Question: I read that spring flowering shrubs should be pruned soon after they are through blooming. Why can’t I wait until fall to prune after they have made their growth for the year?
Answer: Spring flowering shrubs like lilac, spirea, viburnum, forsythia and azalea begin developing flower buds for the next season in late summer and early fall. If you wait until the end of the growing season to prune you will remove most of these flower buds and there will be very little bloom next spring.
We have about a two month window for pruning spring flowering shrubs. If you want to reduce the size of plants, prune right away. If you would like to let plants make some new growth before pruning wait a month or so.
Some plants like lilacs and forsythia may need to be pruned twice to keep them within bounds. I like to prune forsythia down to an arbitrary height of about 6 feet. Then I shorten subsequent growth by about half a month later.
For old lilacs that have become very woody, I remove one or two of the oldest main branches back down to the ground each year. This stimulates new growth from the ground. I let at least a few of these new sprouts grow. Then I prune the remaining branches back to an arbitrary height of about 6 feet. This stimulates rapid growth of side branches which I shorten by about half about a month later. If new branch growth is especially heavy, I may remove some of them.
Spirea, viburnum and azaleas are less vigorous in growth. Removing flower seed heads and a small amount of connected growth will usually reduce them in size enough so they will be about the same size after they have made their new growth.
Do not use power clippers or shears to prune flowering shrubs. Shearing stimulates multiple side branches and destroys the natural shape and thickness of plants. Instead prune individual branches just above a side branch. If no side branches are present at the height you want to prune, then prune just above a side bud.
I like to start at the bottom of a shrub. The lowest branches may need very little pruning. As you move up the shrub shorten branches more and more so you have a tapered look. If lower branches are pruned too short they will be shaded by the upper branches and will gradually lose their leaves. This results in a bare or chicken-leg look.
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