Japanese white pine

Identification and origin

Pinus parviflora belongs to the Pinaceae family. It is native to Japan, Korea and the Kuril Islands. It is a high-elevation pine, adapted to the harsh conditions and short growing seasons of the mountains.

It stands apart from other cultivated pines by its needles, which come in bundles of five. They are fine, soft and slightly twisted, 1–3 in long. Their outer face is blue-green and their inner face carries a white stripe that gives the foliage a silvery sheen, which is where the name “white pine” comes from. The cones are ovoid, reddish brown to light brown, and ripen in fall.

The bark stays gray, smooth and understated for a long time, then turns scaly and darker with age. It looks nothing like the thick, fissured bark of Japanese black pine or the flaking bark of red pine. On a bonsai, it takes about thirty years on average for the bark to look mature, so young trees keep a juvenile appearance for a long while.

Growth is slow. In cultivation the tree usually reaches 19.5–49 ft tall with a similar width; the largest specimens exceed 39.5 ft, with a spread of 13–26 ft, over twenty to fifty years. In its native habitat it can grow much larger. When young it is dense, conical to columnar; its branches spread widely, then the crown flattens with time and the tree takes on a picturesque outline.

There are many cultivars:

• ‘Glauca’: especially blue needles;
• ‘Aoi’: narrow, upright habit with curved blue-gray needles in tight bunches;
• ‘Azuma’: silver-blue incurved needles;
• ‘Aoba Jo’: short, curved, bright blue needles;
• ‘Gimborn’s Pyramid’: a rare compact form that reaches about 10 ft;
• ‘Kokonoe’, ‘Zuisho’ and ‘Myojo’: dwarf forms prized for bonsai.

Light

Full sun suits it best: at least six hours of direct sun a day, ideally with a south-facing exposure. The more light it gets, the shorter and denser its needles stay; in shade they lengthen and the branching thins out. It tolerates both sheltered and exposed sites, but its wood can suffer in strong winds, so avoid windy corridors for a young tree.

Watering

Once well established in the ground, Japanese white pine tolerates drought. Water mainly during the first years after planting and during long dry spells, after checking that the soil is dry deep down. Too much water harms it far more than too little: soil that stays soggy starves the roots of air.

In a container it is the “driest” of the common bonsai species, because its roots need as much air as water. Soak the whole root ball, then let it dry out thoroughly before watering again. Watch the soil rather than a calendar: in hot weather it dries quickly, in winter very slowly. The tree is sensitive to water quality: hard water or water with a high pH turns the needles yellow, while clean, slightly acidic water keeps them well colored.

Potting mix

In the ground it accepts clay, loam or sandy soils, acidic, neutral or alkaline, and even poor soils, as long as they drain well. It also tolerates salt and, to some extent, urban conditions. Drainage remains the key requirement: avoid low spots where water pools.

In a container, choose a very airy mineral mix, for example equal parts pumice, lava rock and akadama, with the finest particles sifted out.

Temperature

This is a very cold-hardy tree, grown in USDA zones 4a to 7b, that is, down to about -29 °F in winter. It needs a real cold winter rest: without this dormancy it uses up its reserves within a few years and loses buds, then branches. It is never grown indoors.

Humid heat does not suit it; it grows better where summers are cool. In a container it tolerates temperatures well below 19 °F if the pot is large enough, at least 12 in wide; the roots in a small pot must be protected from hard frost. It also tolerates intense heat above 100 °F, but it moves water slowly, so light shade helps it through those periods.

Repotting

In the ground, plant a young tree in its permanent spot, in well-drained soil; its slow growth leaves time to plan the space it will fill over the years.

In a container, repot in spring, just before or as the buds begin to swell. Cut back only a limited part of the roots and keep at least half of the root mass intact: it holds beneficial fungi, mycorrhizae, which pines depend on.

Pruning

A mature tree needs little pruning.

• Leader: remove any competing top shoot to keep a single main stem.
• Lower branches: on a mature tree, raise the crown if needed by removing a few of the lowest branches.
• Bonsai: the new shoots, called candles, are shortened by hand; branch pruning is done once the new shoots have hardened off or in early fall, always leaving enough foliage.

Because it produces only one flush of growth a year, never remove all of its candles, unlike Japanese black pine.

Propagation

• Seed: the main method. Sow in trays in a cold frame in late winter, then keep the young plants in pots for at least two years before planting them out.
• Grafting: cultivars with desirable needles are often grafted onto more vigorous Japanese black pine rootstock.
• Cuttings: rare; some cultivars such as ‘Zuisho’ can be rooted this way.

Common problems

The same symptom can have several causes: yellowing can come from the natural fall shedding of old needles, overwatering, hard water, lack of light or a pest. Look at the tree and its soil before treating anything. Planted in a cool spot with good drainage, it has few problems.

• Adelgids: these small, aphid-like insects covered in a waxy secretion attack candles and buds as they open in spring.
• Other pests: conifer aphids, sawfly larvae, pine shoot moth, scale and spider mites.
• Diseases: honey fungus and needle cast diseases, which brown the foliage and make it drop; root rot in soil that stays too wet.
• Wind: branches can break in storms.

On the other hand, it resists deer well.

Toxicity

No toxicity is reported for dogs or cats: Japanese white pine does not appear among the plants listed as toxic to these animals, while ponderosa pine, a close relative that is listed, is classed as non-toxic to them. Still, keep children and pets from chewing the needles or cones.