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How to water a bonsai tree: Improper watering is probably the largest cause of problems with bonsai, in Japan it is said that it takes years to learn to properly water bonsai. First of all don't water your tree(s) on a schedule, check your trees everyday and only water as necessary. Dig into the soil with your finger (the deeper the better without scratching up the roots too much) and test to see if the soil is damp.
Another method is to insert a kitchen skewer or chopstick to the bottom of the pot and leave it there. Take it out daily to check it; if the skewer has soil stuck to it don't water. Touch the wood to your cheek or underside of your wrist, if it is damp there is no need to water. This method is particularly useful for trees that are in heavier organic/peat based soils. It becomes less effective when proper bonsai soils are used. After a while you will get used to the requirements of your particular trees. You will find that temperate trees will need less water in late fall and winter than they will in spring or summer.
Okay, if the tree needs watering, water using a watering can that has a fine rose so as not to disturb the soil. There is an old saying that for bonsai it always rains twice, meaning that a single watering is not sufficient to thoroughly wet the soil. Water the entire surface of your tree until the water drains freely from the bottom of the pot, wait for about 10-15 minutes and repeat. Some even recommend a third watering, and if you have the time and are so inclined it certainly will not hurt. You will never over water your trees in this way. Correct watering is not about the quantity of water used but the frequency with which it is applied. The goal of watering is to completely saturate the soil each time you water, repeating only as necessary. A little water often is the wrong approach.
Another technique that can be used is to immerse the entire pot in a sink full of water for about 15 minutes. This is not done as a method of routine watering but rather under special circumstances. It is especially useful after re-potting to ensure that the dry soil is completely saturated the first time. It can also be used as an emergency measure if a tree has inadvertently been allowed to dry out. Sometimes commercially prepared bonsai are potted in such a poor soil medium that immersion is required to thoroughly wet it. If this is the case consider it a warning sign that the soil is in poor condition and should be replaced in short order. Immersion is also time consuming and becomes increasingly impractical as your collection grows.