Japanese black pine is the epitome of bonsai. Few trees can convey the stoic power or the subtle profundity of bonsai to the degree that a black pine can. Black pine is a tree that takes many years to achieve the mature look of a superior specimen bonsai. For this reason, it is important that those who would choose to grow them be steadfast in their attentive and uncompromising care of the tree. Growing black pine for bonsai carries with it a healthy responsibility to prepare and maintain good material for future generations to work with.
Black pine is a strong tree that responds well to the techniques used in the creation of bonsai. Working with black pine is a balancing act and its growth characteristic is such that it needs ongoing and careful maintenance in order to stay in bonsai trim. Left on its own, a black pine will develop long, leggy branches that emerge in whorls from a leggy trunk. The branches will have lollypops of foliage at the branch tips. As pines are apically dominant (like most trees), the upper branches will get most of the tree's energy, leaving the lower branches weaker in comparison. All of these characteristics run counter to the bonsai aesthetic.Notes:
Soil: Japanese black pines like a well-drained soil. A good mixture is akadama and pumice mixed 50/50. Younger trees may enjoy more grit and older ones more akadama. Akadama by itself is a good mixture as it drains well. A free-draining soil is essential to preventing root rot that can come from a waterlogged soil. An open mixture also allows oxygen to get to the roots.
Watering: Keep black pine evenly moist, but not damp. They like to be a little on the dry side and can endure a bit of a drought. If you have them in a free-draining soil, it will be hard to water too much. When you water, be sure to water thoroughly, two or more times to fully saturate the soil. This ensures that all of the soil gets wet and facilitates an atmosphere exchange in the pot - the air is refreshed with new oxygen. Make sure that your water is of a ph level between 5.5 and 6.5, as pines enjoy this range. They can tolerate other extremes, but thrive in this range.
Exposure: Black pines enjoy full sun all day long, but if you live in an area where the daily temps in summer are in excess of 100F, you should probably place them under shade cloth for the hot part of the day. If your trees get full exposure during the summer in hot locations, their color may tend toward yellow-green. If you keep them under shade cloth or in partial shade, their color will be a deeper green.
The heat of summer can bake the roots if the pot gets too hot. One solution is to cover the pot with a cover or sun-block of some sort. This helps to keep the temperature reasonable in the heat.
Pines that have been pruned drastically in the spring, like nursery stock that you have cut back, must be protected from harsh heat for their first summer. If you don't put them under shade cloth, the current year's needles can get sun scald and the tree will be weakened considerably.
Fertilizing: Pines don't usually use fertilizer quickly like deciduous trees. They enjoy organics like Hollytone or Bio Gold beginning in late March through September and will do well with an occasional supplement of vitamins and chelated iron.
Pruning: Branch pruning can be done in late fall or winter to reduce sap bleeding from the wound. After cutting off the branch, reduce the cut around the edges with a sharp knife or chisel and seal with a putty cut paste. Don't use the viscous kind as it does not react well with the sap. You might consider removing major branches in spring as the large wound will heal faster.
Pines should be reduced slowly, over a period of years. Drastic pruning is stressful and should only be performed on very strong and well-fed trees. Keep recently pruned trees out of temperature extremes and in partial shade for 2 to 4 weeks.
Rough nursery stock that is in good health can be pruned hard in the spring so that you can begin to define the shape. If you do this in early spring, go ahead and root-prune the tree and repot. If you do heavy pruning in late spring, don't repot until next spring and treat the tree with extra care during the growing season.
Shoot trimming is done in the growing season to force ramification. Pruning in the fall can be effective in forcing back buds - but only on strong, well-fed trees.
Wiring: Wiring of pines is probably best done in the late winter. It is more risky to wire any other time, as during the growing season, the cambium is full of sap and is literally floating on the xylem and can be easily damaged, killing the branch. There is, however, a school of thought that says that wiring in the growing season is best, as damage to the branch is more easily repaired.
Black pine is limber in comparison to many trees. Even large branches can be wired and trained into different shapes if care is taken. Don't wire a weak or otherwise stressed pine. It may be too weak to live through the inevitable damage of wiring. Don't let a wired pine freeze.
Repotting: Depending on where you live, pines can be repotted from December to March. Most information on black pine says to repot young trees every other year and older trees every 3 to 5 years. Some have found, however, that repotting every year can be useful for spurring the tree into more vigorous growth and for keeping tabs on the root zone. Keep in mind that if you do so every year, you cannot remove much of the roots.
Black pine do not generally appreciate too much root removal at repotting. Be conscious of the root feeder channels of the trunk when trimming roots. Don't drastically trim a root that feeds a large lower branch. The root structure will likely mirror the branch structure with large roots emerging under large branches. Also, keep in mind that feeder channels do not always go straight up the trunk as many trees' trunks are twisted.
Each time you repot, take time to arrange the roots carefully. Black pine, like most bonsai, need a good surface root structure as part of the necessary elements of quality and aesthetics. Use bent wire as staples to hold down bowed-up roots. Use chopsticks or stones to raise and separate roots that need to be positioned. In time, they will correct themselves just like a wired branch.
After repotting, place the tree in bright shade or dappled sunlight for a few weeks and protect from temperature extremes and wind. Don't let the new soil dry out completely.
Pests: The most problematic pest for black pine is the red spider mite. They usually attack trees that are weak or stressed for some reason, usually in late spring and summer. Get into the habit of checking for them on a weekly basis and observe you trees for signs of weakness. A pine infested with spider mites will have needles that appear speckled upon close inspection. The best way to check for them is to hold a piece of white paper under a branch and shake the branch. Look on the paper for needle-point sized specks that move. Although dangerous, they are easily killed with any mitacide and even a strong spray of water can remove them. It can take a couple of treatments to rid a tree of them.
Other common pests include aphids and mealy bugs. These are also easily controlled by common pesticides. Some advocate weekly spraying of insecticide for one's entire collection, but this practice kills the beneficial organisms, too and can hamper your organic fertilizer program. As pests will usually attack only weak or stressed trees, keeping close tabs on them and keeping them well cared for will obviate most insect control.
In winter, it can be useful to apply a treatment of lime-sulfur and water, mixed 1:30 and sprayed on the foliage and bark. This keeps fungus from getting a foothold in the damp, cool dormant period.