Thuja

Light and exposure

Thuja grows equally well in full sun or partial shade, which makes it one of the few conifers that copes with a north-facing spot. It gives its best foliage, dense and full from top to bottom, once it gets at least 4 hours of direct sun a day.

In a hedge facing north or squeezed between two walls, the shadier side tends to thin out at the base over the years, something many gardeners in the region notice after 8 to 10 years. Spacing the plants a little further apart at planting, or choosing a less shaded spot, avoids this thinning.

Watering

A mature, well-rooted thuja copes very well with summer drought and is happy with Belgian rainfall the rest of the year. The real risk lies elsewhere: during the first 2 summers after planting, the root system is not yet developed enough to reach water deep down, and a dry summer easily scorches, then kills, a whole hedge planted the previous spring.

So water generously, once or twice a week, for young plants during their first 2 summers, especially on sandy soil that drains fast. Running a hose at the base for an hour, rather than a quick pass with a watering can, encourages the roots to grow down deep instead of staying near the surface.

Soil and repotting

Thuja adapts to most garden soils, from the loam typical of the Herve region to heavier clay, as long as water does not sit around the roots for long. A soil that stays cool and slightly moist suits it better than a dry, poor one, but the opposite excess is more dangerous than the shortfall.

On compacted clay soil, or in a low spot where water collects after the autumn rains, it is worth loosening the ground deeply and raising the planting line slightly. Permanently waterlogged soil opens the door to phytophthora, a root rot that browns and kills plants one by one along the hedge.

Temperature and humidity

This is a non-issue in our region: thuja is fully hardy and shrugs off winters in the Herve region without damage, late frosts included. It is in fact planted precisely for this toughness, in spots where more tender conifers would struggle.

It is, however, entirely normal to see the foliage turn an orange-brown or bronze shade in winter, especially on the plants most exposed to wind and frost. This is a natural protective response, not a sign of disease, and the green returns as soon as spring arrives. Only browning that persists after growth resumes in April should raise concern.

Pruning and upkeep

A thuja hedge is trimmed twice a year to stay dense and neat: once at the end of May or in June, once the spring flush is finished, and again at the end of August, before the wood hardens for winter. A hedge trimmed only once a year opens up and thins out faster.

The rule that shows no mercy: never cut into brown wood with no green needles left. Unlike hornbeam or beech, thuja has no dormant buds on old wood, so it never regrows after a cut that goes too far, the branch stays bare and dead for good. Always trim so that a little green foliage remains on every shoot.

Pests and diseases

The thuja bark beetle mainly attacks plants already weakened by lack of water or overly compacted soil, boring galleries under the bark that brown, then kill, whole branches. A vigorous, well-watered thuja resists it far better and is rarely troubled.

Phytophthora, encouraged by permanently waterlogged soil, causes browning that starts at the base and works upward, often one plant at a time along the hedge, which sets it apart from simple drought stress by its steady progression. Summer dieback, on the other hand, mainly affects young, unwatered hedges during heatwaves, with widespread, rapid scorching rather than localised browning.

Propagation

Cuttings are by far the most common way to propagate thuja, including in nurseries. In autumn or winter, take a semi-ripe side shoot about 15 centimetres long, ideally with a small heel of bark at the base, and plant it in a well-draining mix of potting soil and sand. Rooting is slow, expect several months before new growth appears.

Sowing from seed is possible but gives uneven plants that take much longer to reach a usable hedge size. It suits curious hobby growers more than anyone wanting a hedge quickly. For a new hedge, it is better to buy container-grown plants already shaped than to attempt propagation yourself.

Month-by-month care calendar

Green Hub tips

• Never cut into brown wood with no green needles: unlike hornbeam or beech, thuja does not regrow from old wood, and the branch stays bare for good.
• Water thoroughly for the first 2 summers after planting. A thuja hedge that scorches suddenly in summer is almost always a case of roots not yet settled in, not disease.
• Foliage turning bronze in winter is not a disease, it is a natural response to cold and wind. The green comes back with the warmer days.
• Space plants 60 to 80 centimetres apart at planting. A hedge planted too tightly from the start chokes itself and thins out at the base faster.