American sweetgum bonsai

Placement

American sweetgum is an outdoor tree. Give it full sun, in a spot where the air does not stay too dry. The more sun it gets during the growing season, the brighter its fall color: yellow, orange, red and purple then mingle on the same tree. During the hottest weeks of summer, though, give it light shade around midday and in the early afternoon.

In the ground, the tree handles cold easily. In a pot, its roots are far more exposed: protect it from hard frost and strong winds. It tolerates heat well.

Soil mix

Choose a standard, free-draining bonsai mix with a neutral to slightly acidic pH; a mix with a pH above 7 does not suit it. The roots grow vigorously and soon fill the pot: give them some room. A mix of equal parts akadama, pumice and lava rock provides the drainage it needs.

Watering

Water thoroughly as soon as the soil starts to dry, until water runs out of the pot’s drainage holes, without following a schedule: the look of the surface decides. The tree tolerates moderate drought, but in a pot the root ball should not be allowed to dry out. It prefers slightly acidic water, with a pH of about 6 to 6.5: if your tap or well water is too alkaline, use rainwater, alone or mixed, and check the pH.

Fertilizer: during the growing season, a solid organic fertilizer once a month or a balanced liquid fertilizer once a week. Avoid fast-acting, high-nitrogen fertilizers, which produce oversized leaves. On a tree that is already developed, three feedings a year are enough to keep growth healthy and fall color good.

Pruning

Timing matters a great deal with sweetgum. Prune the thicker branches in spring, after the new shoots have emerged, and cover the wounds with wound sealant. Avoid any pruning during winter dormancy: it causes twig dieback, and the wounds heal poorly.

During the growing season, let shoots extend 4–6 in, then cut them back to one or two leaves. In spring, remove the large terminal buds so the side buds develop. Watch the apex, which is very vigorous: trim it carefully so the weaker lower branches can catch up. Defoliation is not needed on this species; to keep leaves a reasonable size, it is better to avoid excess nitrogen. The tree sometimes produces a few oversized leaves.

In the wild, sweetgum forms a narrow pyramid on a single, straight trunk. That silhouette points the choice of style toward upright forms. Much of the shaping can be done by pruning alone, with wiring completing the work.

Wiring

Wire twigs and new shoots in early and midsummer, while they are still flexible. Old wood is stiff and hard to bend: to lower a thick branch, use a guy wire instead, a thin wire anchored to a strong root, a sturdy branch or the pot, padding the contact point with a piece of rubber.

Use anodized aluminum wire, suited to deciduous trees, about one-third the thickness of the branch and wound at roughly 45°. Wire the whole tree before bending, working from the trunk to the main branches and then the secondary ones; to bend a branch down, run the wire underneath it, to bend it up, run it over the top, and bend with pressure on the inside of the curve to reduce the risk of splitting. Check the wire regularly, because it 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, while the tree is still dormant; older, larger trees can wait longer. To tell whether it is time, slide the tree out of its pot: if the roots are circling the root ball, repot; if they are still contained within the soil, wait until the following spring. The roots grow vigorously and tolerate normal to heavy root pruning. After repotting, keep the tree out of wind and scorching sun, and wait until growth resumes before feeding it.

Propagation: from seed, root or stem cuttings, root suckers or air layering.

Pests and diseases: in a good spot and with proper care, sweetgum usually stays healthy. A weakened tree can be attacked by borers and weevils, tent caterpillars, fall webworms, bagworms, leaf miners and several scale insects. Also watch for canker, bleeding necrosis and leaf spots. For a serious attack, use a suitable treatment, but above all improve its growing conditions.