By Larry Hodgson
Late spring is the time to prune your dwarf conifers
in order to slow their growth.
Most of these conifers are mutations of the original wild plant, usually a tree. And most have a rounded, dense habit, without a terminal leader. But that doesn’t mean they’re really dwarfs. Instead, it’s their development that’s slow.
Take for example a popular dwarf conifer: dwarf mugo pine (Pinus mugo pumilio). The label probably shows height and spread of about 4 feet by 4 feet (1.2 m × 1.2 m). But if you plant it thinking it will stop at 4 feet (1.2 m) in height and spread, you’ll be in for a shock. It will reach about that size in just 10 years. And rather something closer to 6 feet (1.8 m) in height and 6 to 10 feet (2–3 m) in spread after 20 years. And the size of a one-car garage in 60 to 75 years. Wow! Not so dwarf, is it?
It’s important to understand that the dimensions printed on the label of a dwarf conifer give its dimensions at 10 years, not its dimensions at full maturity. At age 20, it will be about twice as tall and wide as that. And in general, after 20 years, it usually needs to be removed, because it will really take up far too much space.
Slow Conifers by Annual Pruning
To prevent dwarf conifers from taking up too much space in your garden—and to maintain their dwarf habit—, gardeners typically prune them every year. For that, you need to know what category yours belongs to. So . . . is your plant a whorl-branched conifer or a random-branched conifer? I’ll bet you don’t know!
1. Whorl-Branched Conifers
These conifers, including pines (Pinus spp.), spruces (Picea spp.), firs (Abies spp.) and Douglas firs (Pseudotsuga spp.), have only one growth cycle per year. It occurs in late spring. After, the plant stops growing until the following spring. This gives them gives the pyramidal, whorled, Christmas tree effect we tend to associate with conifers.
Whorl-branched conifers are easily recognized by the “candles” (narrow annual spring shoots) that appear at the end of each branch in spring. In pines, they even look a bit like asparagus stalks! And it is through these candles that you can limit their development.
Typically, you would cut back the candles by a third or even half when they are still young and soft. So, before the needles fill out fully and become tough. In many temperate climates, that would be somewhere between mid-May and late June.
On the other hand, you have to let each candle produce at least a few new needles. So don’t eliminate it completely. Consider cutting back two thirds of the length of the candles as being the limit.
You can snap candles by bending them with your fingers, or pinch them between your thumb and forefinger. However, that can leave your fingers very sticky because of the sap the cuts give off.
Many gardeners find it more convenient to prune candles using pruning shears.
After you prune, needles from the unpruned part of the candle will expand and cover up the wound, but no new candles will appear. Less noticeably, small buds will form just below the cut, maturing over the summer. They’re the buds that will produce next spring’s candles.
2. Random-Branched Conifers
These conifers, including arborvitae (Thuja spp.), false cypress (Chamaecyparis spp.), hemlock (Tsuga spp.), juniper (Juniperus spp.), larch (Larix spp.), Russian cypress (Microbiota decussata) and yew (Taxus spp.), don’t produce candles, but rather great numbers of small shoots from lower stem axils. And although they grow fastest in the spring, random-branched conifers still continue to grow all summer.
You normally prune these for the first time when their spring growth is complete, usually between late May and late June. Again, pruning consists of removing between one third and two thirds of the growth at the end of the branches. This time you won’t be able to use your fingers, but pruning shears or a hedge trimmer!
However, random-branched conifers continue to grow, slowly, until the beginning of September. As a result, even after a very neat, careful trim just before summer begins, they start to look unequal again just as summer comes to an end. There isn’t a huge amount of growth. However, it’s enough to encourage perfectionist gardeners to prune a second time at the end of August. Don’t prune much later than that, as this can stimulate late growth which will not have time to harden off before the onset of cold weather. And that can result in dieback and brown stem tips.
The Result of Annual Pruning
For both types of conifers, this annual pruning will reduce the plant’s growth considerably and make it denser and more attractive. And the conifer will remain dwarf much, much longer!
If You Don’t Want to Prune
I used to prune the very first dwarf conifers I grew—two beautiful mugo pines!—but then, I was young and enthusiastic. After a few years, I was less of both. And tired of scraping my pruning shears free of all that nasty sap. I don’t anymore.
Let me share a few tips on how to enjoy dwarf conifers without having to do more than the occasional pruning, like to remove a damaged branch or a reversion.
Among the things you can do to profit from dwarf conifers without having to prune them every year, there are:
- Choose miniature conifers rather than dwarf ones. Miniatures, by definition, grow less than 1 inch (2.5 cm) per year. Dwarfs grow 1 to 6 inches (2.5 to 15 cm) per year. And 6 inches (15 cm) is quite a lot! Choosing one that is naturally very slow to develop will give you more time before you need to react;
- Start with younger specimens . . . which, moreover, are less expensive;
- Leave ample room for future plant growth. For example, multiply the diameter listed on the label by 2 ½ and use that number when you’re planning the spacing for your new plant.
Finally, when your conifers finally do outgrow your needs, perhaps in 25, 30 or 35 years, pull them out and start over with young specimens . . . or recommend that your grandchildren do so!