Repotting guide

When & how to repot Japanese Arborvitae (Thuja standishii)

Also called Japanese Arborvitae, Standish's Arborvitae.

More about japanese arborvitae

About Japanese Arborvitae

Thuja standishii · also called Japanese Arborvitae, Standish's Arborvitae · flowering

Japanese Arborvitae is a graceful, slow-growing conifer native to subalpine forests of Japan's Honshu and Shikoku islands. Its flat, bright green aromatic foliage sprays and broadly pyramidal form make it an elegant specimen tree. Rarely seen in Western cultivation, it prefers cool, moist conditions and well-drained soils, and is one parent of the popular 'Green Giant' hybrid.

Mature size: 6–10 m tall, 2–4 m wide in cultivation; up to 18 m in habitat; slow-growing (15–25 cm per year)

Watch for — Root Rot in Wet Soils: Phytophthora cinnamomi and related water moulds cause crown and root rot in waterlogged conditions. Japanese Arborvitae is sensitive to prolonged wet feet. Ensure excellent soil drainage at planting; avoid low-lying areas. Raise planting beds if drainage is poor.

How to tell japanese arborvitae needs repotting

Repotting on a calendar is less reliable than reading the plant. For japanese arborvitae, watch for these signs:

Thick roots out of the drainage holes, or circling the surface and lifting the plant.
The pot dries out unusually fast and japanese arborvitae wilts between waterings it used to shrug off.
The plant is visibly top-heavy and tips over easily.
Stalled growth and small new leaves over a full season — though with a big specimen, top-dressing is often the better first response before a full repot.

For the underlying biology of a pot-bound root system and why it stalls a plant, see our guide to spotting and fixing a root-bound plant.

How often to repot japanese arborvitae

Every 2–3 years; top-dress in the in-between years. Japanese Arborvitae's growth habit — broadly pyramidal to conical evergreen tree; flat, horizontal foliage sprays of bright green, aromatic, scale-like leaves with glaucous whitish markings beneath; fibrous, reddish-brown bark; slender branching structure — sets the pace. Japanese Arborvitae is a graceful, slow-growing conifer native to subalpine forests of Japan's Honshu and Shikoku islands. Its flat, bright green aromatic foliage sprays and broadly pyramidal form make it an elegant specimen tree. Rarely seen in Western cultivation, it prefers cool, moist conditions and well-drained soils, and is one parent of the popular 'Green Giant' hybrid.

What size pot to step japanese arborvitae up to

Move up exactly one pot size. A heavy japanese arborvitae dropped into a vastly bigger pot sits in a reservoir of wet soil its roots cannot reach, which rots them and destabilises the plant. In the years between repots, lift off and replace the top 3–5 cm of soil (top-dressing) instead — it refreshes nutrients without the shock of a full repot.

Not sure of the exact diameter? Our pot size calculator takes the current pot and root spread and tells you the right next size — it deliberately recommends a single step up, never a big jump.

The best time of year to repot japanese arborvitae

Early spring, just as new growth restarts, is the ideal window for japanese arborvitae. The plant is moving into its strongest growth phase and re-roots into fresh soil quickly. Avoid repotting in winter dormancy or, for flowering plants, while it is in bud or bloom — recovery is slowest then and you risk dropping the flowers.

Step-by-step: repotting japanese arborvitae

Consider top-dressing first. If japanese arborvitae is not badly root-bound, scrape off and replace the top 3–5 cm of soil instead — far less shock for a big plant that hates moving.
Get help and one size up. For a full repot, choose a pot just one size larger. A heavy plant needs two people and a stable, free-draining pot.
Ease it out on its side. Lay the plant down, slide the pot off, and gently loosen the outer roots. Do not bare-root a mature specimen.
Repot at the same depth. Add fresh moist, well-drained, slightly acidic loam or sandy loam beneath and around the rootball, keeping the original soil line. Firm it so the trunk is stable and upright.
Water and leave it put. Water thoroughly, then leave japanese arborvitae in the same spot and light — moving and repotting at once is what makes it drop leaves.

Aftercare

Leave japanese arborvitae in exactly the same spot and light it was in before — moving and repotting at the same time is what makes a big specimen drop leaves. Water it in well, then let the top of the soil dry before watering again so the larger volume of fresh soil does not stay sodden. Do not fertilise for about 4 weeks — fresh mix already carries nutrients and feeding freshly disturbed roots scorches them.

The right soil mix for japanese arborvitae

Japanese Arborvitae wants moist, well-drained, slightly acidic loam or sandy loam. Grows best in fertile, humus-rich, slightly acidic soils (pH 5.5–7.0) with consistent moisture. Tolerates a range of well-drained soil types but dislikes compacted, waterlogged, or strongly alkaline conditions. Incorporate organic matter at planting to aid moisture retention and drainage. Always use fresh mix when you repot — reusing old, broken-down soil reintroduces the compaction and poor drainage you are repotting to fix.

Repotting japanese arborvitae — frequently asked questions

How often should you repot japanese arborvitae?

Every 2–3 years; top-dress in the in-between years for japanese arborvitae. Fully repot japanese arborvitae only every 2–3 years; in the in-between years just top-dress the top 3–5 cm of soil. Step up one pot size in spring with moist, well-drained, slightly acidic loam or sandy loam. It is heavy and hates being moved, and a vastly oversized pot holds water against the roots and rots them.

What size pot does japanese arborvitae need?

When is the best time of year to repot japanese arborvitae?

Should you top-dress or fully repot japanese arborvitae?

For a big, heavy japanese arborvitae, top-dressing — replacing the top 3–5 cm of soil — is the gentler option most years, with a full repot only every 2–3 years. A mature specimen sulks and drops leaves when fully repotted, so do it as rarely as the roots allow.

Should you fertilise japanese arborvitae after repotting?

Not immediately. Wait about 4 weeks after repotting japanese arborvitae. Fresh mix already contains nutrients, and feeding freshly cut or disturbed roots burns them. Resume your normal feeding routine once you see new growth.