Japanese black pine bonsai

Placement

Japanese black pine is an outdoor tree. Indoors, it only survives for long in a greenhouse. Give it full sun throughout the growing season, for as many hours of the day as possible: in shade, needles grow longer, inner branches die back and the tree loses vigor. It handles heat very well, above 100 °F in humid air. In the height of summer, just shield the pot from direct sun, since roots can overheat in an exposed container.

The tree is hardy, but its roots are exposed to cold in a pot. Wrap the container as soon as frost nears 25 °F, even briefly; if the cold lasts below that threshold, shelter the tree in a cold greenhouse. While the root ball is frozen, also shield it from freezing winds.

A well-grown pine usually stays healthy. Watch for aphids, woolly adelgids, scale and caterpillars; spider mites leave small reddish specks under the needles and can be knocked off with a jet of water. Fungi cause tip blight and needle cast, and a soggy mix rots the roots. Pine wilt, a nematode spread by long-horned sawyer beetles drawn to weakened trees, turns the whole tree yellow and then brown within weeks: there is no cure. The same needle drop can have several causes, so look closely before treating.

Soil mix

Japanese black pine needs a very free-draining mix. Equal parts akadama and pumice suit it well. Young trees in development like a higher share of coarse grit; older trees, more akadama.

Watering

This pine is thirsty, more so than Japanese white pine, yet it hates a mix that stays wet. Don’t water on a fixed schedule: when the soil starts to dry, water thoroughly until it runs from the drainage holes, then let it drain and dry slightly before the next watering, without letting it dry out completely. During the summer’s second flush, shelter the tree from long rainy spells: too much water at that stage makes the new needles longer. After decandling, keep moisture steady even though the tree carries less foliage.

Fertilizer: feed heavily in spring, as soon as the candles extend, with solid organic fertilizer renewed at least three times at four-week intervals. Stop when you decandle, then resume four weeks later and continue until early fall. A tree still in development can be fed more freely, with a break during the hottest weeks.

Pruning

Pruning black pine centers on decandling, which triggers a second summer flush with shorter needles, makes branching denser, encourages new buds on old wood and balances vigor.

• When: in early to midsummer, on healthy trees. Timing depends on climate: earlier where summers are cool, later in hot regions. Do not decandle a weak, underfed or spring-repotted tree, or one whose trunk you still want to thicken.
• How: cut each spring shoot squarely at its base, leaving a short stub with a few pairs of needles and sparing the surrounding needles. Leave longer stubs on strong shoots and shorter ones on weak shoots. On an unbalanced tree, work in three rounds about ten days apart: weak shoots first, then medium, then strong.
• In fall: keep only two shoots at the end of each branch. Pluck old needles and thin the new ones: leave about three to seven pairs on strong areas and seven to twelve on weak areas.
• Heavy cuts: make them in fall or winter, when sap flow slows. Reduce the height of a nursery tree gradually, never more than half the growth per year; an old pine tolerates only one major intervention per growing season.

Styles: it suits every style except broom: informal upright, slanting, windswept, literati, semi-cascade or root-over-rock.

Propagation: sow seed in early spring, keeping only the seeds that sink in water; cultivars are grafted in late winter.

Wiring

Wire from early fall to early spring, before bud break, or just after decandling, before the new buds appear. Choose wire about one-third as thick as the branch, coiled at roughly 45°: aluminum on young trees, annealed copper on thick branches and trunks. Check it and remove it before it bites into the bark, cutting it at every turn rather than unwinding it. For a branch too stiff to bend, use a guy wire, protecting the bark where it is attached.

Repotting

Repot in spring, when the buds begin to swell, and only when roots fill the pot: about every two years for a young tree and every three to five years for an established one. Pine roots live in partnership with beneficial fungi called mycorrhizae: never bare-root the tree, keep at least half the roots, and replace no more than a third of the old mix at a time. Cover the drainage holes with mesh, work the mix between the roots with a chopstick, then keep the tree out of strong sun and wind for three to four weeks, without fertilizer. A tree repotted in spring is not decandled that same year.

The Japanese black pine profile covers growing it in the garden.