My friend in Carbondale, CO has a lilac bush (no known type) that is budding out. They had their yard person trim it way back last year and clean out a lot of the interior. Since it is budding out, she thinks it is best to have her yard person trim off the top to send the new energy downward to help fill out the interior. However, I am not sure if this is best to do at this time of early spring. Please advise.
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Pruning is usually recommended when the plants are dormant, however, this can be done in the spring, or during the growing season depending on the type of plant. Pruning can encourage regrowth- but one of the risks could be that you would lose your floral display for the year if you remove growing buds. Pruning during the growing season can potentially expose your tree to insects and disease, but there are practices that you can take to minimize these risks.
That being said, corrective pruning can be done at any time of year, at your own risk. Early spring blooming shrubs (like lilacs) can be pruned after flowering to promote flowering for the following year.
Here are a couple of publications that go into greater detail about pruning timing and best practices (these also go into details about achieving a desired structure and shape in your shrubs):