How to trim arborvitae
How and when to prune arborvitae (thuja) to keep them beautiful and healthy all year round
Arborvitae, also known as thuja, are very popular evergreen conifers, which growers use in their gardens in the form of hedges, as individual solitaries, or in combined plantings with other trees or shrubs. The most popular arborvitae species are emerald green and green giant. They can also be grown in their natural form, but to maintain them as hedges or shaped forms, arborvitae require regular pruning to create the nice, dense appearance they're known for, and to protect them from damage caused by heavy snow during the winter.
An arborvitae hedge is a very popular way to create privacy in your garden or to separate your property from the street or the neighbours. Arborvitae trees make a dense, fast-growing and evergreen hedge, making them an ideal solution for many gardens. Proper trimming of arborvitae will not only help them maintain the perfect hedge appearance but also improve the health, strength and resilience of the thujas against the elements. With a little care, your arborvitae will always be green and dense from top to bottom, just the way growers would like them to be.
When to trim arborvitae
Many gardeners have no idea when to prune arborvitae, and so they often opt for early spring pruning, along with their deciduous trees. However, pruning conifers during the dormant season isn't ideal. You can prune at any time during the growing season, but the best time to trim arborvitae is from late summer to early autumn. An alternative is to prune during late spring, after the new spring shoots have already matured. If the arborvitae are young and growing rapidly, you can prune them twice a year. In any case, you should avoid pruning your thujas during the hottest days of summer and during the dormant period.
Start pruning your arborvitae when they are still young
A common mistake when growing arborvitae as hedges is not pruning them until they reach a certain height and width, but this will actually prevent them from being as thick and strong as you would like. Pruning should therefore start as soon as the arborvitae have taken root after planting, once they're showing new growth. It is important to trim thujas only very lightly at first, cutting new shoots that are an inch (2.5 cm) or more in length. Such a cut will encourage the plant to sprout more lateral shoots, creating a denser growth form. It is always advisable to trim back young arborvitae a little and let it grow back, repeating this cycle until it has grown to the desired size. One to two trimmings a year are needed to achieve a dense form.
How to trim arborvitae
In order to produce green needles, the arborvitae conifer needs light, so the wider the top of the plant, the less light will reach the lower parts of the plant, which will produce more woody branches without needles. It is therefore necessary to prune arborvitae from a young age so that the lower part is wider and gradually tapers upwards. A very slight (5-10°) slope is sufficient to ensure that the whole plant gets enough light, but it should start right from the lowest parts of the plant. A long stick can be used as a pruning aid by leaning it against the hedge at the desired angle and trimming off any branches that extend beyond the stick. A common mistake when pruning arborvitae is to trim the sides perpendicularly and to use tapering only on the top of the plants.
If you follow this pruning principle, the arborvitae hedge will have healthy, green growth down to ground level for as long as possible. The plant will have more horizontal than vertical branches pointing upwards. This form of pruning, with a wider base and narrower top, will also ensure that snow and ice will slide off the hedge more quickly without damaging the plants, so it is also a form of protection.
Arborvitae hedge pruning rules
• Start trimming from the bottom edge of the hedge, as there are usually fewer new growths at the bottom, and use the bottom to determine the upward angle of the hedge (see principle above). Ideally, trim the plants from more than one direction (bottom-up, side-to-side and top-down). This will ensure a full, healthy and dense hedge appearance without long shoots.
• Avoid trimming too far. To achieve the classic green brush effect (no protruding branches, but no areas without green needles), never cut arborvitae to the brown parts without needles, as they will never turn green again. This is another reason to start pruning your hedges while they are still young, because if you let them grow without maintenance, it is almost impossible to prune them later without creating bare spots.
• The upper part of the arborvitae hedge always grows faster, so it is natural to prune more of it than the lower part. This ensures that more nutrients, water and light will reach the lower parts of the plants.
• The top part of the arborvitae hedge should be trimmed last (after the sides have been trimmed). To create an even arborvitae hedge with the same height, you can use a long stick marked at the desired height, and by placing it against the hedge from the ground, this tool will determine the desired height of the hedge.
Tools for pruning arborvitae
A useful tool for pruning arborvitae is a hedge trimmer. They come in electric or battery-powered models, with varying blade and handle lengths, so you should be able to find one to suit your needs. Hedge trimmers with extended handles are suitable for trimming taller hedges. Manual hedge shears can also be used as an additional tool for maintaining smaller hedge areas.
Author: Daniel Weber