Hardy bonsai include all varieties that naturally occur in temperate regions of the world. Therefore, these trees require four distinct seasons including a cold winter dormancy. Hardy bonsai include Juniper, Pine, Larch, Maple, Hornbeam and others. These are generally not suited for growing indoors.
Proper watering is critical! Feel the surface of the soil daily. If the surface is dry or only slightly moist it is time to water. Water from above and thoroughly soak the soil. Watering the foliage as well as the soil is also beneficial for your tree. Allow excess water to drain away quickly and never let your tree sit in water for extended periods. During hot dry weather or under windy conditions your tree will need frequent watering. Very small bonsai in small pots can sometimes dry out in a day or less. Most do not like their soil to become really dry. Consistent bone-dry conditions are likely to kill your tree.
Conifers such as junipers, pines and spruces thrive in direct sunlight and can gradually be acclimated to full sun. Deciduous trees such as maples and elms need to be grown in very bright light to part sun. Hot sun and wind can cause leaf burn, so you may need to provide some protection from hot midday sun. Very small bonsai in small pots do better in some shade.
Hardy bonsai need a cold winter dormancy to grow well. Once temperatures are consistently above freezing in spring, your bonsai should be placed outside and allowed to remain there all spring, summer, and fall. Once night temperatures drop below 35 degrees your tree will need some winter protection from the cold and wind. An unheated garage, basement, cold frame or a very, very cool room in the house are all suitable locations. Bonsai that drop their leaves can be placed in the dark. Even though dormant trees use much less water, completely drying out can still be dangerous so remember to check regularly for water.
Hardy bonsai do best with frequent feeding during the growing season (spring, summer and fall). Organic feeding can be done with Green King pellets. Miracle-gro or Schultz water soluble fertilizers can be used but should be mixed only half as strong as the directions recommend. The idea is to keep your tree healthy and maintain very moderate growth. Do not feed in the winter.
Most bonsai need to be repotted and/or root pruned every 2 to 3 years. It is best done in the spring once your tree has started growing well again. Normally a quarter to half of the root mass is removed to allow for fresh soil and new roots to grow which will reinvigorate your tree. Occasional trimming and wiring will also be necessary to maintain form. A good book or bonsai professional might be a good consultant.