Prune spring blooming shrubs now

June 13, 4:11 PMPittsburgh Gardening Scene ExaminerRose Field

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This neglected Rhododendron is hoping to be thinned after the blooms fade.

At the risk of sounding like a TV commercial, “Now, for a limited time only, prune your spring blooming shrubs. This offer won't last!”

Sorry if I shouted too loudly, but it is a slim window of opportunity and if you don't prune spring bloomers very soon, you'll lose some of next year's flowers.

Most of the spring flowering shrubs commonly grown in the Pittsburgh area such as lilacs, Forsythia, mock orange, Rhododendrons and Azaleas bloom on what's referred to as “old wood”. This means that stems that grew in the previous year already have flower buds dormant in them over the winter and that's why they're all ready to bloom as soon as rising spring temperatures encourage them. After they finish flowering, new growth continues, and by late summer, next year's flower buds are formed. If you prune them too late in the season, it's bye-bye to next spring's display.

Some gardeners err on the other side, being so eager to get out and get muddy as soon as the snow melts, that they hack into spring flowering shrubs in March. Late winter pruning works for a few shrubs, but those which are about ready to bloom aren't among them. We'll discuss those that like late winter pruning in a future article.

Traditionally, July 4th marks a turning point where it becomes too late to prune early bloomers. Whenever the flowering finishes up, pruning can begin, so for most spring blooming shrubs, late May and early June are ideal.

Maintenance pruning for Rhododendrons

As we enter the third week of June, most Rhodies have finished flowering and two leaf buds of new growth are visible on either side of the dead flower heads. It can be a rather delicate operation to deadhead the spent blooms without breaking the very vulnerable new growth. The payoff is a much neater looking plant and increased vigor, as energy isn't wasted setting useless seeds. Before long, new flower buds will emerge, so take the time now to thin the shrub and improve its shape.

Begin by taking a careful look at the Rhodie. Notice any crossing branches, or the notorious “three D's”: dead, diseased or damaged branches. (The “three D's” can be removed at any time on any type of shrub, obviously because they are nothing but liabilities.) When two branches cross, see if one is going in a direction not in keeping with the shrub's natural shape, and take that one out, down to where it joins a main stem. The interior of the shrub should be relatively open to air circulation and light, so some thinning cuts are beneficial. Take them down to an outward heading branch. To shape, cut back to a lower whorl of leaves taking care not to leave a long leafless stub. This type of light maintenance pruning can be done every year to keep a Rhododendron in good shape.

Forsythia may require heavy thinning

Look in any neighborhood in the Pittsburgh area and you will readily find Forsythia pruned in exactly the wrong way: sheared into unfortunate round or square shapes. While there are only a few plants which should ever be sheared this way to begin with, there's probably no other which is less amenable to the bad practice than Forsythia.

Before you protest that the plant would be too large for it's spot if not treated that way, let me say, then it's the wrong plant for that spot. These poor things are surely the most often abused shrubs in our gardens and when pruned wrong, they are frightfully ugly. Shearing causes a “bird's nest” effect where a single branch tip produces two, which get whacked, and those produce four, which get whacked, and those produce eight..., well, you get the idea. Eventually, you have an outer shell of leaves over a leafless, unhealthy interior. In spring these garden disaster victims produce just a few flowers and for the other 50 weeks, they look awful.

Forsythia should be allowed to grow into a natural vase shape, and thinned to encourage healthy new growth and keep the interior open to light. Each year, try to remove about one third of the stems, taking them out all the way to the ground. While blooms occur on what we defined as “old wood”, it's still the younger, more vigorous branches that bloom best, those just a couple years old. Thinning in this way will also eliminate the wildest, longest branches and give a more tidy look without shearing.