Most conifers can be pruned, however, they respond differently from other plants, so you need to know what type of tree you are working with and how best to trim it.
Do You Need to Prune Conifers
You don’t need to prune any plant, and evergreens are no exception. Most conifers have a distinct shape that keeps them rather attractive with no pruning at all.
Why You Might Want to Prune Conifers
There are a few instances where trimming coniferous trees and shrubs might be worthwhile. As with any plant, pruning out diseased, broken, or dead branches is recommended. Disease in any branch can quickly spread throughout the tree or shrub and dead branches are open invitations for disease and pests to move in.
The most common reason to prune conifers from an aesthetic point of view is to get a fuller plant. A small amount of pruning at the right time can result in a denser, bushier plant, which can be very attractive. However, it is very easy to ruin the shape of a conifer, by pruning to drastically or at the wrong time.
While many plants can be pruned to keep their size in check, this is hard to do with conifers because most grow from a central leader. Practices such as topping a tree by cutting off the uppermost part of the trunk should be avoided at all costs. Pruning back this center stem will reduce the height of the plant, but the width will continue filling out, leaving you with an oddly shaped tree, and it will eventually die. If a tree is too tall for the space, it should just be removed and replaced with another smaller species.
Another common pruning practice is limbing up or removing the lowermost branches. You shouldn't do this just to make space underneath, but if lower branches are dying then this kind of pruning is appropriate. You are much better off researching the growth habit of your tree or shrub and planting one that won’t outgrow the space you have.
When to Prune
Most conifers are pruned either while dormant in early spring before new growth starts, just as new growth is starting, or when they are semi-dormant, in mid-summer. Early spring is preferable since the new growth will quickly fill in.
The new growth on conifers is referred to as "candles" because of the candle-like shape of the branch tips. Cutting the candles back halfway, before the needles unfold, will keep the tree more compact.
Candling should occur between late March and mid-May, depending on the area and the weather.
Don't try to prune once the needles have opened fully or you may end up with a misshapen plant since most evergreens cannot replace their growing tips.
A general rule of pruning that also applies to evergreens is the rule of thirds; never remove more than one-third of a plant at any time. Removing too much growth all at once is very stressful to a plant and it will take them a long time to recover.
How to Prune Some Popular Conifers
When deciding how to prune, you will need to know the growth habit of the conifer’s needles. There are two basic growth habits:
- Whorled: Whorled branches, as the name implies, grow in somewhat of a circle around the branch. Examples of these would be Douglas-fir, firs, pines, and spruces.
- Non-whorled or random branching: Some randomly branched evergreens can re-grow from old wood. For example, you can prune yews well past the green needles into the center of the plant and they will still re-grow. Granted, it can take a few years before they look presentable again, but it is a good way to rejuvenate old shrubs. Examples of randomly branched evergreens include arborvitae, cedar, hemlock, juniper, yew.
Pine tree needles grow in little bundles, like a feather duster. They generally grow in groups of two, three or five needles, depending on the species. There is one flush of growth each year, at the terminal ends of the shoots. You won’t see new growth on the inner stems.
If you want to prune pines, do so as the new growth starts to emerge by pinching out 1/3 to 1/2 of the candle. Do not simply cut back the branches, because they do not regrow needles.
Spruce (Picea) and Firs (Abies)
Spruce trees tend to naturally form a pleasing shape and should need very little pruning. Their needles have what is called a “peg” where they join the branch. The pegs remain on the branches even after the needles drop, which is why spruce branches feel so rough and bumpy.
If you’d like to prune your spruce trees, to enhance their conical shape, do so in the spring, just after new growth has started. Trees grown for Christmas trees are simply sheared, rather than pruning individual branches. It’s not the best method, but it works.
Since spruce trees can be multi-branched, you can also prune back to a lateral branch or a bud.
Arborvitae is one of the evergreens that can handle heavy pruning. There are buds in the crotches between branches that will develop new growth. However old shrubs tend to get a dead zone in the center that loses these buds. Be careful not to prune too far into older arborvitae.
You can prune arborvitae in early spring or mid-summer, but heavy pruning is best done in early spring so that new growth can fill in.
Junipers can have either scale-like needles or scratchy needles, which makes them an extremely unpleasant plant to prune. Like arborvitae, they get a dead zone in their centers, from lack of sunlight. Don’t prune past green growth, or you will not get new growth filling in.
Prune junipers in early spring, before the new growth even starts. Shearing leaves junipers looking unnatural. It’s better to use hand pruners and slowly make cuts in the branches, stepping back to see how the plant looks. Many junipers have lateral branches, so you can cut back to one of those and maintain a somewhat natural look.
Yews are arguably the most forgiving evergreens to prune. They will regrow from old, woody branches. It takes a while for new growth to fully fill in, but yews are very long-lived plants.
You can shear or prune yews. Wait until the new growth has almost changed from bright chartreuse to a darker green. Shearing makes for quick work, but be prepared for new errant branches to pop up. Yews put on two flushes of growth each year.
If You Plan to Trim Your Conifers
It’s best to research the species of conifer you plan on pruning, before starting. You don’t want to do something you can’t correct—and pruning is almost never correctable. Whatever you do will permanently affect the tree. But as mentioned, your best bet is to choose an evergreen that naturally has the size and shape you are looking for, so you can let it grow undisturbed.