American Arborvitae Bonsai Care

Thuja occidentalis

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Zones 3 to 7, outdoor. The native arborvitae of eastern Canada and the northern United States. Most are lost to overwatering.

Cupressaceae (the cypress family) · Evergreen conifer; outdoor bonsai

Cultivars with their own guide: Fire Chief Arborvitae, Gold Arborvitae.

American Arborvitae quick reference

USDA zones	3 to 7 [s3] [s4] [s5]
Indoor / outdoor	Outdoor, in a sunny place; shade over midday in the hottest weeks [s12]
Light	Full sun to part shade; in full shade the foliage thins out dramatically [s3]
Water	Keep the root ball evenly moist; it must not dry out, and it needs free drainage [s3] [s12]
Soil	Moist and well drained; wild stands grow at pH 5.5 to 7.2, often on limestone [s1] [s3]
Repot	Every 2 to 3 years in early spring [s12]
Frost	Hardy to zone 3 in the ground [s3]; in a bonsai pot it needs protection from strong frost [s12]
Mature height	40 to 60 ft (12 to 18 m) tall and 10 to 15 ft wide in the landscape [s3] [s4]
Growth rate	Slow [s3] [s5] [s8]; the Morton Arboretum rates it moderate [s4]
Season of interest	Year-round scale foliage; bronzes in winter [s3]
Leaf reduction	Not rated by the sources we checked
#1 cause of death	Root rot on poorly drained, wet sites [s10]; in a pot, a mix that stays wet carries the same risk

Overview & Character

American arborvitae is the native arborvitae of eastern Canada and the northern United States [s1] [s2], a slow-growing evergreen of cedar swamps and limestone uplands [s1]. Many dwarf and golden cultivars are selections of this species, among them 'Congabe' (sold as Fire Chief), 'Golden Globe', 'Danica', 'Bobozam' (sold as Mr. Bowling Ball) [s4] and 'Rheingold' [s5]. More than 120 ornamental cultivars exist, differing in foliage color and growth habit [s1].

The plant is toxic: its foliage and cones contain thujone, and eating them causes stomach pain, vomiting and diarrhea in people and poisons animals [s13] (see Toxicity).

Thujas are not often used for bonsai [s12], but the species is workable. Pinching the new shoots while they are still soft builds ramification [s12]. The trade-offs are that the foliage fans tend to grow too large for the tree's scale, so trimming never stops [s22], and that pruning has to leave green foliage on every branch (see Pruning). Treat it as a hardy outdoor conifer that wants steady moisture [s12].

Identification

Foliage. Scale-like leaves about 2 mm long in flat, fan-like sprays [s9]. The upper surface is darker green than the paler underside, and the foliage turns bronze or yellow-brown in winter [s3] [s9]. Crushed foliage has a sweet, tansy-like scent [s5] [s15].

Bark & trunk. Grayish brown to reddish-brown bark with shallow furrows, peeling on the trunk and branches of mature trees [s3]; gray on the surface and reddish-brown in the furrows [s15].

Growth habit / form. A dense, pyramidal tree in the landscape [s23]; many cultivars are globes or cones [s4].

Cones / fruit / seeds. Oblong cones 8 to 13 mm long, pale cinnamon brown when ripe [s1], with 4 to 5 pairs of scales [s14]. In northern Michigan they ripen from August to September [s1].

Look-alikes & how to tell them apart: Oriental arborvitae (Platycladus orientalis, often still labeled Thuja orientalis) holds its sprays vertically, has leaves nearly alike on both sides, and bears fleshy, blue cones 1.5 to 2.5 cm long with thick, hooked scales [s17]. Western redcedar (Thuja plicata) has shiny dark green leaves with white "butterfly" markings underneath [s18]. Sawara cypress (Chamaecyparis pisifera) shows distinct white markings below and small, round cones about 6 mm wide [s20]. Eastern redcedar (Juniperus virginiana) often carries sharp needle leaves on the same branch as scale leaves, and dark blue, berry-like cones about 6 mm wide [s19]. American arborvitae's leaves are simply paler green below [s9], and its cones are brown and oblong [s1].

Climate & USDA Zone

American arborvitae is hardy in USDA zones 3 to 7 [s3] [s4] [s5], and some sources extend it to zone 2 [s8] [s9]. In the wild it grows best on neutral to slightly alkaline soils derived from limestone that are moist but well drained [s1]. It does not tolerate dry, exposed, windy sites, where the foliage winter-burns yellow-brown [s3]. In a bonsai container it needs protection from strong frost, and it needs more light during that winter protection than other conifers [s12]. Snow and ice can break its limbs or leave stems leaning [s1].

