Unlike a houseplant, bonsai trees use a "free draining" type of soil because their roots cannot tolerate "wet feet". In addition, they are grown in significantly less soil and, therefore require more watering. Factors such as tree location, temperature, lighting conditions, quantity of soil used, and the changing seasons will determine the frequency of watering. You can get to know when your tree needs to be watered by observing the foliage, testing the soil with your index finger just below the surface, or just by the weight of the pot. (The drier the tree, the lighter it will feel.) To take the guesswork out of watering, an inexpensive moisture meter which works very much like a thermometer comes in handy. Insert it into the soil and the movement of the needle will tell you if it is time to water.
Rainwater is best for watering plants, but tap water that has stood for a few hours is adequate. In summer, trees should be watered in the early morning or late afternoon to avoid the midday heat. This will prevent the leaves of the finer bonsai from burning. In winter, water early to permit any excess to drain before the night frost. Plunging the pot into a bowl to soak is ideal for recently potted trees, small collections and for trees that have dried out. Be sure to drain properly, however!
All trees grow in more humid conditions than our homes, offices and dormitories. So what can we do to provide this essential humidity ? Misting the tree is only beneficial for a short time, so what we recommend is to place the tree on a humidity tray and add water to the tray. As the water in the tray evaporates it creates a humid environment around the tree 24 hours a day. When the water in the tray is gone, add more water. It’s a good idea to separate the pot from the water in the tray by adding some pebbles to the bottom of the tray. This will prevent any roots from sitting in the water.
Because bonsai trees are cultivated in limited amounts of soil, adequate feed is very important. As a general rule, a small amount of feed is given in the spring and a larger amount in the fall. Feed for bonsai should contain three principle ingredients; nitrogen, phosphoric acid, and potash. It is also a good idea to use a fertilizer containing "chelated" iron. Water before fertilizing your tree and then apply at half the strength recommended by the brand’s manufacturer. Rotate the use of brands since different manufacturers add different amounts of trace elements and minerals. You may find it easier to use slow release fertilizer granules whose nutrients are released with each watering.