When you see that the plant needs water, you must water thoroughly, which means “watering until all the soil is completely soaked.” To achieve this, a watering can with a rose with very fine holes is needed, so that the water comes out smoothly.
The bonsai are watered from above first, so that the humidity opens the pores of the soil. After a few minutes they are thoroughly watered again, until the water comes out of the drainage holes.
The tray that accompanies the indoor bonsai is used so that excess water does not drip on the furniture. After watering this tray should not be full of water. The level should not reach the holes in the pot. A bonsai that is always wet rots its roots. On the other hand, there are bonsai trees that, because they have a very pasty soil, cannot be watered well by this system, since the water slides down the sides of the pot and does not penetrate the soil. In this case, it would be necessary to make a transplant and change the soil.
You can find plastic and ceramic trays that prevent damage to furniture. It is not recommended that the bonsai stay permanently with the roots submerged in water.
Another option is to fill the tray with volcanic stone or Akadama, which creates environmental humidity with the remaining water.
So that the soil of these bonsai trees is well soaked, we can put them in the plastic tray that accompanies the indoor bonsai and leave them for about ten minutes. It is also necessary to water this way the bonsai that, by carelessness, have gone dry and do not soak well.
Never water with a vaporizer used to remove dust and give plants moisture. They are not used for watering.
They only moisten the surface of the pot, and the water does not reach the roots. In short: You have to water with an appropriate watering can or do it by immersion.
The small holes in the watering can rose help the bonsai better absorb water by imitating rainwater. You can find watering lances that adapt to hoses, or special watering cans for bonsai.
More information about watering cans, trays, hydrobons here.