Hello,
When can I prune my Japanese lilac, weigela, and viburnum bushes? They are under a mature oak canopy and so produce a lot of long shoots from the top all summer. Also, as a result, they have become too open. They have small leaves appearing now as well as flower buds. I wanted to take them down 1/3 last fall but read that it should be done in the spring. I am in west TO. Etobicoke master gardeners haven’t answered my email from 2 weeks ago. Is it too late?
I hope you can answer soon.
Thank you.
Rosemary
Thank you for your question.
The care of a Japanese lilac tree is easy because it maintains its lovely shape without extensive pruning. If yours is a shrub, it may need renewal pruning every few years. It flowers on old wood, meaning the flower buds were formed the previous season.
To control the height of your lilac tree, the branches should be trimmed to roughly one foot below the desired height in spring, right after the tree finishes flowering for the year. When trimming a branch, cut it back to 1/4 inch above a bud, or swollen section of the branch or stem so that you do not remove next years flowers. If possible, deadhead spent blossoms to encourage additional growth the following year.
Read more at Gardening Know How: Japanese Lilac Information: What Is A Japanese Lilac Tree https://www.gardeningknowhow.com/ornamental/shrubs/lilac/japanese-lilac-information.htm
Similar to your Japanese Lilac, Weigelas flower on previous years growth so pruning should be carried out immediately after flowering. Prune out any dead wood. With clean, sharp secateurs your aim is to start pruning from the inside of the bush, working your way out, removing dead/damaged wood, and thin out dense growth areas inside the base, to promote good air circulation. When pruning any large shrub use the measure of “one third” as a maximum amount of plant material you remove.
Viburnum pruning depends on the variety grown. In many cases, a good time to prune is just after flowering but before the setting of seedpods. Again, pruning up to a third of the shrub can achieve natural-looking results without harming the viburnum. This link from the New York Botanical Garden will provide more detail about pruning your Viburnum.
The Virginia Cooperative Shrub Pruning Calendar posted by the Halton Master gardeners gives an excellent summary of when to prune any shrub.
Good Luck!