- David Hale
Watering your bonsai trees isn't always as simple as many people think. There are a lot of things to think about when deciding whether your tree needs water, so I've created this Bonsai Watering Checklist to help!
In this post I'll take you through the ins and outs of bonsai water so you'll know the best way to water your bonsai tree.
Here's a list of what we'll cover:
- Why watering bonsai is hard
- Why do we apply water
- The soil system and watering
- Environmental factors and the weather
- Which species need more water
- Watering checklist
Why is watering a bonsai tree so hard?
Water is essential to keep your bonsai tree alive and healthy. Trees take up water through their roots, which is why we need to ensure the soil your bonsai is growing in gets plenty of water.
What most people don't realise is that roots also need air in order to thrive. Just as the tree will die if we never water the soil, it will also suffer and die if we keep the soil too wet (although this will take a lot longer, in months rather than days).
This is where things can get a bit tricky. It can be really difficult for beginner (and experienced) bonsai practitioners to know whether they are keeping their trees too dry or too wet. But that's the problem I'm hoping to help you solve, so keep reading and we'll try to set out a foolproof system to keep your trees properly watered!
Some bonsai tree species are more tolerant of sub-optimal watering practices than others, so take this into account when buying your first trees.
Why do trees need water?
Just like us, trees need water to survive. It is a vital component for many physiological functions in the plant and having even a simple understanding of these can improve the way you water.
To create their food
Most of us are aware that plants create their food through a process called photosynthesis. Photosynthesis is a process where plants use sunlight to convert carbon dioxide and water into sugars. Without a good supply of water through their roots, your trees will not be able to create the food they need to grow and develop.
To help them breathe
Needles and leaves on trees have tiny holes, called stomata, that allow air to flow. This air flow is essential for the tree as it allows carbon dioxide to enter the system and oxygen and heat to exit. Stomata are held open when their walls are full of water, kind of like an inflatable pool lounger. This means that if your tree doesn't have enough water, it's stomata will start to close. This can cause problems with photosynthesis and overheating.
To cool themselves
If it's a hot day, trees can't just get up and walk to the shade if they are feeling a bit warm. If your tree gets too hot its tissues can start to break down, causing irreparable damage. In order to keep themselves cool, trees rely on water. As water molecules move from the roots, through the trunk and branches and out to the leaves they pick up heat from the tree. The water then evaporates out through the stomata, taking the excess heat with it.
The soil system and watering
When I talk about the soil system, what I mean is everything contained within and acting on your bonsai's soil. That includes:
- The pot/container/slab
- The soil
- The roots of the tree
- The microbial and fungal organisms that live in the soil
- The top dressing/moss on top of the soil
I'll address a few of these in turn so we can see how they impact the water that your tree needs.
Bonsai are grown in small containers to restrict root growth and create a tree with a small and nicely scaled canopy. Containers are generally very shallow, and sometimes slabs are used to created a really rugged aesthetic.
Containers should also have a good number of drainage holes to help remove excess water from the system.
The size of your container is going to directly impact how quickly it will dry out. As you would expect, a smaller container will dry out faster than a larger one, which can make your bonsai harder to care for.
However, this doesn't necessarily mean a bigger pot is better for your tree. As mentioned above, keeping the soil too wet can kill your bonsai just as easily as keeping it too dry.
When it comes to watering, the best container for your bonsai is one that fits the tree. The exact size will depend on many factors, such as the size and species of the tree, the amount of foliage it has and the state of its root system.
Keeping your bonsai's roots aerated is just as important as keeping the watered. Since we are growing bonsai in a small soil space as we just discussed, we need to make sure we maximise the potential of that space to grow a healthy root system.
If you are new to bonsai, you may not be aware that the healthiest roots are grown in an aggregate soil mix. What this means is we use a type of soil that has very good drainage and plenty of pores to allow air to circulate through the soil system.
The ideal soil mix depends on the species, your environment and the components you have available to purchase. In the Western world, most mixes will involve a combination of pumice, lava rock and akadama, though there are many different substrates that you can experiment with to find the best results for your environment.
