Don’t torture rhododendrons with poorly executed pruning

Every so often Marjorie and I will help a friend prune an overgrown rhododendron. In addition to bringing out the best of the teacher in both of us, these “miniworkshops” are great therapy. It is a joy to bring out the best in a plant, to nudge its growth in the preferred direction while enhancing its beauty.

Sadly and despite our efforts to teach proper technique, much butchery is still done in the name of pruning. Rhododendrons are often the victims. The weapon of choice is the electric or gasoline-powered hedge trimmer.

Just this past weekend we lamented the torture of several rhododendron shrubs in the landscape of a local business. It was obvious that someone lacking knowledge of the natural form of the species, a prerequisite to success in pruning any plant, had been intent on forcing them into rounded meatballs of foliage. Equally obvious were the decapitated stem sections poking out in all directions, mangled leafless stubs bearing witness to the indiscriminate cuts made by the shears as they chewed through the plant.

Never use shearing to reduce the size of a shrub! The cuts made by shearing, called “heading cuts,” involve the shortening of numerous stems. Buds on the stem sections that remain and that would have stayed dormant are prompted by shearing to grow with greater vigor than if the stems had been left alone. The buds nearest the cuts grow into vigorous, upright shoots that soon make the shrub taller than before the shearing. Thus the act of shearing defeats the intent of reducing the size of the plant. At the same time, the repeated shearing necessary to control height results in a dense tangle of branching, an unnatural and unhealthy form for any shrub.

Heading cuts should be used on rhododendrons only when the intent is to encourage growth from a selected young stem. To fill a void left by loss of a branch, for example, a carefully directed heading cut or two will do the trick. The pruning tool of choice for such heading is a pair of hand pruners, not hedge shears.

Control the size of rhododendrons with “thinning cuts.” Instead of cutting off part of the stem, a thinning cut removes the stem completely, typically cutting it back to a larger branch. The plant response is nothing, at least not near the cut. Remaining shoots on the plant may grow more, simply because stored energy for growth is distributed over fewer stems and leaves, but the overall effect will be to reduce the height and width of the shrub for a much longer period of time than with shearing.

Depending on the size of the stem to be removed, thinning cuts are made with hand pruners, a pruning saw or loppers. On an old rhododendron, the saw gets most of the work. Be sure to make each cut close to the permanent stem but without cutting through the wood of the permanent stem.

Make every pruning cut a product of thoughtful consideration. Walk slowly around the shrub, looking at it from all sides, and identify one branch that is too tall or too wide. Remove that branch with a thinning cut. Often this involves crawling into the heart of an old shrub on all fours in order to find the point where the offending stem joins a larger branch. Repeat this process until the height and width of the shrub are reduced to the desired level. While you are at it, use thinning cuts to remove any dead, damaged or diseased branches.

Remember that your rhododendron is going to grow each year, perhaps more vigorously than before the pruning, so reduce the overall size enough to allow for a few years of new growth before having to thin again.

Rhododendrons can be thinned in late fall, early winter, or early spring. Avoid pruning them in late summer or early fall when there is still a chance for pruning-stimulated new growth. This new growth may not have time to harden and could be damaged by the first freeze of winter.

Send queries to Gardening Questions, P.O. Box 418, Ellsworth 04605, or to reesermanley@shead.org. Include name, address and telephone number.