How to prune Thuja occidentalis columnar: training and care

Why prune Thuja occidentalis columnar?

Thuja occidentalis cultivars like 'Emerald Green' or 'Degroot's Spire' are valued for their slender columnar form and fast growth. They reach 15 metres tall but stay only 2-3 metres wide. This makes them ideal for hedging, privacy screens, or formal gardens.

But without pruning they spread wide and lose their neat shape. The front becomes thick and woolly, sides grow out crookedly. With patient pruning you keep them compact, densely foliaged, and architecturally clean. The advantage: Thuja grows quickly and responds extremely well to cutting.

Growth pattern of Thuja occidentalis

Thuja occidentalis is a conifer with feather-like, scaly foliage. It grows upward very rapidly - sometimes 30-50 cm per year. The natural form is columnar, but without pruning the sides spread out. This happens especially on older plants and in shade.

Healthy Thuja:

• Grows straight up
• Has dense foliage from bottom to top
• Does not brown out internally
• Responds well to pruning

Struggling Thuja:

• Grows crooked or spirals
• Becomes hollow inside
• Develops brown patches
• Responds poorly to cutting

When to prune Thuja occidentalis

Thuja pruning happens best twice yearly:

• April-May: First pruning. Winter is past, tree grows fast. This is the main pruning window.
• August-September: Second pruning. Control summer growth. Gentle cutting without major wounds.
• October-March: Minimal pruning. Tree growth is slow. Large cuts heal slowly.

How to prune Thuja columnar

Basic inspection

Walk along your Thuja. Look for:

• Brown or dry patches (usually inside)
• Branches growing at odd angles
• Bird droppings (sign of insects)
• Lopsided thick areas

Hedge shear pruning

Pruning Thuja with hedge shears is fast and efficient. This is preferred for hedging and columnar forms.

Procedure:

1. Draw the shears in one smooth motion from bottom to top
2. Cut all branches back roughly 10-15 cm from your intended shape
3. Work from one side to the other, checking regularly with your eye

A well-sharpened hedge shear makes work far easier. Dull tools will frustrate you.

Hand pruning for detail

For finer work (especially shaping young plants) use secateurs:

1. Snip individual side shoots growing out of line
2. Work from bottom to top
3. Step back regularly to check symmetry

Control height

If your Thuja grows above bounds:

• Carefully cut the apex back, no more than 20-30 cm
• This encourages side branching instead of continued upward growth
• Next year the top will develop more branches

Build fullness

After a few seasons the interior of your Thuja can become open. Prevent this by:

• Not cutting interior side shoots completely away
• Leaving more branches on the sides than on the outside
• This builds dense silhouette over time

Troubleshooting issues

Brown patches in the interior

This happens when branches lose light. Once brown, they do not regrow. This is normal in dense hedging. It mostly stays invisible from outside.

Solution: Check if insects are the cause (aphids). Treat with insecticide if needed. Otherwise, accept this as part of normal hedge life.

Crooked growth or wavy form

A Thuja that does not grow straight can be corrected by gently cutting back asymmetrical side shoots. It takes a few years but gradually it straightens.

Growth favoring one side

This can happen from:

• Too much shade on one side
• Wind blowing from one direction
• Poor water reach on one side

Solution: Cut the strong side back gently, let the weak side grow more. Over two seasons it balances.

Step-by-step

Step 1: Inspect

Walk along your Thuja. Check for asymmetry, brown patches, and untidy branches.

Step 2: Prepare hedge shears

Sharpen your tool well. Check blades are parallel.

Step 3: Start from bottom

Cut carefully from bottom to top, with smooth strokes. Leave 10-15 cm extra for safety.

Step 4: Finish side faces

Work the left and right sides with the same technique.

Step 5: Check and finish top

Check the apex for symmetry. Trim the top carefully if needed.

Step 6: Step back and inspect

Assess your work from a distance. Does the form look straight and dense?

Seasonal tips

April-May: Growth starts. Wounds heal fast. Ideal for significant reshaping.

June-July: Heat. Ensure your Thuja gets adequate water after pruning.

August: Second pruning can begin. More cautious, less aggressive.

September: Final pruning. Less growth, so conservative cuts.

October-March: No pruning unless essential. Growth is slow, wounds heal slowly.

Frequently asked questions

How fast does Thuja occidentalis really grow?

Average 30-50 cm per year in height. In width they grow 10-20 cm per side yearly. This is fairly rapid for a conifer. In ideal conditions (good water, nutrition) it is faster.

My Thuja develops lots of brown interior - what do I do?

Usually normal background die-off in dense hedging. As long as the outer layer stays green, it is invisible. However, if the entire tree browns, check for insect problems or drainage issues. Thuja dislikes waterlogging.

Can I save an unhealthy hollow Thuja?

Difficult. A Thuja that is completely hollow inside will not fill in again. You can reduce pruning to make the most of remaining growth, but a true rescue is tough. Better to prevent this happening beforehand.

How often must I really prune?

For hedging: twice yearly (May and August). For single columnar plants: once yearly (May) suffices. So if you want your hedge to grow fast, prune less. If you want it compact, prune more frequently.

Can I prune Thuja into deadwood?

No. Thuja does not regrow from dead wood. If you cut into brown wood, that spot will not fill in. Always cut into green wood or the border. This differs importantly from, say, boxwood.

Frequently asked questions

How long does Thuja occidentalis live?

Healthy Thuja can reach 100+ years, especially if well maintained. They are tough and slow-growing once mature.

Can I relocate my Thuja?

If young (first 3 years): yes, carefully. Dig with large root ball, plant quickly, water well. Older than 5 years: risk is high. Better not unless absolutely necessary.

Thuja smell - why so strong?

Essential oil in Thuja foliage. This attracts insects and gives characteristic scent. Nothing wrong - it is natural. Some find it pleasant, others do not.

Discover your own garden design

At [gardenworld.app](https://gardenworld.app) you can upload your front yard and see how Thuja columnar fits - as a hedge, privacy screen, or accent plant. Plan your landscape before you start pruning.