Japanese black pine

Identification and origin

Pinus thunbergii belongs to the pine family, Pinaceae. Native to Japan and Korea, it grows in groves along the seacoast and even on rocky cliffs. It has become locally naturalized in the eastern United States.

In its native range, this large evergreen conifer can reach 98.5 ft, with a spreading, umbrella-shaped crown. In cultivation it usually stays between 20–60 ft tall and 12–20 ft wide, with a medium growth rate. Young trees have a flat-topped pyramidal shape without a strong central leader; with age they become irregular and asymmetrical, often twisted, with a broad crown of drooping branches and a curved trunk.

The needles, held in pairs, are 3–5 in long. Dark green, stiff, slightly twisted and sharply pointed, they are bound at the base by a silvery sheath and stay on the tree for four to five years. The best identifying feature is the terminal bud: a silky, silvery white “candle” about 1 in long that stands out against the dark foliage. The egg-shaped cones, shiny and light brown, are 2–3 in long and take two years to mature; they stay ornamental through fall and winter. The thick, gray-black bark becomes furrowed and breaks into plates.

Cultivars include ‘Thunderhead’, compact with a cloud-like outline; ‘Majestic Beauty’, a vigorous grower; and ‘Oculus Draconis’, with variegated needles.

Light

Japanese black pine needs full sun, meaning at least six hours of direct sun a day. Don’t shade it: when light is lacking, the needles grow longer and some branches eventually die back. It handles both exposed and sheltered sites, including the seashore.

Watering

Once well rooted in the ground, it tolerates drought and is content with soil that is moist to occasionally dry, or even very dry. Above all, it needs soil that does not stay waterlogged.

In a container, check the mix with your finger and water thoroughly, until water runs out of the drainage holes, when it starts to dry deeper down. Japanese black pine uses a lot of water but dislikes a mix that stays soggy: let it drain and dry slightly between waterings, and shelter it from long rainy spells if water stands in the pot. A pine left thirsty for long periods is also more exposed to the insects that seek out weakened trees.

Potting mix

In the ground it accepts clay, loam or sandy soils that are acidic, neutral or alkaline, even poor ones, as long as they drain well. It tolerates heat, sand and salt spray, which makes it a top choice for stabilizing dunes and planting along the coast.

In a container, use a very free-draining mix of coarse mineral materials, such as equal parts akadama and pumice, in a pot with generous drainage holes: roots rot in a mix that stays wet.

Temperature

It is hardy in USDA zones 5a to 8b and handles heat very well, above 100 °F in humid air. A tree in a container is more exposed to cold than one planted in the ground: protect the pot, by wrapping or sheltering it, as soon as frost approaches 25 °F, and move it into a cold shelter if the cold lasts below that threshold. Also shield it from freezing winds while the root ball is frozen.

Repotting

Young container-grown trees are repotted about every two years, in spring, when the buds begin to swell; older trees only every three to five years. Pine roots live in partnership with beneficial fungi called mycorrhizae: never strip the roots bare, and keep part of the old mix. Do not remove more than half of the root mass. After repotting, shelter the tree from scorching sun and wind for a few weeks, and hold off on fertilizer.

Pruning

In the garden it needs no pruning to grow well.

It does, however, respond very well to the shaping used in Japanese gardens (niwaki) and to bonsai. Most of the work is done on the “candles,” the young spring shoots. Shortening or removing them at the right time makes the branching denser and the needles shorter; thinning shoots and plucking some of the old needles in fall balances vigor between the top and bottom of the tree. Make large cuts preferably during the cool seasons, when sap flow slows and little resin bleeds.

Propagation

The species is mostly grown from seed. Sow in spring in a light, sandy mix, in full light. Before sowing, drop the seeds into water: those that sink are viable, while those that float are usually empty. Germination can take from two weeks to three months. Transplant the seedlings once a second set of needles has appeared. Cultivars, on the other hand, are propagated by grafting in late winter.

Common problems

Young trees are usually trouble-free; problems show up mainly on older trees, often after about twenty years.

• Pine wilt: the most serious disease. This tiny nematode is carried by long-horned sawyer beetles that lay their eggs in weakened or dying pines. The tree yellows, wilts and then turns entirely brown, and no longer oozes resin when wounded; it often dies within thirty to forty days, and a whole planting can be lost in a single season. Since there is no cure, remove affected trees and avoid the stresses that attract these insects.
• Diplodia tip blight: it often starts by stripping the lower branches of older trees. Other fungi cause needle cast, rusts and cankers; honey fungus can attack the roots.
• Pests: conifer aphids and adelgids, scale insects, sawfly larvae and caterpillars, pine shoot moth, weevils and borers. Also watch for spider mites, which leave small reddish specks on the needles.

Yellowing or falling needles can have several causes: needles only live four to five years and the oldest eventually drop, but overwatering, root rot, drought or a pest produce similar symptoms.

Toxicity

Japanese black pine is mildly toxic to dogs and cats. Chewing or swallowing needles, cones or sap irritates the mouth and digestive tract, causing drooling and vomiting that are usually mild and short-lived; the essential oils it contains are too dilute to cause serious poisoning. Contact a veterinarian if a pet has eaten a large amount or shows lasting symptoms.