Q My PJM rhododendrons, my lilacs and my azaleas all bloomed beautifully this spring. Should I prune them now that they're done blooming?
A Yes. Any pruning should be done shortly after spring-flowering shrubs are through blooming. They set their minuscule buds, which will become next year's blossoms, within a few weeks of blooming. If you wait to trim them, you'll remove potential flowers, no matter how carefully you prune.
But before you begin, ask yourself whether the plants really need to be pruned. Neither rhododendrons nor azaleas grow rapidly in our climate. So until they're approaching the size you want them to be, you should limit your pruning to removing faded flowers and doing minor shaping by cutting back stems that are wildly out of place. Dead or broken branches may be removed at any time.
Lilacs can be a different story. Some remain quite compact, but many varieties can get huge and produce flowers so high off the ground that they're hard to reach. A little light pruning every year, right after the flowers fade, helps keep lilacs from becoming supersized.
If you already have an overgrown lilac, try "renewal pruning." Cut a quarter to a third of the oldest, woodiest stems right down to the ground in early spring for several years running. This kind of pruning encourages new shoots to grow from the roots. By the time you remove the last batch of thick old stems, some of the younger ones will be mature enough to bloom, and the entire plant will be a more manageable size.
You could prune the entire lilac heavily, but it would take years before the plant would bloom again. Unfortunately, I learned this the hard way.
Deb Brown is a garden writer and former extension horticulturist with the University of Minnesota. To ask her a gardening question, call 612-673-7793 and leave a message. She will answer questions in this column only.