Why Has My Bonsai Got Yellow Leaves? (and How To Fix It)

Bonsai trees can often get yellow leaves which can be quiet worrying.

There are all sorts of reasons why this happens, sometimes it’s normal and sometimes might be a sign that something has gone wrong.

I cannot say exactly what has caused your leaves to turn yellow, but hopefully you can use the information in this article to try and work out what might be happening and correct it.

Low Light Levels Cause Yellow Leaves

A tree needs light to survive and if the leaves do not get enough light they will start to turn yellow.

Leaves are filled with chlorophyll (which is used in photosynthesis to make energy).

Chlorophyll is what gives the leave its green colour.  When a leaf does not get enough light the leaf will stop producing chlorophyll and the leaf will start looking yellow.

If your tree does not get enough light you will see this happen.  This can be a common problem if you are growing your tree inside, without enough light.

You might also see this on the inner or lower leaves of your tree. These leaves can be shaded out by the ones above it. As they are not getting enough light they will start to turn yellow. This isn’t really a problem, but if you want these to grow you will need to trim your tree to allow more light to these inner areas.  

(You can read more about – Grow Lights for Bonsai)

Watering Issues Cause Yellow Leaves

Watering is essential for your tree. However, if you mess up the amount you give your trees it can cause your leaves to turn yellow.

When you underwater a tree it will wilt and eventually dry out, the leaves will start to die and turn yellow, before going brown and dropping off. This can happen quite quickly on a hot day, so make sure your tree does not dry out at any time.

You can also cause yellowing of your leaves if you add too much water. This problem takes a little time to occur, so one heavy watering isn’t going to be an issue. The problem usually comes from a lack of drainage. If you roots are just swimming constantly in water they will start to rot.

The roots need a balance of air and water, so being constantly submerged is like they are drowning.

As the roots rot and die, the tree will no longer be able to send water up into the tree, so it starts to struggle and turn yellow.

You need to address either of these water issues quickly or it will kill your tree. The yellowing of the leaves is a warning sign that your tree is in a lot of trouble.

(You can read more about – How To Water A Bonsai)

Root Problems Cause Yellow Leaves

As I’ve mentioned above, over watering can cause root rot. However, other issues with your roots can also cause your leaves to turn yellow.

If you tree is root bound, or your soil is too compact it can prevent water getting to your roots. This then means your tree is basically under watered and can cause yellowing of the leaves.

If your roots are eaten by insects, it will cause a similar effect to having them rot off and again can cause yellowing of the leaves as the tree can no longer take up any water properly.

You should inspect your roots for problems if you think this could be the cause of the problem. It your roots are root bound, you will need to repot your tree in spring. Until then you will have to make sure you tree is getting enough water by giving it more and trying to really soak the root ball to make sure it gets enough.

Nutrient Deficiencies Cause Yellow Leaves

This is a common reason why trees in nature get yellow leaves, but I believe it is unlikely the cause in bonsai trees.

Bonsai trees tend to be fed a lot and are also repotted often. This means that having a deficiency in a certain nutrient is unlikely.

However, it is not impossible. If a tree is planted in a purely inorganic soil and has been in the pot for a while, it could possibly be lacking in nutrients. The main ones that usually cause yellowing when they are deficient are magnesium and iron.  

A deficiency in nitrogen can also cause yellowing; however, as we feed our trees fertiliser so often I really doubt this is the cause of your issue.  

If you do think this is the problem, make sure you rule everything else out first before feeding the tree. You do not want to try and feed a sick tree as it will probably kill it. If your tree is actually lacking nutrients, a quick feed should green it up rather quickly.      

(You can read more about – Fertilising Bonsai)

Stress Will Effect Leaf Colour

A tree can be stressed caused by any number of factors and the tree often shows it is suffering from stress by having yellow leaves.

Basically being exposed to any kind of environmental extreme will cause a tree stress.

Heat/Sun

Too much sun and heat is a very common stressor. Sometimes the leaves will just burn and go crispy and brown, other times the tree will dry out from lack of water.

Either way this very stressful for the tree and you should try and avoid these extremes. If you live somewhere that is often hot and sunny you should try and reposition your trees to a more sheltered area to get some shade at the hottest part of the day.

Insects

An insect attack can stress a tree massively and sometimes the first sign that you might have an infestation is when you notice your leaves turning yellow.  

You should be checking your trees often for signs of insects and treating them before they get out of control. If you try and ignore it it will only get worse and the tree can die.

Moving Your Bonsai Tree

Moving your tree about too much can also cause stress.

Trees are used to being in one location their whole lives; when you suddenly move them around they don’t like it. Subtle changes in the environment can totally mess up a trees balance and cause them to be stressed.

This is mostly commonly seen when you acquire a new tree and it can initially start to go yellow until it settles into your gardens environment.

This can also happen when you bring a tree from inside to outside. The sudden change in environment is too much of a shock and can stress the tree and causes yellow leaves.

You can’t really do much about this, you will just have to leave the tree in one position and care for it as best as you can until it settles.  

Repotting/Root Work

Repotting can be pretty stressful for a tree, especially if you do it at the wrong time, or you go too hard.

A sign that you might have removed too many roots is when your tree starts to struggle in the following days. If your tree has leaves when it is repotted they may turn yellow.

If you repot in the correct way at the correct time you shouldn’t really have this problem.  (You can read more about – When To Repot A Bonsai)

Too Much Extreme Work/Man Handling

Working on a tree can be very stressful if you do too much at once, such as wiring. Extreme work can actually set a tree back and the stress caused may cause the leaves to turn yellow.

However, if you do the work at the right time, you shouldn’t have this problem.

Sometimes you may just accidentally damage a branch from bending it too forcibly. If you man handle a tree like this and the leaves turn yellow, there is a high chance you have actually damaged the branch so much that it has died.

Cold Weather Will Change The Leaf Colour

The onset of autumn can cause yellowing in leaves. However, depending on the species this is totally normally and you can sit back and enjoy the colour show.

However, if you have a tropical tree that gets a little too cold, it can get damaged and turn yellow. This is not good for the tree and it can potentially kill it.

You need to check what species you have and if it can handle cold weather or not and if it loses its leave sin the winter.

Evergreens

Although evergreens stay green all year round, they can still drop old leaves from time to time. This happens on both conifers and broadleaf and even tropical’s.

After a few years some of the foliage will just become too old and will turn colour and drop off. This is totally normally.

This is usually one or two leaves at a time. If all of your leaves suddenly do this, it might be a sign that there is actually a problem and you should try and figure out what is wrong.

How To Fix Yellow Leaves On A Bonsai

Once you have noticed your leaves have turned yellow, there is probably not a lot you can do to save them. You will just have to let them fall off.

You can try to gently knock them off. They should fall off fairly easily. If you want, you can cut them off. The leaves can be easily replaced.

However, you need to fix the problem that caused them to drop in the first place. Whatever you think is wrong, do the opposite. If you do not correct this, the tree will just die.  

If your tree is stressed try and care for it as best as you can in order for it to rebalance its self. Remember that if it has no leaves, it will probably not need as much water, but you should still make sure it is damp at all times.

If the tree is still alive you should start to see new buds form within a few weeks and it should re grow new leaves and return to full health.

Conclusion

Yellow leaves can be a sign of trouble, but if you react quickly you can save the tree and get it back to full health within a few weeks.

The important thing is that you do not ignore it and you access what might be causing the problem so you can act appropriately if needed.