When it comes to growing Japanese maple trees in containers there is very little scientific research on the subject, however we can learn a lot by studying the methods used in bonsai. The horticultural techniques used in the art of bonsai can be applied to growing Japanese maple trees in containers or a small garden.
To begin you must first select a Japanese maple cultivar that will thrive in a container. Since maples are strong growing trees you will have plenty of cultivars to choose from. Pacific Coast Maples has many outstanding trees that grow well in containers. Aratama, Beni Maiko, Chishio Improved, Crimson Queen, Fireglow, Garnet, Inaba Shidare, Kamagata, Mikawa Yatsubusa, Osakazuki, Sango Kaku, Red Dragon, Seiryu, Shaina, Sharps Pygmy, Shin Deshojo, Shishi Yatsubusa, Shishigashira, Tamukeyama, and Winter Flame are trees we grow and all do very well when grown in a container
When selecting a tree to re-pot into a container it is important to check to make sure that the tree is not root-bound by tipping the growing pot and looking underneath. The trees root structure should be healthy and must not have large woody roots circling the root ball.
Before replanting your little tree it is important to select a container that has several holes in the bottom for drainage. If your tree is being planted indoors and your container does not have drain holes you'll need to make sure to watch the amount of water you use so that your tree does not drown.
Maples prefer acidic soil with a high air content. The soil must also have good drainage. The soil should be very low in soluble salts and should have a Ph between 5.5 and 6.5. The soil anchors and supports the tree while it grows in the container. The soil also provides a medium to deliver nutrients, moisture, and oxygen to the tree. Sand based soil is preferred so that your maple is able to develop fine fibrous roots that support the tree. Peat Moss, Perlite, and barky mulch are best for holding air and moisture.
It is important that you do not use composts derived from animal waste because it may burn your maple's roots. If you live in an area that has less than 18 inches of rain per year (see our article- Growing Japanese Maples in Southern California, the Desert, or Other Hot, Dry, Climates.) and/or your water is hard or alkaline it is recommended that you use an Azalea Mix type soil or another type of potting mix that has a slightly higher acid content. These can be purchased from Home Depot or your nearest garden center.
Japanese maple trees are not heavy feeders so it is important not to over fertilize your tree. Be especially careful in using nitrogen fertilizers and fertilizers that have a high percentage of ammonium nitrate. Another alternative that is very effective for many people is rose food and fish emulsion. Rose Food can be purchased almost anywhere. Pacific Coast Maples highly recommends using "Dyna-Gro- Grow 7-9-5" which we sell in small to large quantities. We use Grow 7-9-5 and ProteKt on all of our maples and have been very satisfied with the results.
When growing your maple in a container, root pruning and repotting should take place in the early spring prior to the emergence of new leaves. Root pruning is not difficult and is necessary for the health of your tree. Young trees need to be transplanted into the next size pot when the roots are touching the sides and bottom of their container. Root pruning is not necessary during this stage, however it is important to cut roots that are becoming large and woody. Root pruning is important to the overall health of older maples that have reached their optimum size and should be done every two to four years. After root pruning you may replace your maple in the same pot.
When removing your tree from its container you may notice that the roots have grown to become one solid mass and that there is little space for air or water. If this is the case remove all of the old or dead root material with a knife, pruning shears or saw. It is also important to cut any woody roots pushing up into the tree's root ball. If the root ball is large and dense you can cut straight through the bottom fourth of the root ball. It is also important to cut 1/8 to 1/4 of the mass on the sides of the root ball. Trimming the trees roots will encourage new growth. Fresh soil should then be placed in and around the edges of the container. After replanting the tree in its new soil it should be watered thoroughly followed by very consistent watering for the next few months.