A Guide to Pruning Boxwoods - How & When

Pruning boxwoods is a job that's easy, and easy to put off. A conundrum indeed. If you've put it off for years, believe it or not, you can bring your boxwoods back to their original charm with one or two terrifying hard prunings.

Over the past 20 years or so since I planted them, these boxwood topiaries on either side of my backyard gate have been getting bigger and bigger and more and more bossy.

Ditto for the boxwood bushes that edge my fence.

I prune them every year but I was always afraid to prune them really hard worrying that if I did I'd end up with a leafless skeleton of a boxwood ball.

There was no need to worry. You can prune boxwood back to a nib practically and it'll bounce back into whatever shape you give it.

Now, with arrival of the dreaded box tree moth in Ontario, which obliterates boxwood I figured now was the time to try a hard pruning because what did I have to lose?

Boxwoods are dropping off the Southern Ontario landscape from the moth larvae.

Box tree moth are moths that lay eggs on boxwood plants, which turn into larvae that eventually defoliate the plant, then start eating the bark. This will kill kill the entire boxwood hedge or plant if it isn't treated.
Mixing 12 ml of BTK in 4 litres of water and spraying the plant foliage and stems a few times a season is the best treatment against the box tree moth larvae.

Signs of Box Tree Moth

Before pruning first check to see if your boxwoods have evidence of box tree moth larvae. You'll see webs on the leaves and if they've been eating you'll also see larvae and completely brown leaves.

If you look at your hedge and it is a dead, crispy mound of brown this is also an excellent sign that you have box tree moth.

Hard Prune - Winter

The best time to hard prune (hack back) is in late winter or early spring while the boxwood is still hibernating, before new growth begins.

The worst time to hard prune is in late summer because new delicate shoots will emerge after pruning and then promptly get killed by the frost.

For successful hard pruning to reshape a boxwood the plant needs to be dormant.

Light Prune - Summer

Regular light pruning can be done throughout the year but the best time for a small trim (just a little off the top to tidy things up) is early summer after the new shoots have emerged.

If you're planning to do major reshaping or cutting back significant portions of the shrub stick to the dormancy period in winter and early spring.

Even if you WANT to do the trimming in the fall because it looks fun and you can't wait and there's nothing left to do in the garden - do not do it.

I felt the same way last year but I forced myself to wait until very late winter in 2024 before I hacked my boxwood topiaries into submission.

Hard Late Winter Prune 2024

Using the hand pruners I had just sharpened in March I took off about ⅓rd of each topiary to decrease their size and fix their scraggy shape.

Before

After

I'll cut them back and shape them once more this winter which should get them back to their original small spherical shape and the same size by next summer.

Shaping The Plant

As you can see I also have a square boxwood hedge along my fence which is trying its best with the help of BT to fight off a box tree moth invasion.

Now that I know how well hard pruning works I'm going to also reduce the size of the hedge this winter.

• For rounded or natural shapes: Use hand shears to take off excess then pruners or loppers to make branch cuts that follow the natural form of the plant.
• For formal hedges or topiaries: I use electric hedge trimmers for getting straight lines on my boxwood hedge. To keep them straight I follow the edge of the fence boards but you can also run a string line.

TIP When you're shaping any tree or hedge it's easy to get too focused on a small section up close. STEP BACK and take a look at your work constantly so you're keeping the overall picture in mind.

Very much like pruning a poodle topiary.

Tips For Proper Pruning Techniques

Proper pruning techniques ensure that your boxwoods remain healthy and vibrant. Here are some tips to follow:

• Use sharp, clean tools: Dull blades will crush the stems instead of cutting them which leads to disease. (learn how to sharpen blades here)
• Prune on dry days: Rain and damp weather equals the spread of fungal diseases. Always prune when the foliage is dry to reduce this risk.
• Make selective cuts: Instead of shearing off large sections at once, make smaller, selective cuts of branches from inside the plant.
• Thinning: Regularly thin out dense sections by cutting back branches all the way to the main stem. This improves air circulation and helps with healthier growth.

To recap

The frequency of pruning depends on the type of boxwood and the look you want. Typically, boxwoods need to be pruned at least once a year. For formal hedges or topiaries like mine, you'll also need to lightly trim a couple of times. 

• Annual pruning: Your big hard prune (if you're trying to reshape or rejuvenate) should be done in late winter or early spring. Only take ⅓rd of the plant off each year until you've reached the size you want.
• Touch-ups: If you want a pleasantly perfect topiary, plan to do light pruning every 6 to 8 weeks during the growing season to maintain shape.

So if you, like I, are worried your boxwood has gotten a little too big and bossy or leggy and languishing fear not. Mark your calendar for late February or March and then you can successfully hack attack it.