Light

Give it full sun to part shade [s3] [s4]. In full shade the foliage thins out dramatically [s3], and even partial shade can leave it thin, open and much less appealing [s15]. In the hottest weeks of summer, shade it over midday and early afternoon [s12].

Watering

Moist, never dry, never waterlogged. The root ball should always be slightly moist and must not dry out [s12]. In the landscape it does best in consistently moist, well-drained soil [s3] [s16], and it does not develop well on extremely wet or extremely dry sites [s1]. In a pot the risk runs both ways: the root ball must not dry out [s12], and root rot is common on poorly drained, wet sites [s10], so a pot that stays wet is a risk.

Soil & Root Care

Use a well-draining bonsai mix such as akadama, pumice and Kiryu [s12]. Wild stands grow on soils from pH 5.5 to 7.2 [s1]; the bonsai guidance is neutral or slightly acidic water and soil at pH 6 to 7.5 [s12]. Repot every 2 to 3 years in early spring with normal root pruning [s12]. Give it deep, well-drained conditions and correct any drainage that lets water stand around the roots [s10].

Feeding

Feed through the growing season: a solid organic fertilizer every month, a liquid fertilizer once a week, or both [s12].

Pruning & Styling

Keep green foliage on every branch you cut back. Extension sources agree that arborvitae does not regrow foliage on bare, leafless wood: once the foliage is gone from a branch, new growth will not come back there [s5] [s6]. The University of Missouri adds that an overgrown arborvitae cannot be cut back more than 20 percent [s7]. A bonsai reference says thujas can be pruned back into the old wood and will bud again [s12], but it is written for the whole genus and conflicts with the extension advice, so the careful rule is the extension one.

Pinch the new shoots while they are still soft to build ramification [s12]. The plant tolerates shearing [s4] but does not recover from severe pruning [s8], and shearing thickens an outer shell of foliage that shades out the interior [s6]. The bonsai reference gives late autumn for heavy pruning [s12]; the extension guidance for random-branched conifers such as arborvitae is early spring, when new growth will cover the cuts [s21]. Branches can be wired all year round [s12].

Propagation & Cuttings

Ease: Cuttings root readily [s24]. Methods: seed or cuttings [s12]; cultivars by cuttings [s1].

Cultivars are propagated by cuttings [s1]. Semi-hardwood and hardwood cuttings both work, and late fall or early winter is a good time to root conifers [s11]. The seed has only slight dormancy [s1], fall sowing is preferred, and whether it needs stratification is not clear [s14]. Seedlings grow slowly, only about 3 in (8 cm) a year at first [s1].

Pests

Leafminers, which scorch the foliage, are among its principal insect pests, and bagworm, juniper scale and spruce spider mite can significantly damage ornamental plants [s1]. Mealybugs and other scales are occasional visitors [s3]. On bonsai, cypress tip miners and Japanese beetles are also reported [s12]. Outdoors, deer find it highly palatable and often overbrowse it [s2].

Disease, Fungus & Infection

Root rot shows up often on poorly drained, wet sites: the foliage discolors and the plant eventually dies [s10]. Tip and shoot blight (Phomopsis juniperovora) attacks foliage and shoots in humid weather [s1]. Cypress canker and thuja blight are also reported on bonsai [s12]. Winter burn from drying wind and sun browns exposed foliage [s3] [s23].

Most Common Causes of Death

The ways these trees are most often lost, in no particular order:

Root rot from poor drainage. Use a free-draining mix [s12] and correct standing water [s10].
Drying out completely. The root ball must not dry out [s12].
Cutting back to bare wood. A branch stripped of its foliage stays bare [s5] [s6].
Hard frost on the pot, or drying winter wind. Protect the container from strong frost and the foliage from exposed, windy sites [s3] [s12].
Too much shade. Even partial shade leaves it thin and open [s15].

Seasonal Calendar

Spring: Repot every 2 to 3 years as growth starts [s12]; structural pruning, leaving green on every branch [s5] [s21]; feeding begins with the growing season [s12].
Summer: Pinch soft shoots; keep the root ball moist; shade over midday in the hottest weeks [s12].
Autumn: Heavier pruning if needed [s12].
Winter: Protect the pot from strong frost, in good light [s12]; winter bronzing is normal [s3].

Toxicity, Safety & Handling

Thuja branches and cones contain essential oils with thujone. Eating parts of the plant causes abdominal pain, sometimes bloody vomiting, and diarrhea, and the plant is toxic to animals, which should not be fed it [s13].

Sources

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