Bonsai trees are immensely popular plants, but they aren’t always the easiest to care for, and many people find that their tree does not stay healthy for long after they have purchased it. If your bonsai tree has started to look sick, let’s explore the reasons why it happens.
Lots of things can cause leaf yellowing in bonsai trees, including improper watering, poor lighting, or over-trimming, which will stress the tree out and leave it unwell. Too much or too little fertilizer can also cause issues. Bonsai trees are sensitive and need good care if they are to thrive.
Reason 1: Not Enough Sunlight
Light is essential to all plants, but bonsai trees really suffer if they are not given enough light to thrive. Because they are miniaturized, they may be less resilient to poor conditions, so if your bonsai isn’t getting enough light, its foliage may turn yellow as the tree starts to die.
The bonsai needs to be healthy in order to produce enough chlorophyll for its leaves, and as the chlorophyll is what makes the leaves green, a sickly tree will often have pale or yellow foliage.
This problem is unfortunately self-perpetuating because the yellow leaves will not be able to photosynthesize as effectively. This in turn means that the bonsai has less energy, gets sicker, and its leaves become even yellower.
How To Solve It
Most bonsais need a minimum of five hours of sunlight per day, but it will vary according to the kind of tree that you have. Spend some time researching your particular tree, and find out how much sun it needs. Trees that are native to woodlands may cope with less light, while those that grow in open spaces may need more.
Once you know how much light your tree needs, work out how you can provide this. You could try moving it closer to a window, opening curtains/blinds, or installing a grow lamp to help the tree get enough light. Hopefully, with more light, the yellowing will disappear and your tree will produce healthy, green foliage again.
Reason 2: Underwatering
Because they do not have deep root networks, many bonsai trees are vulnerable to being underwatered, especially if they are kept in restricted containers. The small space and the dense root ball combine to mean that it is easy to underwater them.
Trees need water to take up nutrients from the soil and keep their leaves healthy, so if your bonsai has not got enough to drink, yellowing will occur. The plant won’t have the nutrients necessary to keep producing chlorophyll.
Because they are kept in such small containers, many bonsais need to be watered more frequently than other houseplants. Check the soil of your bonsai regularly to make sure it isn’t thirsty. Some people check this every day.
How To Solve It
You should first press your finger into the soil around a bonsai’s roots to see whether the tree is actually thirsty. If the soil is dry, give it a drink, but be aware that sometimes when you water a bonsai, most of the water will run to the edges of the container, rather than soaking the tree’s root network.
To solve this problem, give your bonsai a really good drink until you see water trickling into the tray beneath its container. This should mean that the soil is thoroughly saturated. Tip the excess water out of the tray so your bonsai isn’t standing in water, and it should perk up within a few days.
Reason 3: Overwatering
Overwatering is a common problem with houseplants, and unfortunately, it can also produce leaf yellowing. A plant with constantly wet roots will not be getting enough oxygen and is also vulnerable to root rot. Ironically, root rot prevents it from drawing up water, so your plant may be thirsty even though its pot is wet.
If your plant’s pot doesn’t drain well or you water it too often, your bonsai will soon start looking sick. Equally, if it is planted in heavy clay, water will sit around the roots, encouraging rot.
How To Solve It
There are two solutions to overwatering: the most obvious is to decrease the frequency with which you water the plant. However, this won’t solve the issue if your plant is not draining properly, so you should check that the container and the potting medium are both suitable.
The container should have some generous holes in the bottom, and you may want to add a layer of fine gravel beneath the soil. This will encourage water to run out of the bottom of the pot, drying your bonsai’s roots out, and preventing water from pooling around them.
You should also aim to use bonsai soil or some other fine potting medium to promote good drainage. You can add perlite to the mix if you are having issues.
If your bonsai has got very wet, make sure you repot it using some dry medium. You can take this opportunity to inspect the roots, too. Any that have turned brown or mushy should be cut away using sterile scissors.
If the root ball is very wet, soak away some of the moisture with clean paper towels, or allow the roots to dry for a few hours before you replant your bonsai in fresh, well-draining soil.
Reason 4: Natural Yellowing
It’s important to be aware that some bonsai trees do yellow naturally. Any tree that is deciduous and sheds its leaves in the fall will do so in its bonsai form too. It may lose all of its foliage during the winter, and this is nothing to worry about, provided it is natural for that tree.
Even evergreen trees will lose old foliage as they grow and age, and this may turn yellow before it drops off. The odd yellow leaf is not a cause for concern.
How To Solve It
This is a natural process for the tree, so there is no need for a solution. Being aware of whether your bonsai is deciduous or evergreen should help you to determine whether it ought to lose its leaves in the fall.
Reason 5: Poor Pruning
Bonsai trees need to be trimmed and wired in order to retain their shape, and this should not cause leaf yellowing unless you do it too often or too heavily. Being cut does cause stress to the tree, so excessive pruning may make it sick.
Additionally, when you make cuts to the tree, you expose it to both pests and diseases, and it’s a good idea to be aware of this. Either could cause leaf yellowing.
How To Solve It
Learn how to prune a bonsai correctly, and keep pruning to a minimum. You should not be trimming your bonsai tree’s roots and leaves often – only enough to maintain its shape. The more you cut the tree, the more stressed it will be, and the more vulnerable it will become to diseases and pests.
There are many useful guides on pruning bonsais, so familiarize yourself with the correct method before you start shaping your tree. Always use sterile equipment, and wash your hands before you touch the plant, especially when making an incision. This will further reduce the risk of introducing infections and should help to keep your tree healthy.
Reason 6: Fertilizing Issues
Again, the small pot means that it can be difficult to balance the fertilizer needs of your plant, and too much or too little can lead to leaf yellowing. Most bonsai trees will need fertilizing during their growing season, but you do need to be cautious about overdoing it.
In general, it’s better to give a tree too little fertilizer, rather than risk giving it too much.
How To Solve It
An under-fertilized bonsai tree is easy to fix; simply dilute some fertilizer suitable for that type of tree, and add it to the pot every few weeks during the growing season.
An over-fertilized tree may prove more challenging. Try using soil testing kits to confirm whether it has too much of a particular nutrient in the soil, and flush the pot out with clean water if you have given it too much food.
There are plenty of explanations for yellow leaves on a bonsai tree, but most have fairly easy fixes. Identify the issue and act quickly to solve it, as these plants are delicate and may not survive for long in poor conditions.