As with many trees and plants, watering your Bonsai properly is crucial to keeping it healthy. The existence of dehydration due to lack of water or low humidity is the leading cause of death among Bonsais.
There are many different variables in place that affect the amount of water each individual plant needs. All soils have different water retention properties, the different species of Bonsai differ in individual water requirements, and even the different styles of pots can affect the rate at which the soil dries out. Drying out isn’t the only problem, over watering can cause serious damage as well, the key is to find the right balance of soil, water and air that you will develop a healthy root system and as a result, a healthy bonsai.
Water does much more than simply provide moisture for your tree, it also carries nutrients your plant needs and flushes out the excess salts that might build up otherwise. Another thing that water does that is an unseen benefit is to push out old gasses and pull in a fresh atmosphere.
Most people falsely think that the transpiration process happens only in the leaves but if this were true then plants that were completely cut back would not be able to grow back from the roots. It is the topsoil or first few inches that are the most important to the functions of any plant or tree, even huge oak trees get the majority of their nutrients from the top portion of the soil around them. This is actually the factor that makes the growing of Bonsai trees in shallow pots possible. The soil of a Bonsai has to be very porous to allow for better gas exchange and water flow which helps the root system grow to support the huge foliage mass you see on Bonsais.
According to Japanese custom you should water once for the pot, then for the soil, and finally for the tree itself. This assures that there is enough water for the tree after water has been absorbed by the pot and the soil. You should also pour water on the foliage as it also absorbs water and the process removes dust and pollutants.
Don’t rely on rain, water your plants anyway and just make sure the pot is draining properly. If you are watering with a hose, make sure you have an adjustable nozzle that can be set to mist and check the temperature before you put the water on the plant. Remember that watering is the most critical part of raising Bonsais.