Bonsai watering calculator

A bonsai doesn’t have a watering schedule, and the most common answer to “how often should I water it” — every seven days — is the one most likely to kill it. What a calculator can work out is how often that shallow pot is likely to dry out, so you know how often to go and feel the soil. That’s what this does.

Where does it live?
Which kind of tree is it?
What’s the weather doing where you are?
How big is the pot?
What’s it planted in?

Why this gives you a check-in rhythm, not a schedule

A watering schedule assumes the soil dries at a constant rate. It doesn’t. A bonsai lives in a shallow tray of coarse, fast-draining soil, so it can go from properly moist to bone dry in a single hot afternoon — or stay damp for a week in cold weather. Follow a calendar and you will inevitably water a soaking pot in November and skip a parched one in July.

So the answer is to water when the top centimetre of soil has gone slightly dry, and to check often enough to catch that moment. The interval between waterings is an outcome, not a setting. This tool estimates the checking, and the finger test makes the actual call: push a finger about 1 cm into the soil near the edge of the pot; slightly dry means water now, still moist means wait.

Where the numbers come from

The ranges are the ones in our own guide, How often to water a bonsai tree: once or twice daily for an outdoor tree in hot summer weather, every one to three days in spring and autumn, weekly or less in winter, and every two to three days for an indoor tropical. The result is held inside the published range for your situation, so no combination of answers can push it somewhere the guide doesn’t support.

Pot size, soil mix and species then shift it within that range, because those are three of the five things the guide names as changing the drying rate — a shohin pot of gritty akadama holds almost no reserve, while a large, deep pot with plenty of organic content stays wet far longer, and a pine is comfortable running on the dry side where a maple or an azalea is not. Those shifts are an estimate, not a measurement. A heatwave, a draughty room or a windy balcony will all move the real answer, and the soil is the only reliable source.

If you’re not sure, check anyway

Overwatering and underwatering are both possible, but they are not equally dangerous. Constantly wet soil starves the roots of oxygen, and by the time the leaves yellow and drop the roots have already rotted — recovery is slow and uncertain. An underwatered tree droops, tells you within a day, and usually forgives you after a good soak.

That asymmetry is why this tool never errs towards waiting longer. Looking at a pot costs nothing; only watering it does. If the check says the soil is still moist, you have lost two seconds. That’s the whole point of checking rather than pouring.

The method is simple. Remembering across a collection isn’t.

Which tree you soaked this morning, which you skipped because the soil was still damp, and whether the maple has now gone three days in a heatwave. Bonsai keeps a care calendar for the whole collection — watering, repotting, pruning and wiring — with gentle reminders instead of a nagging seven-day timer. On your device, no account.

Get Bonsai on the App Store →