No plant can survive without water and the need is even greater for bonsai trees. They are grown in shallow containers with little soil and rely on rain and artificial watering for their moisture
Most bonsai trees that die do so from a lack of water or from being in low humidity for too long. They rely on water to grow as the water is absorbed from the compost to the roots and then the nutrients are distributed throughout the tree. If water is not given so that these important processes can ocurr the the tree will die.
There are a number of factors which will determine how much water bonsai trees require:
Some species will require more water than others
Different soils will dry out at different rates
Small pots may dry out more quickly than larger pots and what the pot is made of will also play a part in how quickly it dries out
Plants whose roots a fully grown will absorb more water than younger trees with smaller roots.
What you need to do is take the time to observe and get to know your individual bonsai trees. As you become familiar with them you will learn when they need water by looking at the soil and the color and growth of the leaves. Each tree should be checked regularly and watered when required. Never allow the soil to dry out completely.
Watering is usually required once or twice a day in summer and maybe every three or four days in winter. Even though there may have been some rain, don’t assume that it will have been enough for the tree as it may only have dampened the surface of the soil. Once again, you need to check each plant, testing the soil about an inch below the surface; you can use a water meter to help you or just poke your finger into the soil.
Morning is the best time of day to water the bonsais so they are ready for the heat of the day. Don’t just moisten the soil but give them a thorough soaking so that the whole container is wet and there are no dry spots where the roots could die. In Japan they often water all the trees then go back to the beginning and water them all again to ensure the job is done correctly. Allow the water to drip out of the bottom of the pot and you may need to prop it up slightly so that it all drains away.
Gentle watering is required; if you use an ordinary garden hose you may find the flow of water too stong and the soil will be washed away. Make sure to use a fine mist or a watering can with a fine rose so as not to disturb the soil. As well as watering the soil you can also spray the foliage with water. The leaves can absorb water and the spray will wash any dust or pollutants away.
However, excessive watering can also damage the bonsai tree; the effects of this will not be immediate but can take some time to become apparent. If the roots are continually surrounted by water then they are unable to absorb air which is essential for a healthy tree. The fine root hairs become suffocated by the excess water, they will die and eventually rot. Of course if the tree cannot get air and nutrients from the roots then the leaves and braches will suffer and also eventually die. In the mean time a sick tree is more susceptible to attack by diseases and insects.
Growing bonsai trees is a fascinating hobby and by using well draining, moisture retaining soil and getting to know your plants you should be able to meet the challenges that watering presents.