How often to water a bonsai: checking the soil, light, seasons and indoor trees

TUREFF• •0

Watering kills more bonsai than anything else, both too much and too little. A bonsai pot is shallow, holds little soil and dries out much faster than a garden pot, but soggy roots rot just as surely. The answer is not a fixed schedule. It is learning to check.

Why a schedule does not work

How quickly a bonsai dries depends on the species, the pot size, the soil mix, the weather, the season and how much the tree is growing. A tree that needs water twice a day in a hot summer week may need it once a week in a cool, wet spell. Watering every Sunday because it is Sunday will be wrong most of the time.

How to check the soil

Feel the surface. Push a fingertip slightly into the soil. If the top just under the surface feels dry, it is close to time.
Use a chopstick. Push a wooden chopstick into the soil and leave it a while. If it comes out damp and dark, wait; if dry, water.
Lift the pot. A watered pot is noticeably heavier. With practice, weight alone tells you a lot.
Look at the colour. Many bonsai soils go lighter as they dry.

Aim for soil that is slightly dry on top before you water, but never bone dry right through.

How to water

Water thoroughly, until water runs freely out of the drainage holes.
Water gently with a fine spray so the soil is not washed out.
Wait a minute and water again, so the whole root ball is soaked rather than just the top.

Do not give small sips; that wets the surface while the roots lower down stay dry. A pot without drainage holes is not suitable for bonsai.

Seasons

Spring: growth starts and water needs rise quickly. Check often.
Summer: hottest and driest, especially in sun and wind. Small pots can need water more than once a day. Some growers shade pots in midsummer to stop the roots overheating.
Autumn: needs fall as growth slows.
Winter: dormant outdoor trees use little water, but they still need checking. Trees sheltered from rain under a roof or in a cold frame can dry out unnoticed.

Light, the other half

Water and light go together: a tree in strong light grows and drinks more.

Outdoor trees generally want plenty of sun, with some species preferring afternoon shade in hot climates. Check what suits your species.
Indoor tropical trees need the brightest spot you have, ideally close to a sunny window. Light falls away very quickly a short distance from glass, and a dark corner will slowly starve the tree.
Grow lights can help indoor trees through dark winters.

Indoor trees in particular

Heating dries the air and the soil; check more often in winter when radiators are on.
Keep trees away from radiators and draughty doors.
A shallow tray of gravel and water under the pot (pot sitting above the water, not in it) raises the humidity around the tree a little.
Rotate the tree now and then so it grows evenly towards the light.

Signs something is wrong

Underwatering: wilting, crisp leaves, soil pulling away from the pot edge.
Overwatering: yellowing leaves, soil that stays wet for days, a sour smell, weak growth.
Both can look similar; checking the soil tells you which.

Holidays

For a few days, water thoroughly and move outdoor trees into shade. For longer, ask someone to check them, and show them how to water properly first. How do you judge when to water? Share your method, or ask about a tree that is struggling, in the comments.