by Harry Harrington (http://www.bonsai4me.com/)

Bonsai, as with nearly all other types of cultivated plants, require moisture at their roots to survive. Without a continual source of moisture, the tree is unable to continue its life process, initially losing leaves, then branches and finally the entire tree can die. Without doubt, the quickest way of killing a bonsai is to allow the compost to dry out completely.

However immediate the effects of under-watering are though, over-watering a bonsai probably results in more dying trees. The effect of over-watering a tree on a continual basis takes much longer to become noticeable and is often wrongly diagnosed.

Established plants and trees growing in the ground have the ability to 'adjust' to their habitat and the quantity of water that is available to them. If there is not enough water available to the root system, the roots will spread out into the soil until enough moisture can be reliably acquired. Thus plants growing in relatively dry areas will have far-reaching root systems that will continue to spread out until a reliable source of moisture can be found.

On the other hand, trees growing in damp conditions where moisture is permanently available in the upper levels of the soil, will tend to have shallow root systems as they have easy access to moisture.

In the confines of a pot, a bonsai loses this ability to self-regulate its exposure to moisture. It is unable to govern how much or how little water it has available. The compost in a bonsai pot is also far less stable than soil in the ground; its ability to dry out is greatly increased, and it is greatly affected by the outside influences, such as the weather and the surrounding ambient temperature.

Correctly watering your bonsai is a skill itself and is not as straightforward as one might expect when starting out. It is often said in Japan that it takes 3 years to learn to water correctly. It can sometimes take three years of tree losses before a bonsai enthusiast realizes that it is his/her watering regime that might be the cause!

THE EFFECTS OF UNDERWATERING AND OVERWATERING
Plants rely on a continual flow of water to stay alive and to grow. Water is absorbed from the compost into the roots by a process known as osmosis, the water is then pulled up the body of the plant and is released into the atmosphere through the foliage. This process allows the plant to distribute vital nutrients throughout its structure. However, without a source of moisture at its roots, this flow of water is interrupted, and the plant structure quickly collapses and dries out. Leaves and branch tips are the first areas to be affected, followed by branches. Finally the trunk and roots themselves collapse and dry-out, by which time it is unlikely that the tree will survive. Application of water at this point is probably too late; moisture can actually be absorbed out of the roots back into the wet compost in a process known as reverse osmosis.

As previously mentioned, the effects of over-watering are far more subtle and can take a relatively long period of time to detect. Over-watering creates an environment for the root system that is permanently wet. Roots need oxygen to 'breathe', and the presence of too much water reduces the ability of the compost to absorb air. This in turn causes the fine root hairs to suffocate and die. The immediate effect to the tree is a loss of vigor as parts of its root system are unable to grow and/or dieback.

More worryingly, the dead roots start to rot. Naturally occurring bacteria are able to colonize the dead tissue and, in very wet composts, are able to thrive. As the root system continues to die back from the effects of over-watering, the root-rotting bacteria are able to spread throughout the root system and slow (if not completely stop) the ability of the tree to seal the remaining live root-tips. Gradually the live portion of the root system becomes smaller and, as it does, it is able to support less of the visible top growth of the tree.

Foliage on the tree will start to yellow and drop; smaller branches will shrivel and die back. As the live portion of the root-ball becomes even smaller, it is eventually unable to support the primary branches and the trunk, causing the tree to die.

Root-rot is often only detected at repotting time in Spring. Rotted roots will be found to be black and will disintegrate when touched. The only reliable way of stopping root-rot is to cut away all dead areas of root.

HOW OFTEN SHOULD I WATER?
As has already been discussed, it is important to avoid the effects of under-watering and over watering. So, how do you water a bonsai correctly?

Firstly, NEVER water to a routine. Simply watering on a daily basis, without observation of the bonsai soil first, is often carried out by beginners following the advice of well-meaning bonsai retailers. To allow the tree to go without water for any length of time is disastrous and should be avoided at all costs. Bonsai can indeed require water on a daily or even twice daily basis, particularly in hot weather. However, watering to a routine commonly leads to permanently wet compost at other times. If the compost does not lose some of its moisture content between waterings, it becomes permanently wet, leading to problems associated with over watering.

Instead, trees should be checked routinely (at least on a daily basis), so their water requirements can be observed, and they can then be watered when they actually require it. The surface of nearly all bonsai compost changes color and appearance when it starts to dry out. With careful observation, it is always possible to tell whether or not the compost surface is dry or not. This can take anything from 12 hours to a week or longer after watering, depending on a variety of factors such as the surrounding ambient temperature, plant vigor, pot size and whether it has rained or not. In the UK, trees tend to need watering daily during the Summer, but with lower temperatures and increased rainfall during Autumn, Winter and early Spring, watering needs can change day by day. Never assume that, because it has rained, your tree has received enough water, particularly during the Summer. Often, it only rains enough to wet the upper layers of the compost.

The correct time to water is when the top centimeter of the compost has started to dry out. With regular observation of your trees on a daily basis, you should be able to apply water when it is actually required. Allowing the compost to dry a little between waterings will ensure that they are not over watered.

Different trees have different water requirements. Try to water individual trees in a collection as they require it, rather than en masse.

HOW SHOULD I WATER?
If you allow the compost to dry a little between watering, you will avoid the effects of over-watering. When the tree does require water though, it needs a thorough soaking. Avoiding over watering does not mean just 'moistening' the bonsai soil instead of watering it properly. Each time you water, it is important that the entire root system and body of compost is properly wetted to avoid pockets of dry soil where roots could be left to dry out and die.

The Japanese have an adage for watering; 'For bonsai, it rains two times'. Water should be applied twice. The first watering wets the soil so that any dry soil particles will accept moisture better, as they tend to shun water at first. Water should be applied all over the compost surface until it can be seen to run out of the drainage holes. The second watering should be left for 10-20 minutes, by which time any previously dry areas of the compost will be ready to accept water. For a second time, water thoroughly all over the surface of the compost until water can be seen to run out of the drainage holes of the pot. The compost and root system should now be sufficiently wetted until the next watering is required.

OTHER WATERING PROBLEMS
Bonsai composts should always be free draining. Compacted, poor-draining composts can cause many of the problems associated with under- and over-watering. Bonsai compost should be open enough to allow water to penetrate throughout and to ensure that excess water is able to pass out through the drainage holes immediately.

Compacted soils slow the penetration of water, which will tend to sit on top of the compost surface and run over the sides of the pot or down the inner edges. Once properly wetted, poor bonsai composts can hold too much water and little oxygen, which can eventually lead to problems associated with over-watering.

Extra care should be carried out when watering trees potted in poor draining compost which should then be replaced at the next repotting.