If there's one thing that you have to do to keep your bonsai alive, it's water them.
The trouble is that many bonsai owners are so frightened of underwatering their bonsai trees that they water them a LOT. Unfortunately this can lead to overwatering.
But what does an overwatered bonsai tree look like?
We'll dive into that here, as well as factors that contribute to overwatering and how you can improve them.
Can you overwater a bonsai tree?
You can definitely overwater a bonsai tree. Some common advice I see on bonsai forums is that if a bonsai is potted in well draining aggregate soil then it cannot be overwatered. Sadly this is not true. Root growth is stimulated by the wet/dry cycles of thoroughly watering your bonsai, then letting it dry out a bit.
If you never let the soil dry out, even if you are using an aggregate mix, roots will not be stimulated to grow in the same way.
Why overwatering a bonsai tree is bad
It's important to understand what's happening beneath the soil when you water your bonsai too much.
Tree roots depend on a certain amount of air as well as water to grow well. Airy conditions stimulate the bonsai's roots to grow, so if its roots are kept too wet, new roots won't be produced.
Over time, roots will start to die off. The wet conditions are a breeding ground for bad bacteria and microorganisms, which lead to root rot.
If the tree remains in a poor soil situation it will lose its roots and eventually die. The longer the period of overwatering, the bigger the impact on how long your bonsai tree will live.
Factors that lead to overwatering a bonsai tree
The good news is that pretty much all of the considerations leading to an overwatered bonsai are within your control. Don't worry if you have some (or all!) of these bad habits, we all did at the start 😅.
Organic soil mixes
Many bonsai are sold or repotted in very organic soil, often just regular garden compost. This kind of soil is very dense and holds a lot of water. Over time this soil mix will compact down so the roots have very little air, and the water-retentive nature of the soil means it seldom has a chance to dry out.
This rosemary is potted in compost which is retaining too much water. You can see it is dark and compacted.
Watering too regularly (and on a schedule)
Trees don't run like clockwork. Many things impact how much water a tree is using, from temperature, wind, growth and dormancy. Applying water at set intervals is generally not a good idea, as you run the risk of over- or under-watering. A better way to look after your trees is to keep the same schedule and use a bonsai watering checklist to decide if you need to apply water now or wait until later.
One of my biggest grievances in bonsai forums is the well-intended advice of 'slip pot into a bigger pot' or 'put it in a really big pot to let it grow bigger'. While I understand the sentiment, the impact of putting a bonsai into a much bigger pot than the one it currently occupies can be a negative one. This is because that big pot will be full of soil, not roots. It will take a long time for the tree to fill its new pot with roots, and in the meantime, there is a bunch of soil that has nothing to take the water out of it except for evaporation. The cold, wet conditions for a small tree in a big pot can spell the end of the tree, so if you want to grow your trees bigger I'd suggest planting them in the ground (if you can) or progressively up-potting into slightly bigger pots at each repotting.
This one is much less under our control and is usually not fatal for a tree, but can lead to some ill health. Some areas of the world do experience stupid amounts of rain (trust me, I live in Scotland) that can leave your trees standing in water for far longer than is desirable. There are a couple of things we can do in this situation (discussed below), but the simplest is to move your trees temporarily under some cover if you have some available.
Signs of overwatering your bonsai
An over-watered bonsai tree will show a slow deterioration over weeks or months. The visible features occur as a result of a loss of roots, and will generally be across the whole of the tree rather than focusing on one specific area or branch.
Doctors use something called the 'end of the bed test', which is where they look at their patients from the end of the bed, and if they look very unwell, then they probably are. We can do the same with our trees.
An overwatered bonsai tree will look like it is struggling. You may see some or all of the following:
- Yellowing/browning of the leaves
- Wilting leaves
- Leaves falling off
- The tree becomes looser and wobbly in its pot (please don't wobble the tree too hard when checking!)
- Soil is very wet and looks muddy
- Soil can have a sour smell indicating that it is decomposing
- Growth of weeds that thrive in wet soil conditions (such as liverwort seen in the title picture)
The same rosemary as above. Note the yellowing and brown leaves, as well as the general impression of an unhealthy plant.
