How to grow western red cedar (Thuja plicata)

Identifying Thuja plicata

The leaves of Thuja plicata are scale-like, borne in flat sprays, and give off an aromatic, pineapple-like scent when crushed. Allowed to grow naturally, western red cedar forms a tall, conical-shaped tree. The branches are slightly drooping and made up of mid-green, scale-like leaves. Clusters of small flattened oval brown cones, up to 1.3cm long, are borne in summer and autumn.

Size, height and spread

Thuja plicata and Thuja plicata ‘Atrovirens’ have moderately fast growth of 45-60cm a year. Regular trimming keeps a western red cedar hedge compact, restricting growth to as little as 1.5-2m high. Left untrimmed it forms a tall-growing and broadly columnar tree with a mature height of around 35m with a spread of around 7-10m. The oldest known western red cedar tree grows in the US and is over 60 metres tall and 15 metres wide. Compact-growing varieties are slower-growing and much smaller, typically reaching around 2.5m high and 1.5m wide.

Value to wildlife

Western red cedar hedges and mature trees provide good nesting sites for birds and shelter for wildlife.

How to grow Thuja plicata

Grow western red cedar in sun or shade in good, moisture-retentive soil. Ideally plant in autumn or early spring and water during dry spells during the first two growing seasons. Only trim if growing for hedging – up to three times a year to maintain a neat and well-shaped hedge.

Where to grow Thuja plicata

Thuja plicata and its varieties thrive in fertile, moisture-retentive soil. Avoid waterlogged or dry ground. Site in sun or shade, avoiding exposed sites as cold winds may scorch the foliage. Site according to the variety size and intended use.

How to plant Thuja plicata

Plant into well-prepared soil, preferably during autumn to establish before the next growing season. Space thuja hedge plants 60-75cm apart. When planting Thuja plicata ‘Atrovirens’ as a boundary hedge, place at least a metre away from the property boundary to allow space for the hedge to grow and to give access for trimming. Plant a hedge at least five metres away from a house (for a hedge that will be kept regularly trimmed to a compact size), and clear of drainage pipes. Larger hedges should be further away from a house.

How to care for Thuja plicata

For the first two years after planting, water during dry spells, though take care not to over-water as thuja suffers in wet ground. Protect plants from cold, drying winds.

Thereafter Thuja plicata needs little care. Annual mulching of the root area using an organic mulch keeps the soil in good condition and protects the roots from extremes of temperature.

How to prune Thuja plicata

Use a hedge trimmer or shears to trim the top and the sides of a hedge, two to three times during the growing season. Ideally avoid pruning during the spring-early summer nesting season, but if you need to prune during this time, check for nesting birds beforehand.

Avoid trimming in bright sun which may cause foliage to scorch, as the freshly cut growth is sensitive to bright light and temperature. Slope the sides so the hedge is wider at the base than at the top to give even light distribution and shed excess rain or snow. Only hard prune the top of a hedge and cut around a third to a half of the growth from the sides.

When pruning or handling the cut foliage, wear gloves and clothing that covers arms and legs as contact may aggravate skin allergies.

Pests and diseases of Thuja plicata

Thuja blight is caused by a fungal disease and may affect western red cedar, particularly in enclosed or overgrown places with poor air circulation. Affected young trees may die completely and foliage of older ones browns and dies in patches. Symptoms of yellowing, then browning and dead foliage, appear in late spring and summer. If the disease appears, cut out and dispose of affected foliage, and collect up fallen dead leaves that are likely to carry the infection.

Conifer aphid and scale insect may cause mottling or yellowing leaves. Black sooty mould may be the first obvious sign of these insects: the mould grows on the sticky honeydew that they produce. Healthy trees are usually able to withstand attack. Nature-friendly gardening encourages the natural predators of insects and maintains a natural balance.

Varieties of Thuja plicata to grow

Thuja plicata ‘Atrovirens’ – used for hedging, this may be sold either under the common name of western red cedar or its botanical name. Height x Spread: 35m x 1m (untrimmed).

• Buy Thuja plicata ‘Atrovirens’ from Crocus

Thuja plicata ‘Can-Can’ – this semi-dwarf variety has a slow growth habit and dark green foliage. Perfect for growing as a specimen tree or as part of a slow-growing hedge. H x S: 2.5m x 1m (untrimmed).

• Buy Thuja plicata ‘Can-Can’ from Dobies

Thuja plicata ‘Goldy’ – golden foliage turns orange in winter, providing colour when little else is available. H x S: 6m x 1m (untrimmed).

• Buy Thuja plicata ‘Goldy’ from Primrose

Thuja plicata ‘Stoneham Gold’ – dark green inner foliage and golden yellow outer foliage make this a fantastic specimen tree. Slow growing. H x S: 1.8m x 90cm (untrimmed).

• Buy Thuja plicata ‘Stoneham Gold’ from Ashwood Nurseries

Thuja plicata ‘Whipcord’ – unusual both in leaf type and overall shape, the foliage has a ‘braided’ appearance, green in summer and developing bronze tints in winter, and the plant forms a low weeping dome shape. Ideal for rockeries. H x S: 1.8m x 90cm (untrimmed).

• Buy Thuja plicata ‘Whipcord’ from Thompson & Morgan