Choose a Japanese dwarf garden juniper for a bonsai if you live in a climate that mirrors the type of environment where Japanese dwarf garden junipers thrive. Since bonsai trees are an outdoor plant, juniper bonsai will not thrive outdoors outside of its natural climate range. Japanese dwarf garden junipers need constant daytime temperatures above 40 degrees Fahrenheit. For this reason, they are not good plants for growing in the upper Midwestern United States.
Place your Japanese dwarf garden juniper bonsai where it will get good air circulation and at least six hours of direct sunlight daily.
Water your Japanese dwarf garden juniper daily with a mister and a syringe during spring and summer months, and as needed in fall and winter. Never allow your juniper bonsai to dry out. During hot, dry periods increase branch misting to several times daily. Feed your bonsai with a liquid fertilizer (10-10-10) diluted to 1/4 strength and water with this four to five times per growing season.
Root prune your Japanese dwarf garden juniper every three to five years. To do this, unwire the tree from its bonsai tray. Save the moss or soil covering and set it aside. Support the trunk in one hand and turn the tray over. Tap the bottom of the tray to loosen the bonsai's roots. Gently work the tree loose from the tray.
Loosen the roots from their soil and root ball so that they hang freely. Prune away 1/3 of the roots from the edges of the root ball, and 1/2 the roots on the bottom of the root ball.
Gather coarse sand, loam, clay and humus to create a soil for your container soil. Mix a drainage layer that contains 1/2 coarse sand and 1/2 combination of loam, clay and humus. Strain these items through a coarse sieve with three wires to an inch. Place the fragments caught in the sieve into the bottom of your container.
Spread the roots of your Japanese dwarf garden juniper over the coarse drainage layer. Replace the electrical wire that holds the tree to the base of the tray.
Mix a potting mixture for your tree that combines one part humus, two parts sand, two parts medium-grade clay and two parts loam. Pass this soil through a medium-grade sieve with eight wires per inch. Place the soil captured by the sieve in the container and firm it around the tree's roots with a chopstick.
Create a topsoil layer by mixing fine loam, clay and humus in equal parts. Pass this soil through a fine-grade sieve with 15 wires per inch before covering the roots with it. Then re-cover the soil with moss.
Prune your bonsai by pinching out new shoots with your fingers. Never cut the needles of a Japanese dwarf garden juniper with shears because the cut needles will turn brown. Prune with an eye to keeping and maintaining the tree's cascading shape and controlling growth. Pinching will need to be done continuously through the growing season.
Wrap wire around the trunk of the Japanese dwarf garden juniper to train it into the leaning shape that is typical of the cascading style of bonsai in winter when the tree is dormant. This will allow the tree to become used to its new shape while it is dormant. During the active periods of spring and summer, watch your Japanese dwarf garden juniper for signs that the training wire is girdling the tree (cutting into the bark). Remove the wire as soon as you notice signs of girdling and re-wrap the wire.