By Mike Ecker, Director of Living Collections:
Goodness – it’s cold and you want to prune now? Can’t we wait till it’s nice and warm in July or something? Especially the oaks on Oak Hill! You can’t get much more exposed to the elements on our grounds than on Oak Hill (Glacier Ridge on our maps).
When it comes to pruning oaks, the time to do it is in winter. This is not to punish anyone. It’s because a nasty insect called the two-lined chestnut borer is not around during the winter. This insect, although its name indicates it was a major pest of American chestnut, is attracted to fresh wounds of primarily oaks. Pruning in winter allows the wound to lose its appeal to this insect by early summer once the insect begins flying again looking for sites to lay its eggs.
It’s not just pruning wounds, however. Drought, compacted soil, construction injury to trunk or roots – heck just about anything that stresses an oak is cause for concern with this critter. The black adult has two golden stripes running down its back. Sounds pretty but keep in mind it is in the same genus, Agrilus, as the Emerald Ash Borer and why it leaves distinctly “D” shaped exit holes, just like EAB.
The first sign of an oak having problems with the two-lined chestnut borer is seeing foliage wilt on upper canopy branches in late summer, leaves turn brown and then hang on afterwards. The next year these branches are dead. The insect progresses downward, sometimes killing the tree in a couple of years.
Yes, it’s more than just pruning wounds that attract this insect. But unlike our not being able to control the weather (and let’s hope mankind never gains that knowledge!), we can control when we prune our oaks. So put on your long underwear and get out there!