Water a bonsai when its growing medium needs moisture, rather than on a fixed weekly schedule. Check it daily while learning how quickly its pot dries, and more often in hot or windy weather. Checking does not mean watering every time: a pot that is still sufficiently moist needs a different response from one drying rapidly.
Bonsai describes a growing technique, not one plant species. Identify your tree first. Its species, root condition, container and growing conditions determine the appropriate moisture range.
Build a checking routine
Look at the medium and gently check moisture below the surface without damaging roots. Compare the weight of the pot before and after watering if you can lift it safely. Use several observations rather than relying on a cheap meter or a universal one-centimeter test in every kind of substrate.
Write down when you check and when you water for the first week. Note rain, heat and wind. This gives you a starting pattern for that tree, which you should reassess when the weather changes or the plant moves.
Do not wait for severe wilting or the root ball to shrink from the pot as a routine watering signal. Those can indicate that moisture stress has already developed. Conversely, a dry surface does not prove every part of a deeper root ball is dry.
Water the whole root area gently
When watering is needed, apply it slowly across the medium so the root ball is wetted, allowing excess to drain. A fine rose helps avoid displacing the surface. Check that water entered the medium rather than simply running down a gap at the edge. Empty collected drainage water instead of leaving the pot standing in it.
The RHS bonsai guide explains why shallow containers can need very frequent attention in summer and still need checking during winter. A light surface splash or leaf misting does not replace root watering.
Why the interval changes
A small soil volume, a large active canopy, wind and heat can shorten the time between waterings. Cooler conditions and reduced growth may lengthen it. Indoor heating can also change drying. Use these factors to decide when to check, rather than turning each into another rigid timetable.
Rain does not necessarily wet the entire root ball, especially beneath dense foliage or shelter. Check the pot afterward. An outdoor tree’s winter needs also differ from those of a tropical bonsai kept in a warm room. Do not move every bonsai indoors just to simplify watering.
Match the substrate to the tree
A suitable medium must retain useful moisture while allowing drainage and root aeration. Akadama, pumice and lava rock are components used in some bonsai mixes, but their proportions and particle sizes matter. The name of one ingredient cannot tell you the watering interval.
Freer drainage does not automatically mean less frequent watering. Nor should an already waterlogged mix be watered more often to prevent waterlogging. If the medium remains saturated or has deteriorated, ask an experienced grower about an appropriate repotting plan for the species and season.
Water quality and problem signs
RHS favors rainwater to limit mineral accumulation. The suitability of tap water depends on its quality and the plant; do not assume all treated tap water is poisonous or that every collected rainwater source is clean. For persistent problems, check the water supply and feeding routine with knowledgeable local advice.
Yellow leaves or wilt do not by themselves diagnose overwatering. Check moisture, drainage, recent changes and root condition. Avoid repeatedly soaking wet roots or automatically cutting roots and repotting a stressed tree without identifying the cause.
The useful habit is observation: establish what a properly watered pot feels like, watch how it changes, and respond before serious drying or persistent saturation develops. For general container techniques, see RHS container-watering guidance.
