blow over more easily. The group Plant Amnesty has a great deal of information about why one should avoid topping.

However, topping may be less harmful for x Cupressocyparis leylandii than for other plants, but it is still not a particularly effective solution. The University of Florida Extension concurs that this practice is less harmful to x Cupressocyparis leylandii than it is to other species, but they still do not recommend it.

Peter McHoy's A Practical Guide to Pruning (New York: Abbeville Press Publishers, 1993)also advises avoiding topping, but also notes that if one must top an x Cupressocyparis leylandii, it should be done in midsummer and repeated every few years.

However, in general, it would appear that topping is very much a last resort.

One book, Practical Tree Management: An Arborist's Handbook, by T. Lawrence, P. Norquay, and K. Liffman (Melbourne: Inkata Press, 1993), recommends that "Where a tree requires severe reduction or radical alteration of its aesthetically pleasing, natural growth habit, it is usually far better to consider replacing the tree with a species more suitable for the situation..." Thus, you may consider an initial pruning and eventual replacement.

Date 2017-08-08
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August 01 2017 12:36:01