Conifers Gardening Plant diseases

Strange Growths on a Cedar Hedge

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Cones on arborvitae. Photo: H. Zell, Wikimedia Commons

Question: My cedar hedge is covered with strange yellow-green growths. The hedge is certainly over 15 years old, as it was already well-established when I moved here 12 years ago. This is the first time I’ve had this problem. What are these growths and how can I prevent them?

Donald Hinse
Welland, Ontario

Reply: Those “yellow-green growths” are simply cones. They may not look much like the pine cones you may be more used to, given their much smaller size (only 8 to 13 mm/0.3 to 0.5 in long), but they serve the same purpose: cones produce the seeds that will give birth to the upcoming generation. After all, cedars (eastern white cedar or arborvitae, Thuja occidentalis) are conifers and the word conifer means cone-bearing.

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The cones will turn brown and open to release seeds. Photo: Sten Porse, Wikimedia Commons

By the way, the cones won’t remain yellow green; they’ll turn brown at full maturity in early to late fall, then split open to release their seeds. They’ll hang on over the winter, but will eventually drop off, their job done.

If your hedge plants are only now producing cones after 15 years, it’s because they’re reaching maturity. Although young arborvitae occasionally produce cones, most don’t before the age of 10 and generally they only start to produce them prolifically from about the age of 30 on. Also, they don’t necessarily produce cones every year. Every 2 to 5 years is more typical.

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Female flowers appear discretely in spring. Photo: Hans, pixabay

If you really do want to prevent cones from forming, study the plants in the spring (April to June, depending on the region). In years in which you see pinkish tips to the branches (these are female flowers), prune a bit more heavily and you’ll remove them. Of course, if you prune off the female flowers, there will be no cones that year.

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Pine siskin (Spinus pinus) feeding on arborvitae cones. Photo

Even so, I suggest learning to see cone formation on arborvitae as an added if minor attraction rather than a problem and just letting nature run its course. After all, arborvitae cones are an important source of food for many birds and animals, including chickadees, grosbeaks, pine siskins, nuthatches and red squirrels. Wouldn’t you want to invite them to your garden?

Live and let live: an excellent motto for laidback gardeners!20170908A H. Zell, WC

2 comments on “Strange Growths on a Cedar Hedge

  1. ben keller


    • My guess is that there might be a hummingbird nest hidden inside the branches. It’s a tree often used by hummingbirds for nesting.

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