If you try to use general purpose compost for your bonsai trees, what you will tend to find is it holds water for far too long. Without the free drainage that we get from aggregate soils, water will fester in your pot and reduce the growths of roots from your tree. In the worst case it will lead to the growth of nasty organisms that cause root rot and ultimately the death of your tree. If your soil mix is staying too wet, you should consider repotting your tree.
Top dressing and moss
Moss can look amazing on the soil of your bonsai, but it also serves a really important horticultural function. There are too many for me to go into here but I will tell you about a couple that directly impact the water in your soil system.
Firstly, moss helps to even out the water in the pot. While gravity will pull water to the bottom of the soil, moss can help pull water back towards the surface. Evening out the water distribution through the whole of your container is great for promoting healthy root growth and preventing wet spots in your soil that can fester.
Secondly, moss can help to tell you when it is time to water. Since the top dressing is continually pulling water upwards through the soil, when it is moist you can generally assume there is enough water in the system. Likewise, when the top dressing starts to dry out, it's time for you to consider watering the tree. It's a fantastic way to read the soil system.
Environmental factors and the weather
This is a really big one but it is something that people very often overlook. Obviously it's common sense that if it's raining you probably don't need to go out and water your tree, but there are other factors you need to consider.
Wind will dry out your bonsai
Trees lose water through evapotranspiration, meaning the water turns to gas and floats off into the air. The more wind there that's around, the more water your tree is going to be losing.
It is very easy to get caught out on a cool windy day thinking your trees won't need watered as much because the sun isn't out, but in fact they can dry out very quickly in a strong wind.
Sunshine and heat
Sunshine and heat will increase the amount of water your trees need. This is true even for bonsai in the shade, because the heat causes trees to lose more water through evapotranspiration.
Trees rely on water as their cooling system, so be careful on hot, sunny days to make sure your trees have all the water they need.
Which species need more water
There are far too many species of tree for me to go over each one, but there are certain broad categories of tree that require more water than others. There are also certain trees that favour more oxygen in their soil system for root growth, which is worth bearing in mind when watering.
In general, you will find that when in leaf, your deciduous bonsai such as birch require more watering than your conifers.
Deciduous trees tend to have a very thin cuticle on their leaves and a large surface area. This means water is pulled through the tree from root to leaf much faster. As a generalisation, deciduous trees prefer to have wetter soil than conifers and will require more watering through the summer.
However, since deciduous trees lose their leaves in autumn, they also largely lose their ability to move water (some evaoptranspiration still occurs through the bark). This means their watering requirements through the winter will drop more than that of a conifer.
Deciduous conifers such as larch and Dawn Redwood will also need more water than regular conifers.
When it comes to conifers, you will tend to find that elongating species such as Spruce and Fir (Abies) will prefer wetter soil than junipers and pines. Pines and junipers prefer to have more air in their system than other trees, so you can let them get a bit drier before watering.
Tropical species such as jade generally require less watering that deciduous or coniferous trees.
I've created this checklist to remind you of all the things you need to think about when deciding whether your bonsai needs watering or not.
Before we get to it, one final thing to consider is your lifestyle. While there are ideals for doing things in bonsai, we ultimately need to fit it all in around our day to day lives. This means that if you are not going to be able to check on your trees again for a few hours then you're going to need to water some trees that may not need it right at that moment. This is a compromise we all have to make.
I'd recommend that each time you come to water your trees, think about each of these items before deciding to apply water. Forcing yourself to stop, think and be intentional with every action you perform on your tree can make a massive difference in their health and progression. Bookmark this page so you can come back each time you water for at least a month. As you gain experience this will become second nature and you'll start to do it without thing about it.
Size of pot compared to the tree (will dry out quicker)
Soil (broken down or compost-type soil will dry out slower)
Top dressing (Wet moss means water in the soil system)
Weather (hot, sunny or windy days mean more watering)
Species of tree (Deciduous trees need more water than conifers)
When can I next check the tree (If it's going to be a long time, have a lower threshold for watering)
I hope you find this checklist useful. There are a lot of things to think about when you're looking after bonsai trees, so I really find it helps to have things written down so I don't forget.
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