Japanese Black Pine (Pinus thunbergii)

For a beautiful evergreen in the garden with twisted stems and open-tiered branches with an airy texture, add the Japanese black pine to your landscape. The best part is you can even grow them as bonsai trees with their intriguing form to form a curved trunk. The dark green needles stand out with the white candles in spring. It is an adaptable tree, and native species come from Japan. The striking form with expressive needles stands out in any landscape or home.

I. Appearance and Characteristics 

Pinus thunbergii (syn: Pinus thunbergiana), the black pine, Japanese black pine, or Japanese pine, is a pine tree native to coastal areas of Japan (Kyūshū, Shikoku and Honshū) and South Korea.

It is called gomsol (곰솔) in Korean, hēisōng (黑松) in Chinese, and kuromatsu (黒松) in Japanese.

Black pines can reach the height of 40 metres (130 feet), but rarely achieves this size outside its natural range. The needles are in fascicles of two with a white sheath at the base, 7–12 centimetres (2+3⁄4–4+3⁄4 inches) long; female cones are 4–7 cm (1+1⁄2–2+3⁄4 in) in length, scaled, with small points on the tips of the scales, taking two years to mature. Male cones are 1–2 cm (1⁄2–3⁄4 in) long borne in clumps of 12–20 on the tips of the spring growth. The bark is gray on young trees and small branches, changing to black and plated on larger branches and the trunk; becoming quite thick on older trunks. It is a widely adapted plant with attractive dark green foliage.

One characteristic of the Japanese black pine that makes it desirable for bonsai, is the possibility of inducing a second flush of new growth and improved ramification in a single growing season. Unlike most pines, which are single flush plants, the Japanese black pine can be induced to produce new buds at the base of each spring candle by simply cutting the candles at the base as they elongate, a technique called decandling. This technique will result, in a few weeks, in the flush of multiple new buds at the base of the cut candle; each of these new buds will result in turn in new candles and branches.

In North America this tree is subject to widespread mortality by the native American pinewood nematode, Bursaphelenchus xylophilus, spread by means of beetle vectors. Subsequently, blue stain fungus invades the plant, leading to a rapid decline and death. This nematode has also been introduced to Japan accidentally, leading to the species becoming endangered in its native area.

II. How to Grow and Care

Sunlight

Japanese black pine should get at least six hours of direct sunlight daily. Some light afternoon shade is acceptable.

Temperature and Humidity

This tree generally does well in USDA zones 5 to 8. Winter burn will seriously damage the tree at temperatures below minus 10 degrees Fahrenheit, turning needles dry and brown. Humid conditions may foster fungal diseases, especially if pruning is done during wet periods.

Watering

For newly planted seedlings or new plants, water once every morning and evening during hot spells in the summer. Do not water midday, as this can cause root burn or strangle. Water plants according to your individual climate and rainfall in other seasons. For mature plants, only water when they are dry, keeping in mind that these plants are drought resistant. For indoor potted plants, spray water on the surfaces of the leaves once every morning and evening when it is dry. Be careful not to provide excess water – this hinders air circulation in a pot, leading to the rotting of roots and the withering of branches and leaves.

Soil

The soil must be moist but well-drained. Sandy loam is ideal; Japanese black pine does not tolerate soggy soil and poor drainage. It prefers acidic soil but can also grow in slightly alkaline soil.

Fertilizing

The japanese black pine likes fertilizer and should be fertilized frequently, with just a small amount each time, during its growth period. It should be fertilized once a month in late spring, early summer, and fall. A fermented organic fertilizer is most effective for promoting growth.

Generally, do not apply a nitrogen fertilizer, such as urea or human urine, because pine needles already absorb nitrogen from the air, and pine roots are sensitive to nitrogen. You would be best off with a liquid fertilizer, applying this when the soil is dry in the afternoon. Water the plant again after fertilizer application, which will help with root absorption.

Do not use fertilizers that haven’t been fermented, or those with a higher concentration; the former will burn the roots and the latter will lead to the spindling of needles and more root damage, and could even cause the back-flow of sap, leading to water loss and the withering of the plant. No fertilizer should be applied in midsummer, during severe winters, or in the rainy season in the spring.

Generally, plants in gardens should be fertilized twice during the growth periods in spring and fall. Apply an organic fertilizer once before germination in the spring and apply slightly more fertilizer in the fall to promote robust growth. Stop fertilizing after midsummer so as to prevent spindling.

Pruning

For an indoor potted plant, any dead, diseased or damaged branches should be removed. You can then adjust the tree’s shape by pruning branches, pinching buds, and trimming leaves, giving you a better ornamental effect. Prune the plant before all of its needles fall off, so as to obtain a compact shape, richer lateral and side branches, and a better form overall. Pruning should be done during the dormancy period, so as to prevent excess loss of sap and damage to the plant’s vigor.

For a plant in a garden, dense lateral branches should be pruned so as to improve the survival rate. Remove excess lateral branches during the vigorous growth period based on needs, focusing on encouraging the trunk to grow tall and straight. Cut off any diseased or dead branches right away, so as to prevent the spread of pathogens.

