Q: Our new home came with some beautiful old rhododendrons around the porch. They're getting way too tall, however. What's the best way to cut them back? — Martine Godin, Portland, OR
A: Have no fear. These plants only look like they are hard to prune. Here's a good rule of thumb for a slow-growing shrub like rhododendron: Never remove more than 25 percent of the foliage in a single season. To shorten your giant rhodies, go after the tallest branches. Make the cuts about a foot below your ideal height for the plant. Gently spread the branches, then take a look inside to decide where to cut and how many branches to take. If the rhodies are really tall, you may be able to step inside the canopy to study the situation. You could remove as much as half of the too-tall shoots, as long as the total is less than a quarter of the plant's foliage. When you are reasonably sure of what needs to be done, make one cut, take the branch away and step back to contemplate the result. Don't cut just anywhere along a branch: You should cut near where one branch joins another or just above a bud. Make the cut about a quarter-inch above the branch or bud so you don't risk damaging it. A cut like this will punch a hole in the plant's canopy. As sunlight shines into the opening, dormant buds lower on the plant will awaken and grow. In years ahead, these will become the new upper branches of the plant. At the same time, older branches surrounding the hole will grow laterally to fill and soften the harsh opening you are looking into now.
Next, cut a second branch from another part of the plant's canopy, following the same procedure as above. Or move on to the next plant and remove its tallest branch. This approach is sometimes called “thinning out.” It preserves the natural layered look plants form as individual branches jockey for sunlight. You can also use this technique for shortening sideways growth on rhodies. When pruning lateral growth, it's important to know that upper branches will direct their new shoots downward to fill gaps. And if you trim back a branch so severely that the rest of it is completely shaded, it will die. In that case, better to take the whole branch out rather than shorten it. Ideally, your rhodies should get a light pruning every year, for the same reason we get regular haircuts. Spring is the ideal time to prune them, either during bloom or immediately afterward, because next year's flower buds begin to form soon after the old blossoms drop. Pruning after buds begin to set is okay, but this will reduce the display next spring. After the bloom is finished, it is a good idea to remove the spent flower heads so the plants don't put a lot of energy into making seeds. You'll be rewarded with heavier bloom the following spring. Simply hold the spent flower head between your thumb and forefinger just above the emerging foliage buds on either side of the stem, and break the stem off. This method is easier than using pruning shears and saves a great deal of time.