Never doubt that the quickest way of killing a bonsai is to allow the compost to dry out completely.
However, though the effects of under-watering are immediate, over-watering a bonsai also causes ill-health in trees. The effects of continual over-watering takes much longer to become noticeable and can often be difficult to diagnose.
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 accesses. 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.
HOW OFTEN SHOULD I WATER?
As has already been discussed, it is important to avoid the effects of under-watering and overwatering. So how do you water a bonsai correctly?
Firstly, NEVER water to a routine. Simply watering on a daily basis without first observing the condition of the bonsai soil is often carried out by beginners following the advice of well-meaning bonsai retailers. Bonsai can indeed require water on a daily or even twice daily basis, particularly in hot weather or early Spring. 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 means it is permanently wet, leading to problems associated with overwatering.
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 composts change colour 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 vigour, 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 centimetre 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 overwatered.
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 overwatering. When the tree does require water though, it needs a thorough soaking. Avoiding overwatering does not mean just 'moistening' the bonsai soil instead of watering it properly. Each time you water, it is important that the entire rootsystem 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.
SUITABLE WATER FOR BONSAI
Water your bonsai with plain tapwater. In areas where the tapwater is hard, occasionally watering with rainwater is useful to rid the soil of any build up of salts, but is not essential, unless the tapwater is particularly hard and white salt deposits start to appear around the pot or trunkbase.
Rainwater can be collected in a water butt attached to the downpipe of a shed or house, though it would be difficult to collect enough water to meet the needs of a large collection on a daily basis.
Do not use water obtained from water softeners; many water softening systems increase the volume of salts diluted in the water to great detriment to a bonsai.
WATERING BY IMMERSION
Some bonsai sources willl recommend watering a bonsai by immersing the bonsai pot in water for a while. This is not a recommended way of watering your trees.
Watering by immersion is a way of getting water to penetrate compacted, very poor quality organic soils. If a bonsai needs to be watered by immersion, it is in trouble and particularly prone to the effects of overwatering and weak roots.
If a vendor recommends that you water by immersion, suspect that this is because the tree is in poor soil and is therefore difficult to water properly. Also suspect that the tree will be weak, slow growing and very possibly have root-related problems.
Make holes in the soil around the edge of the pot using a chopstick or similar, to allow water to penetrate the soil and repot at the earliest opportunity (most commonly during the following Spring) into a better quality (preferably inorganic) soil.
