Growing Japanese Maples in Containers

 

Japanese maples have been favored bonsai subjects for centuries.  The extreme diversity within this single plant species has led to 100’s of individual varieties which differ as much as separate species in other plant families.  The ease with which Japanese maples adapt to container growth means that they are one of the best subjects for this method.  And Japanese maples are ideal subjects for containers on patios, porches, walkways, alcoves, or even window ledge gardens. 

 

In addition to the Japanese maple, Acer palmatum, we also include its close relative, the Full Moon maple, Acer shirasawanum; and it’s other close relative from western North American the vine maple, Acer circinatum.

 

SIZE    When planted in the ground, different varieties of Japanese maples have growth rates ranging from inches to feet per year, and final sizes ranging from 3 to 60 feet.  The smaller varieties are perhaps more suitable for container growth.  However, all maples adapt to the confinement of containers with a slowing of growth so that almost any variety may be grown in a container, especially if a minimum amount of pruning is done annually.  As an example, the popular Coral Bark Maple, ‘Sango Kaku’ is one of the larger maples when planted in the ground, eventually growing to 30-50 feet.  In a container, however, it may be kept at almost any size with a only a little pruning.

 

SOIL               Japanese maples prefer a evenly moist soil with good drainage and organic matter.  Although they are quite adaptable, they don’t like dense clay soils with poor drainage.  A mixture of about 1/3 soil, 1/3 well-composted organic matter or coarse bark dust, and 1/3 coarse sand works very well.  There should be at least one large drainage hole in the bottom.

 

EXPOSURE               In containers, the ideal situation with almost all varieties is full sun in spring and fall—this will result in optimum leaf color during these periods.  During those hot summer days, the containers may be moved to a situation with afternoon shade.  These plants don’t like hot drying winds but if you have this situation the container may simply be protected from the wind by a fence or the house itself. 

 

WATER.            Maples are fairly drought tolerant but don’t like to go ‘bone dry’.  If they do, they respond first by obvious leaf wilting.  Watering at this time will bring a quick return to normal.  If drought proceeds beyond this point, the uppermost leaves will start to dry at the tips and this will progress to complete defoliation if no water is given.  This situation is usually not fatal though, at least the first time.  If you forget to water and the leaves dry and shrivel, a second set will grow to replace them.  However, be very careful now, because a third set will not be available for a long time.  No water is required for outdoor maples during the long ‘rainy period’ of fall, winter and spring.  However, when the temperature is in the 90’s, daily watering will probably be necessary.  This may be provided by a drip system set to go off daily during hot weather, or by placing the pot in a shallow (1/2-1 inch) tray filled with water during this period.  This water may be allowed to evaporate before the plant is watered and is especially useful if you will be away during hot summer weather.  Be sure to remove the tray when cooler weather returns.

 

FROST AND COLD             Japanese maples are very cold hardy in western Oregon and Washington and the roots, exposed as they are above ground, are the first to freeze which might occur below about 15-20 Farenheit.  If these temperatures occur, it is best to either bury the pot in the ground or in mulch, or to bring the plant into an unheated area such as a garage.  The plant is dormant and leafless at this time and doesn’t need light or heat.  By no means should they be brought into the house where warm temperatures could stimulate premature growth.  The exception to this cold hardiness is late spring or early winter frosts.  If temperatures drop to 25F or below at this time, the new growth can be killed back to older twigs.  In itself, this does not present a problem as the dead twigs can simply be pruned off later.  However, with certain varieties, mentioned below, there is the possibility of fungal infection through the dead tissues which can lead to serious problems if it extends into the living tissue.  These varieties are best protected overnight if these frosts seem likely to occur. If you do get black tips, then pruning back into a healthy area is called for.

 

PRUNING & REPOTTING             Japanese maples can be lightly pruned at any time of the year. We prefer to prune 1-3 inches above a branching point. The remaining twig is then allowed to turn brown and die at which point it can be simply snapped off at its natural separation point. Larger branches should be pruned in spring at the beginning of the growth period so that there is the maximum length of time for the wound to heal. Again, we prefer not to cut too close to a live branch, so that natural dying of the stub can occur. This is optional, however. Japanese maples don't seem to mind being rootbound as long as they have occasional fertilizer. Repotting or moving to a larger container can occur every 2-5 years. If keeping in the same pot, remove about the outer 1/3 of the root/soil mixture all around the pot, add new soil, and place back into the pot. If moving to a larger pot, just loosen the outter root-bound area and place into a somewhat larger pot. This, like major pruning, is best done in spring to allow new root growth.

 

FERTILIZER            Since the goal of container growing is for the plant to adapt in size and habit to its container, only minimal fertilizer is required and this can be given in late spring after the new growth and again in early fall before dormancy.   In the western Pacific NW, maples often have two growing periods, the first in spring, and then a second in later summer or early fall when temperatures drop and humidity increases with the first cool, rainy temperatures.  The goal is NOT to promote vigorous growth, so a low nitrogen fertilizer is best.  Top dressing with a slow release organic fertilizer works very well. The more fertilizer the plant receives, the more growth, and then perhaps the need for pruning. If the plant looks thrifty and healthy, it is probably getting enough fertilizer.

 

Although almost any Japanese variety may be grown in a container, we here list some varieties that represent a range of growth habits and leaf color and shape which we have found successful. Larger varieties might be best for larger containers, but most maples adapt to whatever their container size as mentioned above. The list includes both upright and cascading or weeping varieties