Many landscape designs include planting arborvitae because of their easy growing nature, low-maintenance needs, and overall good health. Once planted, they are sometimes left to grow into a natural shape or trimmed to maintain a neat, tidy appearance.
If you will be trimming arborvitae trees, it’s essential to learn the right way, avoiding the common mistakes people often make.
When to Prune Thuja Trees
Timing when to prune is as vital.
When working with a living plant specimen, you must consider their growing habits when performing basic care. It’s best to trim thuja trees between early spring and mid-summer; the specific timing depends upon how hard you need to cut back its foliage.
- To maintain an ornamental shape, remove overgrowth in the spring and then wait until mid-summer to fine-tune the form. This timing allows the arborvitae an opportunity to put on new growth before winter.
- If you need to prune your plant heavily, do it before it breaks dormancy, and active growth begins in the early spring. Once spring temperatures rise, the new growth will cover pruning cuts, making them less visible.
- To shorten hedges, wait until they finish flowering in late spring or early summer to tackle the project. Make sure they are cut back before mid-summer to give them recovery time before temps drop in the fall.
When it comes to trimming arborvitae, there isn’t a need for fancy, expensive tools. Basic cutting tools will suffice depending upon the size of the branches. As always, wear appropriate personal protective equipment too.
- Hand pruning shears for branches less than ½” in diameter.
- Lopping shears for branches ½ to 1” in thickness.
- Pruning or bow saw for branches thicker than 1” in diameter.
- Safety glasses to protect your eyes.
Make sure to sharpen your tools are regularly, so the blades cut quickly and smoothly through the branches. Clean pruning cuts — i.e., those without ragged or torn edges — are critical to maintaining healthy plant tissue. Jagged cuts increase the chance of pest or fungal infection.
It’s recommended to clean the blades on your tools after every cut. Realistically, this becomes very time consuming, and most people forego the recommendation. At the bare minimum, always make sure tools are sharp and sterilized before starting pruning and after you finish. This prevents the spread of disease between landscape plants.
How to Trim Thuja Trees
To trim arborvitae properly, trim the terminal tips of the foliage. Make sure not to remove more than 30% of the tree’s foliage. Also make sure to only cut into green, young wood. Work in sections as you cut your tree to make less mistakes. Avoid cutting horizontal branches of your tree.
Arborvitaes are conifers, bearing needles instead of leaves; these fan-like spreads of needles only grow from the buds on the ends of new, green wood. Their growth habit makes it essential that pruning cuts aren’t too far back on the branches, removing the buds. If you cut into the old wood, new growth won’t occur.
If your thuja trees need severely pruned, cut off one foot of growth yearly from the branches’ ends and the top. It slows the process down but ensures you only cut into the young, “green” wood and not the old.
Thuja trees are continually putting on new growth. To keep them from looking overgrown and help maintain their health, you may need to prune them back periodically.
Trimming arborvitae annually or every two to three years, depending upon how manicured you want the tree’s form.
Using your cutting tools, remove vertical shoots that shade lower branches, leaving three leaders per branch. Remove any branches that grow into the center to allow sunlight to filter into the plant’s middle and improve air circulation.
Take off any clubbed growth; these are the twigs appearing as v-shaped shoots at ends of limbs.
Removing Dead Portions
Arborvitae trees are quite resistant to pests and diseases, which is why they are so popular. When a dead or diseased spot does appear, it’s best to prune it out quickly.
Prune dead or diseased branches back to healthy wood. Start by cutting slightly below where the needles are dead; continue moving down the branch, making cuts, until you reach green growth. In extreme cases, remove the branch altogether, cutting it where it comes off the trunk. Discard the cut branches away from other trees or plants; never put diseased branches in a compost bin.
Topping off Arborvitae
The tops of arborvitae trees are often cut off when the height becomes problematic, or people want to prune the tree into a hedge. Removing the top creates a very flat appearance since the shrub will no longer grow upwards after the central leader is removed.
Avoid taking the top off when trimming arborvitae if you are only thinning your tree. Only prune it off if it is severely damaged.
How you approach trimming arborvitae depends on the type and the look you want to create or maintain.
Regardless of the variety, make sure you thin the tree/shrub periodically. This encourages new, dense growth and allows light to penetrate into the lower branches, encourages air movement through the center to reduce disease and pest problems, and prevents ice and snow buildup that causes damage in the winter.
Periodically take a step back and look at your progress. This allows you to see the overall shape better.
Shape shrubs to keep them at the height and form you want. Globe varieties will naturally grow fuller through the center to keep their rounded appearance. A regular trim does help to keep them looking tidy.
Trim the tips off of all the branches to create an even shape. Keep the base or middle slightly wider than the upper part of the shrubs.
With tall arborvitaes, the goal is to maintain their iconic pyramidal shape. Keep the bottom of the plant wider than the top for light and air penetration.
If you are using a ladder to work with really tall plants, wear sturdy boots to secure your footing and exercise extra care to prevent falls.
7 Common Mistakes
- Cutting into the old wood. This creates bare spots that may take numerous seasons to fill in. Only cut into the actively growing branches so that new growth may occur.
- Pruning too much at any given time. Do not remove more than 25-35% of the foliage, cutting back no more than a foot off the branches.
- Cutting horizontal branches. These are integral to the overall shape and structure. Only prune them if they are dead or diseased.
- Making cuts too close to a bud. Try to clip ½’ above the bud. If you cut too close, the bud may die. If you cut too far away, the tip of the branch might die.
- Pruning around the tree instead of working in sections. Working in sections leads to fewer mistakes and keeps your work from being uneven.
- Working with dirty pruning shears. Sterilize your pruning shears before and after pruning to prevent the spread of disease between plants.
- Taking the top off if you want the trees to grow taller. Only cut the top if you want to stop upward growth and maintain a hedge. They will not develop a new leader like deciduous species.