Propagation

As a group, pines are somewhat difficult to propagate by vegetative methods, such as by rooting branch cuttings. For this reason, propagation is more commonly done by seeds extracted from the cones or purchased from a commercial source. Harvested seeds from ripened cones are fairly easy to germinate and grow into seedlings. Here’s how to do it:

• Collect cones in the fall as they begin to fall from the tree, then store them to dry through the first part of winter.
• Harvest the seeds from the cones in late winter when you are ready to start them indoors. Extract the seeds by drying mature cones until the scales begin to separate, then shake them over a sheet of paper to dislodge the seeds.
• After shaking the seeds loose, soak them in water for 24 hours, put the seeds in a plastic bag, and place them in the freezer for four weeks.
• Fill small pots with standard potting mix. Sow the seeds on the surface and cover them with 1/8 inch of vermiculite or fine compost.
• Gently water the pots and place them in a bright location at room temperature. Within 14 days, the seedlings should germinate and sprout.
• Grow the developing seedlings outdoors in full sun, keeping them uniformly moist. Repot them into larger containers as needed. In the first year, they should reach a height of several inches.
• In about two years, the seedlings will be of sufficient size to plant in their permanent locations in the landscape.

Potting and Repotting 

Container culture for Japanese black pine usually is done only when the tree is being grown as a bonsai specimen. Bonsai trees are normally grown in a typical bonsai potting mix of coarse sand, clay, or pumice, and peat in a traditional ceramic bonsai pot.

As with most bonsai plants, Japanese black pines should be repotted every few years to prune back the roots. Japanese black pine, like most bonsai, likes to have its roots rearranged before repotting back in the same container with fresh potting mix.

If you keep it outdoors—or bring it outdoors for the summer—the container must be protected from the hot sun to prevent root burn. In partial shade, the needles will be lighter than a tree grown in full sun. Water it regularly but let the soil dry out to the touch between watering.

Pruning stresses the bonsai tree and causes sap bleeding. Do substantial pruning only between fall and early winter. If you need to do minor pruning during the summer, make sure to move the container into the shade for about a month afterward to minimize sap bleeding.

Overwintering

Like most pines, Japanese black pine can be susceptible to winter burn if grown in regions where it is borderline hardy. Zone 5 gardeners may find that their Japanese black pine develops browned needles on the side that faces cold winter winds. This is most likely to occur with young trees, or in situations where temperatures fall quickly from warm fall weather into freezing winter cold.

Young trees can be protected from winter burn by planting them in sheltered locations and making sure they are well-watered going into winter. Mulching the soil well is also beneficial. If necessary, small trees can be protected with a tent or screen of burlap for the winter. Do not, however, tightly wrap the trees with burlap, as this can trap moisture and foster fungal infection.

Pests and Diseases

Common Pests & Plant Diseases

A variety of pests can affect Japanese black pine, but the most serious is the pinewood nematode, which often kills the tree. These tiny soil worms infect trees through holes bored by bark beetles. Very quickly after infection, the tree will begin to fade color and turn yellow, and the tree often dies within a year or two. There is no cure for this disease, so it is important to diagnose it quickly and remove affected trees to prevent its spread to other pines.

Other possible pests include sawflies, Nantucket tip moths, and bark beetles.

Pinus Thunbergiana: Japanese Black Pi. University of Florida Extension.

Japanese black pine is a fairly problem-free plant when it is young, but as the tree approaches about 20 years of age and begins to set cones, Diplodia leaf blight (a fungal disease) often sets in. Beginning with the lower branches, needles begin to defoliate, giving the tree a shabby appearance. Gradually, cankers may form on branches. Leaf blight that progresses into canker disease is usually fatal.

Other fungal diseases are also possible, including various rusts and cankers. All these fungal diseases can be slowed by good hygiene (sweeping up needles and removing affected branches) and spraying with fungicides each year when new growth is starting. Badly affected trees that have developed cankers from Diplodia can’t be cured. Avoid pruning during wet weather when fungi are easily transmitted. In addition, avoid fertilizing lawns around pine trees, as excessive nitrogen also fosters fungal diseases.

Common Problems 

Other than the all-too-frequent decline of Japanese black pine when the trees become mature and susceptible to common diseases, this is a largely problem-free tree for the first 10 to 20 years of its life. Because it is a very attractive tree, many gardeners are often delighted to use it in the landscape with the understanding that it will need to be removed someday.

Low-Hanging Branches

It’s a common complaint that the tree has low-hanging branches, making it difficult to walk beneath. This issue can easily be rectified by pruning.

Excessive Shedding

This tree is surprisingly messy because it sheds a considerable volume of needles and cones. This is not unusual for a pine tree but P. thunbergii’s very dense growth is messier than most pines.

Brown Needles

Cold winter winds can dry out needles and turn them brown.

III. Uses and Benefits 

Because of its resistance to pollution and salt, it is a popular horticultural tree. In Japan it is widely used as a garden tree both trained as Niwaki and untrained growing as an overstory tree. The trunks and branches are trained from a young age to be elegant and interesting to view. It is one of the classic bonsai subjects, requiring great patience over many years to train properly.

The wood can be used to collect rosin; bark, needles, roots and so on can be comprehensively made into various chemical products; seeds can be extracted for oil.