Bonsai leaves turning yellow: find the cause and fix it

If your bonsai leaves are turning yellow, that's not a problem in itself but a signal. Your tree is telling you that something isn't right. To find the real cause, look at three things: Which leaves are yellow (old ones low down and inside, or young ones at the tips)? How does the soil feel? And what has changed in the last few weeks?

1. Too much water and root rot

This is the most common cause in bonsai. If the soil stays wet all the time, the roots can't get air and start to rot. Sick roots can't take up water or nutrients, so the leaves turn yellow.

• Soil is always damp, often with a musty smell
• Yellowing usually starts low and inside, leaves feel soft
• Water is standing in the saucer

2. Too little water

Sounds odd, but it looks similar. Once the root ball has dried out completely, the tree sacrifices leaves to survive.

• Soil is bone dry and pulls away from the pot edge
• Leaves turn yellow, then quickly dry and crispy

3. Too little light

A bonsai needs far more light than most rooms offer. When it's too dark, the tree can't feed every leaf and drops the inner, shaded ones. Typical in winter and in spots far from the window.

4. Nutrient deficiency (chlorosis)

Chlorosis simply means the leaf is losing its green. A trick to tell the types apart:

• Yellow between green veins on young leaves → usually iron deficiency. Often there is enough iron, but the tree can't reach it because the soil is too limy. This happens especially with azaleas, which need acidic soil and dislike hard tap water.
• Even, pale yellow on older leaves → often a general lack of nutrients, for example because the tree hasn't been fed for a long time.

5. Perfectly normal: autumn and old leaves

Not every yellow leaf is an alarm. Deciduous trees such as the Japanese maple change colour in autumn and drop their leaves. Evergreen trees also replace a few old leaves now and then. As long as the shoot tips look fresh and green, all is well.

6. Pests

Spider mites, scale or mealybugs suck sap from the leaves. Then the leaves look finely speckled or sticky, often with tiny animals or webbing underneath. More on this under spider mites.

7. A sudden change of location

The ficus in particular reacts strongly when light and temperature change suddenly, for example after buying, moving into winter quarters, or even just turning the pot. Many leaves then turn yellow and drop within a short time. It's stress, but usually not a death sentence. A Chinese elm also often drops leaves for a while after a move.

Step by step

1. Check the soil. Push a finger about 2 cm into the soil. Wet? Don't water for now. Dry? Water thoroughly until water runs out of the bottom. How to water properly is explained in our guide to watering bonsai.
2. Empty the saucer. The tree should never stand in water.
3. If you suspect rot, look at the roots. Gently lift the tree out of its pot. Healthy roots are light-coloured and firm, rotten roots are dark, mushy and smelly. If only a few are affected, let the soil dry out and water more sparingly from now on.
4. Improve the light. Place the tree as close as possible to a bright window. Outdoor trees belong outside in summer. Tips for indoors are in our guide to indoor bonsai.
5. Check under the leaves. Hold a sheet of white paper under a branch and tap it. If tiny dots start crawling around, you have pests.
6. Feed only once the tree is healthy. If everything else is right and the leaves stay pale, feed regularly with bonsai fertiliser during the growing season. For azaleas, use low-lime water (such as rainwater) and suitable acidic soil.
7. Be patient. Once the tree has a good spot, leave it there. Give it a few weeks and watch the shoot tips.

What NOT to do

• Don't water more out of worry. This is the most common mistake and makes root rot worse.
• Don't feed heavily. A sick tree with damaged roots can't use fertiliser, it only adds harm.
• Don't keep moving it around. Every move means new stress.
• Don't repot or prune in a panic. That weakens the tree further.

When to see a bonsai nursery

Take the tree (or photos) to a specialist nursery if

• almost all leaves turn yellow within a few days,
• large parts of the roots are mushy or the soil smells rotten,
• nothing improves after 3–4 weeks of better care.

Is it still alive? Very gently scratch a little bark off a thin twig. If it's green underneath, the twig is alive. More basics in our bonsai care guide.