Japanese black pine is best planted in the spring as a container-grown or balled-and-burlap plant. It has a moderately fast growth rate, sometimes as much as 2 to 3 feet per year. Be aware that mature trees may become susceptible to fungal diseases as they approach maturity at 20 years or so. Until then, though, this is an excellent, easy-to-grow landscape tree with a unique appearance.
Light
Japanese black pine should get at least six hours of direct sunlight daily. Some light afternoon shade is acceptable.
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.
Water
After you plant the tree, make sure it gets a full 1 inch of water per week for the first year. After the tree is established, it will be quite tolerant of short droughts and may not require any irrigation beyond ambient rainfall.
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.
Fertilizer
This tree, like most pines, does not usually require regular feeding. If your soil is poor in nutrients, add a complete fertilizer in the spring. Potted bonsai specimens, however, can benefit from more frequent feeding.
Types of Japanese Black Pine
The pure species form of Japanese black pine is very popular in landscape use. There are are also many excellent cultivars of Pinus thunbergii, often bred to be smaller trees. Some favorites include:
‘Majestic Beauty’ is a fast-grower with beautiful deep-green needles. It often reaches 40 to 50 feet in height within 20 years.‘Oculus Draconis’ is a variegated dwarf variety. The needles have a yellow band close to their base. It grows to a mature height of 6 to 8 feet.‘Shirone Jamone’ is another variegated cultivar. It has bright golden or yellow bands on dark green needles. In ten years, it grows up to 10 feet high and 7 feet wide.‘Thunderhead’ is a dwarf cultivar that grows 5 to 10 feet tall in ten years. Its branches are densely needled, giving the appearance of dark storm clouds.‘Pygmaea’ is a compact cultivar with full-length needles. It grows only about 5 feet in ten years.
Pruning
Although pruning is not necessary for the tree’s health, its irregular growth pattern and drooping branches may require regular pruning to make room for people and vehicles to pass underneath the canopy. Pruning can also help the trees remain dense and full when they are planted as a screen. Often, pruning is done to deliberately shape the tree into twisted, exotic shapes, exaggerating its natural growth habit. Its ability to handle this kind of pruning makes it a popular tree for bonsai practice. Major pruning of landscape trees is best done in the spring, preferably during dry weather when transmission of fungal diseases is less likely. When Japanese black pines are grown as bonsai plants, pruning usually involves pinching off the new growth candles in the spring, as well as regular root pruning to keep the tree small.
Propagating Japanese Black Pine
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 (see below).
How to Grow Japanese Black Pine From Seed
Whether purchased from a commercial source or collected from ripened cones, seeds from Japanese black pine are fairly easy to germinate and grow into seedlings. Here’s how to do it:
Potting and Repotting Japanese Black Pine
Container culture for Japanese black pine usually is done only when the tree is being grown as a bonsai specimen—it is one of the most popular of all pine species for this purpose. 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 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.
Common Pests & Plant Diseases
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. But badly affected trees that have developed cankers from Diplodia can’t be cured. Avoid pruning during wet weather when fungi are easily transmitted. Avoid fertilizing lawns around pine trees, as excessive nitrogen also fosters fungal 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 moth, and bark beetles.
Common Problems With Japanese Black Pine
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 fast-growing, growers are often delighted to use it in the landscape with the understanding that it will need to be removed someday. One common complaint with this tree—usually made by owners who did not sufficiently research its growth habit—is that the branches can be very low-hanging, making it difficult to walk beneath. It can easily be pruned to rectify this, however. And you may be surprised by how messy this tree is, shedding a considerable volume of needles and cones. This is not unusual for a pine tree, but P. thunbergii, with its very dense growth, is messier than most pines.
Japanese black pine is also one of the most popular pines to use in bonsai practice.
