Q. I'm wondering how I can bring a row of hemlocks that have grown to about 12 to 15 feet high and 6 to 8 feet wide down to about eight feet high and in a couple of feet without damaging them.
- ---Will in Naugatuck, CT
A. Well, it depends on what kind of hemlocks these are. The basic type—the Eastern or Canadian hemlock—is a slow-growing evergreen that reaches 80 feet at maturity.
But you might have one of the dwarf varieties that stay relatively small and are often used as screening hedges or shrubs. And in his classic "The Pruning Book" (Taunton Press), our limb-lopping expert Lee Reich says that all hemlocks can be trained into hedge-like shapes, and—unlike some evergreens—should regrow nicely from old wood, especially while they're still young. But Lee also cautions that while they SHOULD respond well, "not all trees have read The Pruning Book". So our listener should begin the process of bringing them slowly down in size early next Spring.
Not now. Summer and fall are the worst times to remove healthy branches from any big tree or shrub. Pruning outside of the dormant period in winter stimulates new growth. Forcing that growth in the summer can greatly stress trees. Fall pruning is even worse, as it prevents the plants from going naturally dormant. And you want to be especially careful not to stress hemlocks, as it makes them much more vulnerable to attack by the wooly adelgid—a pest that's been decimating these trees. (If your evergreens ARE attacked by these pests, horticultural oils are the best response.)
So hands off until early Spring. Then I'd give serious thought to just trimming the sides back a bit every Spring. Lee feels that hemlocks can take pruning better than most evergreen plants, but if they are standard trees, the trunks are still going to grow to their normal size, and keeping the tops short could eventually make them look very weird.
All of which makes it hard to give a really good answer here; too much uncertainty in the air. So we emailed Will back to ask what he knew about the trees; how long they've been in the ground and if they've ever been pruned. His response was enlightening…
Will writes: "These bushes/trees (I'm not sure what type they are; just that they're hemlocks) were planted before my parents bought the house 40 years ago. I've been trimming the sides for the past 25 years and the tops every five years or so. But there are brown areas near the bottom that expand each year regardless of how much green I take off." Will included a photo that showed a nice solid wall of green right next to a road—and of course, growing right underneath power lines.
Now, some browning at the bottom is inevitable when evergreens don't get enough sun down low; but the basic secret to keeping these kinds of plants looking nice and full is to shape them slowly and gradually. Just because you suddenly realize you want to do something dramatic doesn't mean the plant is going to like it. Generally, it's just the reverse.
If they're now six feet wide, he should be able to prune a foot or so off of each side every Spring without any harm. Or, if the trunks are really thick—indicating that they are standard sized trees—he might want to instead consider carefully removing a few—especially ones with a lot of browning—and let the sides of the remaining trees grow to fill in those spots. This would also let more light in and reduce the risk of future browning.
But if he tries to reduce their height by almost half in one shot, they could die outright—and they'd certainly look ugly if they did survive. Luckily, his photo shows that they haven't grown too close to the power lines yet, so—with extreme caution—he should be able to achieve what's needed here.
But he'll need to have a safe way to get up there. A rickety ladder is always a bad idea, but a rickety ladder 15 feet in the air with an unbalanced man holding sharp cutting tools under power lines could end really badly. So we emailed again; this time to ask Will exactly how he plans to reach the tops safely.
His answer was very reassuring: "For a long time, we just used a step ladder," he writes, "which was not very safe. Now we use scaffolding, which is safer and helps the job go much faster." That's a relief.
So here's the plan: early next Spring, once he's up there safely—being extremely cautious not to let his head or tools get anywhere near the power lines—he should take his time, shaping the tops as he removes height. Reduce them by about two feet a season and he'll eventually get them down to a manageable ten feet or so without any damage.
Yes, I know he wants them to be eight feet. But the treesdon't CARE what he wants. In fact, they'd much rather just be left alone! So my advice is to spread the task out over a few more seasons than you'd like, let them end up being a little bit taller than you'd like, and the result will be something that you'll really like.