Because constant pruning of bonsai roots and branches relieves a lot of the stress caused by growing, bonsai varieties may live much longer than the same variety of tree growing naturally. Some of the oldest living bonsai in America belong to the Larz Anderson Collection, and were under care in Japan when George Washington was a child in colonial America. Bonsai trees should be trimmed in spring and summer when the plants actively produce new growth, and the foliage should be continually pinched back so that it will regrow more proportionally to the tree's size.
Determine the variety of your bonsai tree by asking the grower at the place of purchase for information about the tree. Another good way to identify your tree is by comparing leaf, needle or bark shape against known tree varieties at an arboretum or through a tree identification field guide. This will help you to determine the best time to trim your tree. Different tree varieties have different optimal times for trimming.
Decide what style of shape that you want your tree trimmed into. Bonsai have several distinct styles of growth, including upright, tilted and cascading. If you purchase a mature bonsai, it will already be shaped. You can determine the shape to give to a tree that you start on your own. The shape of the tree determines how you will trim it.
Head back any branches that are outgrowing the tree's canopy by cutting the branch in front of a bud or alongside the fork where it joins the trunk or another branch. A mature bonsai tree's height should be six times the girth at the base of the tree.
Remove any undesirable branches that do not conform to the style of the bonsai tree. Branches should slope or droop downward to give the appearance of age, and should not be symmetrical on the tree. The branches should not cross the trunk's front. Remove parallel, twisted or tangled branches as well as any branch that is thicker than the trunk in the middle or end of the branch.
Observe the bonsai from several angles after each cut that you make to ensure that the plant is taking on or maintaining a pleasing shape.
Pinch off needles or remove deciduous leaves with pruning shears at the end of the petiole. Leaves and needles will regrow in a smaller size that is more proportional to the smaller tree.
Prune the roots every time you repot the bonsai. Hold the bonsai perpendicular to the tray and gently tease the roots free of their compacted shape. Hold the bonsai pruning shears perpendicular to the bonsai tray and remove one-third of the roots from the edges of the root ball and half of the roots on the underside of the plant.