How to revive an overwatered bonsai tree
Overwatered trees generally recover well if you are patient and apply sound horticultural techniques. Here's what you need to do:
- Only water when the soil is dry
- Leave the foliage alone
- Stop fertilizing
- Tip the pot on an angle
- Don't repot your bonsai until you are in the repotting window of early Spring
Only water when the soil is dry
Trees need moisture in the soil for the roots to take up water and hydrate the foliage. If you inspect the soil and it is damp, then there is plenty of water in the pot for the tree to use. You should only water your bonsai again once the soil has started to dry out.
You need to be patient and fairly strong-willed for this. A lot of people 'love their trees to death' because they give in to the temptation of watering their bonsai too frequently. Taking care of a bonsai tree can be harder than many people think.
If you have a badly overwatered bonsai there may be quite a lot of root attrition in the pot. This means it will take longer for the soil to dry out because there are fewer roots absorbing moisture.
When the soil dries out and you do need to water, then go to town!
You need to drench the soil until water is flowing out of the drainage holes. The idea is to make sure that all areas of the soil mass are equally well hydrated, with no dry spots left behind.
Then you wait until things start to dry out again and repeat.
Leave the foliage alone
The foliage mass on your tree is its solar energy farm.
The energy generated by the foliage via photosynthesis is what the tree will need to use to rebuild its root system. This means you need to leave the tree alone to do its thing.
Let it grow freely, and you can come back when the root situation has been sorted to trim and style your bonsai again.
There is a common misconception that fertilizer is 'food' for plants and can be helpful when they are unwell. Unfortunately, this is not quite true.
Fertilizer is a supplement that can boost the growth of healthy trees. However, fertilizer contains high levels of salt, which can be very damaging to weak or injured roots.
For this reason, you should avoid fertilizing an ill or newly repotted tree. The best route back to health is to water appropriately and let the tree build itself back over time. There is no magic bullet.
Tip your pot up at an angle
If you have a tree weakened from overwatering, it can help to tip the pot on an angle to increase water drainage.
Placing the pot at an angle will increase the height of the gravity column working on your bonsai's soil mass, which will increase the water drainage from the pot. This can help to remove excess water.
This technique is particularly effective if you have overwatered bonsai that you can't protect from the rain.
Leave the roots alone
When a tree is not well, there can be a temptation to lift it out of its pot and inspect the roots. I would strongly recommend that you do not do this.
The rootball of a chronically overwatered bonsai is likely to be very unstable due to the poor root growth and may fall apart in your hands.
Repot the tree when appropriate
If you are within the safe repotting window of late winter/early spring, before the start of foliage growth, then you can repot your tree to help improve its root system. If you're not currently within this time window then just be patient and wait for next year - it will benefit your tree.
When you repot, your aim should be to remove as much muddy and wet soil as possible and replace it with a free-draining aggregate mixture.
The aggregate mix you use can be whatever you prefer and will depend on the species.
For deciduous trees (such as birch) I use akadama, and for conifers, I use a mix of akadama, pumice and lava. You can buy pre-mixed bonsai soil at specialist bonsai nurseries locally or online. They'll be able to advise you on the best mix they have available for your tree.
Be patient and deliberate
I hope this post has helped you understand a little bit more about overwatered bonsai trees. The key thing you need to take away from this is to be deliberate with your watering, and be patient while the tree regains strength.
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Overwatered bonsai FAQs
Is my bonsai overwatered or underwatered?
For a beginner who has a tree that is struggling it can be really hard to know if you're doing the right thing when it comes to watering. To work out if your bonsai has been overwatered or underwatered, think back to the last few times you watered it. If the soil was wet, and you added a lot of water, then it has almost certainly been overwatered. If the soil was dry and you watered only a small amount, then it may be underwatered. If you always water with very small quantities, you may have a rootball that is dry on the inside but damp on top. You can fix this by making sure you thoroughly drench your bonsai each time you water it.
How long does it take for an overwatered bonsai tree to recover?
If you have been overwatering your bonsai tree it can take weeks or months for it to start looking healthier, depending on the health of the tree's roots. While the tree may visibily start to improve, it will likely take a year or more to build up a healthy root system again. Your bonsai may be vulnerable to health issues during this time, so stay vigilant for any signs of fungal infections.