Watering a bonsai is an art in itself. Weather conditions across the country vary from county to county and even in your own garden conditions will vary due to exposure to the sun, wind and available rainfall.
The simple answer to the question "when do I water my bonsai" is of course when it needs it. In the height of a warm summer you will need to water your bonsai much more regularly and at some times daily to keep it at its peak condition. In a rainy spring week you may never have to water it at all.
One tip is to allow a small weed or flower to grow in the bonsai pot alongside the tree - when this wilts you will know that the tree needs watering.
There are a number of factors which determine the frequency of watering -
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- The size of the tree.
- The type of the tree.
- How quickly the tree grows.
- The size and type of pot.
- The amount of available substrate (soil).
- How long the substrate will hold on to the water through absorbtion.
- How compacted the substrate is.
- How much underplanting e.g. moss exists.
- How quickly the tree dissipates moisture into the air through the leaves.
- The exposure of the tree to weather conditions e.g. sun, wind.
- Proximity of solid walls. (which absorb a lot of heat and let it back out again).
- Is it kept indoors.
One thing to bear in mind is that watering a bonsai serves two main purposes 1) to provide moisture and 2) to draw fresh air into the root area as the soil dries out. A by-product of watering is that any nutrients applied as fertiliser will also be washed down through the pot to the roots. Many trees prefer to get a little dry between watering.
So there is no hard and fast rule. Some trees also prefer dryer roots than others (e.g. pine) so knowing your own wee tree makes a difference.
Someone new to bonsai will not yet appreciate how their tree performs so the best advice I have heard is to send it for a "swim" once a week. That is to place the tree (and pot) into a basin of water which covers the top of the pot by at least an inch for around half an hour. No your tree won't drown but it will take in as much water as it needs and the whole rootball of substrate will get wet. When you lift your tree from the water the action of the water leaving the pot will draw fresh air into the roots. This applies to indoor bonsai trees as well.
Be honest with yourself- no god designed a tree to grow in anyone's kitchen or bathroom. So why do we keep them inside? To appreciate them of course! The trees we grow inside most of our houses (as available from many multi-store chains) are normally tropical or sub-tropical species which are used to hot weather but a much more humid atmosphere than we live in. How can we combat that - mist the tree daily, sometimes twice, send it for a weekly swim (see above), don't keep it above a radiator or other heating device.
Type of water
Nope water isn't just H2O. Water out of the taps in the UK has been treated by the local water board and contains levels of chlorine and chloramines which are the chemicals that make our tap water safe to drink for humans. Your local water board web site will normally advise of the levels of chemicals in the water. Rain on the other hand has fallen through the atmosphere and on the way has collected some of the micronutrients that trees love and helps to keep them alive. Using rain water on your tree helps greatly to maintain soil health, that is, the natural balance of the organisms living in the soil. Some trees and in particular Rhododendrons (Azalea) perform much better if only watered with rain water which has been left to stand for a few days.
Does this mean that you can only water your trees with rainwater or water out of a bottle. No, you can water it with tap water but rain water is by far the best. So collecting some in a clean bucket left outdoors makes a lot of sense.
Remember you will only get out of your bonsai tree what you put in to it!