How to Grow and Care for Japanese Black Pine

Japanese black pine (Pinus thunbergii) is an irregular needled evergreen that usually does not grow with a straight central leader, but instead matures into a flat-topped specimen with drooping branches. If there is a predominant leader, it is often a thick twisting structure rather than a straight trunk. The tree has dark-green needles, 5 to 7 inches long, with upright new-growth candles that are a contrasting silky white. The brown cones are 1 to 3 inches long. In ideal locations, the pure species can grow to be quite a large tree, up to 80 feet tall, but many popular cultivars are dwarf varieties, some of which remain only 3 to 10 feet tall. Unfortunately, mature trees are susceptible to a variety of fungal diseases, so large specimens more than 20 years old are rare. Young Japanese black pines are so attractive, however, that they are still commonly planted with the knowledge that they won't last for many decades.

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.

Japanese Black Pine Care

Japanese black pine grows best in sandy loam and in a sunny location. It is a good choice for salty locations, such as seashores, and it does well in both acidic and alkaline soils. If growing as a screen, space them at least 12 feet apart, as these dense trees will quickly fill in. As an isolated specimen tree, give it even more room, as there is the potential for it to reach 50 feet or more in height with a 25-foot width.

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:

1. Collect seeds from mature cones by drying the cones until the scales begin to separate, then shake the cone over a sheet of paper to dislodge the seeds. Cones are best collected in fall as they begin to fall from the tree, then stored to dry through the first part of winter. Extract the seeds in late winter when you are ready to start them indoors.
2. After shaking seeds loose from the cones, soak them in water for 24 hours, then put the seeds in a plastic bag and place them in the freezer for four weeks.
3. Fill some small pots with standard potting mix, then sow the seeds on the surface, and cover them with 1/8 inch of vermiculite or fine compost.
4. Gently water the pots and place them in a bright location at room temperature. Within 14 days, the seedlings should germinate and sprout.
5. 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.
6. In about two years, the seedlings will be of sufficient size to plant in their permanent locations in the landscape. At this point, growth will accelerate rapidly and you can expect as much as 2 to 3 feet of growth per year.
   

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.