One of the main causes of damage or even death in Bonsai is incorrect watering techniques. The most immediate and obvious effect of improper watering is under-watering; simply put, a bonsai whose soil mix dries out will die. A plant that is over-watered will not be as noticeably in danger; the effects of the health of the plant will be more gradual. It will take some experience to get comfortable with the proper way to water bonsai; in fact in Japan it is said it can take up to three years to learn correct watering techniques. By understanding the basics of bonsai watering, however, it may be possible to minimize damage while learning the skills required.
Plants stay alive by a continual flow of water through the plant. The water is drawn from the soil through the roots, up through the plant and converted to gas which is expelled through the leaves. If the bonsai does not have enough moisture to gather from the soil, the first elements of the plant to be affected will be the branches and foliage. Finally the roots will dry out and possibly collapse. Often by this point, it is too late, and watering may not be enough to save the plant.
The effect of too much water in the plant will be that roots will suffocate. As well as water, roots also need air. If there is too much water in the soil, the supply of oxygen will be cut off. While the result will take longer than the damage caused by over-watering, eventually the roots will become unhealthy and can rot. This invites microorganisms which can spread to the rest of the plan. Over-watering will also invite pests.
A common mistake for first time bonsai growers is to water at the same time everyday as part of a schedule rather than inspecting the condition of the soil. Simply watering daily without investigating the soil will lead to over-watering. Observing the top layer of the soil should be enough to determine if the plant will need watering. The amount of watering done will also depend on the season, so again, it is important to keep a watchful eye on the plant and learn the individual need of different trees.
By being sure the soil mix is dry before watering, one can prevent the effects of over-watering. When it is time to water, though, the plant should be watered completely. It is a good idea to water a plant twice, once to moisten the soil, and a second time a few minutes later. The first watering will moisten the soil so that it is ready to accept the water of the second application. The water should be draining out of the drainage holes as the plant is watered a second time. Some enthusiasts will suggest watering by regular immersion; this however is not recommended. If a plant needs to be immersed in water to penetrate the soil mix, it is best to replace the soil with something less dense.