[header] How to trim a bonsai [title] Determine which season your tree should be trimmed during. [step] Bonsai trees should only be pruned during a certain time of year. This is so that the tree can adequately heal the wound created during pruning. 
The ideal pruning season will vary between tree species, but will usually be during the spring and summer when the plant is growing most vigorously. Check online or with a nursery to determine the pruning season for your species.