Now is the best time to prune trees to avoid common problems like Dutch elm disease.
City contractors have been pruning elm trees since the beginning of October and will go right through until the beginning of April. The trees are dormant for the winter, making it the best time to cut branches.
“Because we've got an eight-year pruning cycle, they won't be back for eight years, so they're looking for good strong branching structure, so that it can carry it through the next 8 years,” said Jeannette Wheeler, Urban Forestry Lead for the City of Calgary.
Elms are a big part of the city’s urban forest, making up about 12 per cent of the trees you find in the city. They are vulnerable to Dutch elm disease, a fungus that is spread by the elm bark beetle. Cutting away branches that show sign of infection can help save a tree.
Elms located on private property are the responsibility of homeowners and should also be trimmed of any diseased branches at this time of year as the disease will eventually kill infected trees. Pruned branches must be carefully disposed of.
“If you're pruning elm wood it should go out to the dump to be disposed of there, it's not something you would want to keep around,” said Wheeler.
There are several diseases that can affect elms, which can make it a challenge for people to identify what’s wrong. Another pest causes sidewalks below elm trees to turn black.
“Cushion scale is an insect that lives on the underside of the leaves, then it drops sap which causes the trees to turn black with sooty mold,” said John Duncan, Green Gate Garden Centre. “That's what a lot of people think Dutch elm disease is, but it's actually a secondary infection based off an insect infestation.”
Whatever you think may be infecting your trees, experts maintain that they should not be pruned while the sap is running in the spring and summer, so if you haven’t done it by then, you should wait until next winter.
