Always balance SUN, WATER, AIR, SOIL and NATURAL NUTRIENTS
Spring - Bonsai grow and develop from spring to summer, thus they must store up nourishment in the soil to prepare for the winter. Spring is when most bonsai begin their growth cycle. Monitor your watering to meet the requirements of your plants. Pay particular attention to deciduous bonsai who are at their peak growing stage and coupled with warmer weather may require additional moisture. For flowering bonsai, after the buds have blossomed, remove any remaining flowers and buds from the tree (berries may be left on). Take care to rotate the plants often so that each side receives equal amounts of sun to avoid one side becoming weaker and the other over developing. You should begin fertilizing in April for most plants but not newly transplanted or repotted plants. Evergreens will be happy and flourishing if fertilized from the middle of March on. Wait until the middle of May to fertilize deciduous trees, otherwise the leaves get too big. Start pinching back to control new growth and keep the plant's shape. Pinching terminal buds on a branch will force side branches to grow. Leave on buds that point in the direction you want a branch to grow, removing all others, particularly those on the underneath side of a branch. Cypress and juniper tips can be pinched back with your fingers at the joint. Best not to use shears or the ends will turn brown. Watch for wiring that may cut into the trees.
Fertilizing is particularly important because of the confined area in which bonsai grow. Underfeeding is better than overfeeding. Organic fertilizers such as cottonseed meal, fish emulsion or meal, blood meal, and bone meal are preferable to chemical compounds. Fertilize broad-leafed evergreens when new buds and growth appear in April. Apply small amounts of liquid fertilizer twice a month. Do not fertilize for two months after transplanting or repotting. Give acid-loving plants (camellias and azaleas) iron-based fertilizer. Don not fertilize fruit or berry bonsai with strong nitrogen when fruit is forming.
Summer - Continue rotating the position of the plants, as well as location (from sun to partial shade). Water regularly, preferably in the early morning or late afternoon. Overhead watering is beneficial, but avoid doing this during mid-day and the late evening. Weather can be very hot this month so be careful to guard against sunburn. Feed all fast growing decidiuous trees (except those with fruit, flowers or berries). May and June is the time for transplanting satsuki azaleas. Do not engage in any drastic pruning now that the growth period had ended. Wiring of smaller branches can be done. Continue with the usual fertilizing. Do not transplant or report in late summer.
Fall - The growth period is ending and the plant will not require as much watering, but do not allow their trees to dry out. Try to keep a delicate balance between the minimum water requirement and evaporation due to any hot, dry weather. By mid-September, most conifers and deciduous trees can be transplanted. Begin preparation for winter. Trim any strong growth on the upper portions of conifers and clean off any dead needles. November is the last month that bonsai can be safely transplanted. Focus should be on dormancy, not stimulating growth. On deciduous trees, remove any wires that may be cutting in to avoid losing that branch during the winter. Remove any remaining leaves on deciduous trees. Refrain from fertilizing, transplanting or wiring this month.
Winter - Bonsai are in their dormant stage. They require less watering because of colder weather and rainy days, however if weather is warm, check your plants daily. No fertilizing or transplanting this time of year. January is a good time to shape deciduous trees, but be careful because branches can be brittle. Consider pruning any fruit-bearing bonsai. You may begin grafting on deciduous trees as well as pines and conifers. The best months for grafting conifers are January and February. In general, February to early March is the best time to transplant, replant or change soil. The best indicators of a need for soil change is not time, but the condition of the bonsai. A vigorous, healthy bonsai does not need its soil changed and should not be disturbed.
This handout is not meant to be a formal guide for the care of bonsai. It is only an overview of some general topics and offers a few pointers and tips that may be helpful to the beginning bonsai hobbyist. A highly recommended instructional book is Bonsai Techniques I by John Naka.