When to Prune Thuja? Timing and the Most Common Mistakes

Step by step

1. 1

   First cut in early spring

   Do the main shaping cut in early spring, usually in April, before new growth starts. Remove frost-damaged and broken tips and set the shape. Pick an overcast day, because cutting in full sun and drought encourages the wounds to brown.

2. 2

   Touch-up in June

   After the first spring flush, around June, do a correction. Even out the fresh, light green tips that have grown beyond the desired outline. This trim thickens the plant, because it pushes the side buds to branch out.

3. 3

   Last cut by the end of August

   The last safe date is the end of August. Trim later and the thuja will push out young growth that cannot harden before the first frosts - it will freeze and turn brown. If you miss the window, put the shaping off until early spring.

4. 4

   Never cut into old wood

   This is the most important rule with thuja. Cut only within the green, foliage-covered part of the shoot. If you cut a branch back to the brown, leafless wood deep in the crown, a permanent bald hole remains, because thuja does not regrow from old wood.

5. 5

   Shape hedges into a trapezoid

   Trim a hedge wall so it is wider at the bottom and narrower at the top, in a trapezoid shape. Sunlight then reaches the lower branches too, and they do not go bare at the base. A straight vertical wall makes the bottom of the hedge brown over time.

6. 6

   Check for nests

   In spring and early summer, birds love nesting in dense thujas. Before trimming, look deep into the crown and postpone the job if you find an occupied nest. It is safer to schedule heavier pruning outside the peak breeding season.

7. 7

   Water after trimming

   Thujas have a shallow root system and cope poorly with drought, especially after trimming. Water the plants generously after the job, and repeat every few days in hot weather. A bark mulch helps keep moisture at the roots.

Reducing the height and the Smaragd variety

Lowering a thuja is trickier than narrowing its sides. If you cut the top down into old wood, the top stays flat and brown, because no new shoots emerge there. Reduce the height gradually, cutting while still in the green zone and accepting a slightly flattened tip.

The popular Smaragd variety (Thuja occidentalis Smaragd, also sold as Emerald) grows slowly and forms a naturally narrow cone, so it needs little pruning. Evening out the sides once a year and correcting the height if needed is usually enough. There is no call for heavy shaping if the thujas were planted in a row with proper spacing.

How late you can trim and what to avoid

Thujas take pruning best from early spring to the end of August. Avoid trimming in frost, in heat and in harsh, full sun, because the fresh tips brown in those conditions. The ideal is an overcast, windless day, with the soil watered beforehand.

Use sharp, clean shears or pruners. Blunt tools crush the shoots instead of cutting them, and crushed tips turn brown and become a gateway for disease. Hedge shears work well on larger hedges, but always make sure you stay out of the leafless wood.

How to thicken a see-through hedge

If a thuja hedge is see-through, regular light trimming of the tips stimulates the side buds and thickens it over time - as long as you cut in the green zone. Do not count on bald patches in old wood filling in, though, because those will never grow back.

Shape young thujas from the start, lightly trimming the new growth every year, instead of waiting for them to grow and then cutting hard. A plant trained from a young age keeps a dense, even shape far more easily. Mind the spacing at planting time too, so every thuja gets its share of light.

Frequently asked questions