A Guide to Winter Pruning

When most people think of architecture, they probably think of their town’s coolest new building. But garden plants have architecture, too. Trunks, limbs, stems and branches give plants shape and character.

The best time to see garden architecture is in the winter. It’s also the best time to prune most plants, especially deciduous ones. That’s because without leaves it’s easy to see the shape of plants and to enhance that shape through pruning.

Here’s a winter pruning guide that will help you get your garden in shape and also help you give your plants the cold-weather care they need.

Pruning basics

Be sure to exercise care in pruning winter-flowering plants or early spring blooming varieties, including fruit trees, to avoid cutting off flowering buds. And remember that some flowering shrubs such as hydrangeas can be left un-pruned until late February. Beauty is always in the eye of the beholder, but many people like the winter interest that the large flower heads of plants such as the mop head varieties of hydrangeas add to the winter landscape. Be sure, though, that when you do cut off old flower heads you only trim the plant back to the first set of emerging leaves.

In all cases, pay attention to several pruning basics:

1. Never prune just because you feel like you need to be doing something in the garden. Always have a reason to prune.
2. If cutting a tree limb back to the main trunk, prune the branch just above the branch collar. This is the circular growth against the trunk of the tree from which the limb emerges. Take care to avoid damaging the branch collar.
3. If cutting off only part of a branch, prune back to a set of visible buds. This will eliminate leaving part of a branch or twig, which would create a potential entry point for disease. Remember that new growth will sprout from the bud, not from the end of an empty twig.

Getting started

After the first hard freeze, the top growth of herbaceous perennials – those that die to the ground in winter and re-emerge in the spring — will turn brown. Examples are phlox, baptisia, amsonia and canna as well as hostas and non-evergreen ferns. Cut these to the ground after the first freeze.

It’s important to wait until mid-winter to prune many other plants because this is when they are fully dormant. Cutting plants back before the first frost and before plants are in full hibernation could cause them to activate dormant buds. If they push out tender new growth before the onset of winter, these new shoots will be killed by the inevitable still-to-come freezes.

The dead of winter

The place to start with winter pruning is with dead, dying, and hazardous material. The first branches and stems to go should be those that are diseased, broken or unhealthy. Start by pruning branches that are at head or eye level to avoid getting poked in the eye.

Then move on to structural pruning. For instance, take out any secondary crossing branches or limbs growing in the opposite direction of the main growth. You can also prune for aesthetics — keeping a plant in a size range or limbing one up. But, keep in mind the overall habit of the plant you are pruning — a flowering crabapple doesn’t grow the same as an oak.

Grasses

Ornamental grasses are another type of plant that can be left un-pruned until late February. Many grasses won’t die to the ground but instead will turn brown and keep their seed heads. The seeds will attract birds to the garden and, perhaps, even provide shelter on a frigid night.

Another reason to wait until late winter to prune grasses is because cutting them back too early increases the possibility that they will rot at the growing point. Try to be patient and wait until late February to prune them, cutting them then to just a few inches above the ground. Your patience will be rewarded with reducing the possibility of crown rot and providing a clear path for fresh new green growth in the spring.

Silver and salvias

In general, it’s also a good idea to wait until late winter to prune salvias and anything with silver foliage (some examples: Artemisia, Buddleia and Peroskia). These plants have hollow stems, and pruning exposes their empty inner “tube” to harsh elements. If rainwater builds up inside the tube and doesn’t evaporate, then extreme temperature dips could cause the water to freeze, expand and rupture the stems. Wait to prune these plants until the beginning of March when the weather starts to warm up. Cutting these plants back too early will also rob them of stored sugars they will need to get through the winter.