Northern white cedar bonsai

Placement

Northern white cedar is grown outdoors year-round: like other temperate trees, it needs to go through the cycle of the seasons, and overprotecting it only weakens it. It loves full sun. Even so, during the hottest weeks of summer, give it shade at midday and in the early afternoon, because it does not cope well with intense heat and scorching sun.

In the ground it is very frost-hardy. In a pot its roots are far more exposed, so protect them from hard frost. Wait for the first frosts before adding any protection, so the tree settles properly into dormancy. Where temperatures often drop below 14 °F, overwinter it in a cold frame or an unheated greenhouse, opening it up on sunny days. In a milder climate, with nights around 25 °F, simply set the pot on the ground, out of the wind, wrapped in insulation if you like. One thing to remember: this thuja needs more light in winter than most conifers, so never store it in a dark shed.

Soil mix

The mix should drain very freely while holding a little moisture. A blend of akadama, pumice and kiryu works well; otherwise the usual conifer mix of one third akadama, one third pumice and one third lava rock also does the job. Northern white cedar prefers a neutral to slightly acidic medium, with a pH of about 6 to 7.5. Sift the akadama to remove the dust, which would clog the pot. In a rainy climate, add more lava rock; if you cannot check the tree often, add a little more akadama to hold extra water.

Watering

The root ball should always stay slightly moist and must never dry out. Do not water on a routine: push a finger about 0 in into the mix and water as soon as it feels slightly dry, soaking the whole root ball until water runs from the drainage holes, then repeat a few minutes later. A dormant tree drinks much less, so in winter just keep the root ball cool and moist. The water should be neutral to slightly acidic: rainwater is ideal, and good tap water is fine too.

Fertilizer: throughout the growing season, apply a solid organic fertilizer on the surface of the mix once a month, or a liquid fertilizer every week, or combine the two. A fertilizer richer in nitrogen in spring, balanced in summer and low in nitrogen in fall suits the tree’s cycle; you can also use a balanced fertilizer throughout and gradually reduce the amount. Do not feed during winter dormancy.

Pruning

The foliage grows in fans that quickly become too large for the scale of a bonsai. Pinch the soft new shoots regularly during the growing season: repeated pinching encourages good ramification and dense foliage pads. On many conifers, cutting foliage with scissors leaves brown tips, so pinching between thumb and forefinger remains the main maintenance technique.

Heavy pruning is best done late in the fall. This thuja has a real advantage: it can be cut back into old wood and will bud again there, something few conifers allow. Still, never remove more than one third of the foliage at once, and do not stack major jobs: after heavy structural pruning, wait until the following spring to work on the roots. Seal large wounds with cut paste; a concave cutter leaves a hollow wound that heals better.

In the wild, the tree can reach 49 ft, with a straight trunk under a conical or columnar crown: that natural habit is a good starting point when you plan the shape of your bonsai.

Wiring

Northern white cedar can be wired at any time of year to adjust branch lines. Use wire about one third as thick as the branch, wound at 45° from base to tip; copper, which is harder, suits conifers, while aluminum is easier to handle for beginners. Wrap thick branches in damp raffia before bending them.

To lower a thick or stiff branch, use a guy wire instead: a thin wire tied to a solid anchor point, such as a strong root, another branch or the pot, with padding to protect the bark. A branch takes a few months to hold its new position. During the growing season, check often: wire can mark the bark within one to four months. Remove it by cutting it at each turn rather than unwinding it.

Repotting

Repot every two or three years in early spring, with normal root pruning; large older trees can go longer between repottings. Slide the tree out of its pot: if the roots circle the root ball or water drains slowly, it is time. Tease out the old soil with a chopstick, trim roots that have grown too long without removing more than about 30% of the root mass, cover the drainage holes with mesh, secure the tree with tie-down wires and work the mix in with the chopstick to fill any gaps. Water thoroughly, then shield the tree from full sun, wind and temperature swings while it recovers, with no fertilizer or heavy pruning in the meantime.

Northern white cedar is propagated from seed or cuttings. Watch for the arborvitae leafminer, whose larvae hollow out the scale leaves and turn twig tips brown, a problem that is worse on shaded plants; also cypress tip miners, bagworms, Japanese beetles, aphids, scale insects and spider mites, which leave the foliage looking faded and grayish. Among diseases, canker, tip blight and root rot are the most common. A single symptom can have several causes: browning in late winter often comes from drying wind or sun, and the shedding of old scale leaves near the trunk in spring or fall is perfectly natural.