Keeping Bonsai Healthy

Once you have your bonsai tree, you want to keep it healthy so it keeps growing, looks great and doesn’t die. To do this, you need to ensure it has access to sunlight, water and food, as well as an environment that isn’t too extreme or variable.


Hopefully it’s no surprise that bonsai trees need sunlight. As plants, trees photosynthesise to get energy, turning sunlight and water into sugars to grow (and exhaling oxygen for us to breathe). How much light they need depends on the species and their usual natural habitat – some trees need full sun and others prefer to grow in partially shady positions. In nature, trees don’t tend to grow in enclosed spaces with little access to light, such as caves, so their preference really isn’t to be inside a house. Sometimes we have to keep them in our houses if they wouldn’t survive outside and in this case they should ideally have sunlight through a window or from an artificial light source.


Plants need water, for photosynthesis and to keep their cells turgid (in a state of tension due to water absorption, which gives the cell it’s three-dimensionality). How their water system works is fascinating. If the tree’s living cells dry out, at a certain point they stop functioning and die. Add to that, bonsai trees are usually in very small pots. These can easily dry out within a 24h period, particularly if no water-retaining material is in the growing medium, which can lead to damage or death for the tree. Legend has it that lack of watering is the number one reason why new bonsai owners kill their bonsais within a short period of time. That’s why it’s important to know when and how to water your bonsai trees.

Trees need water, but most trees don’t want to be permanently wet. They need good drainage so that the water from watering flows away and doesn’t drown the roots.


Food for trees is the 17 nutrients which are needed for metabolism. These nutrients are usually obtained from air, water and soil, but since bonsai trees don’t grow in soil, they also need fertiliser. Not all nutrients appear in every fertiliser, so care is needed to find the right one – or alternative sources of nutrients – for your trees. Fungi and bacteria can help trees take up nutrients, these can also be added to the pot to colonise the rhizosphere.


The tree’s environment can affect the way it grows by affecting the nutrients it can take in, or the light and water it can access. A very windy, dry or hot environment can cause evaporation so the tree needs more water. A very cold environment which freezes can damage the roots and buds of trees which are not adapted to the cold. Ideally you want to provide your trees with something approaching their natural habitat, or at least a compromise which prevents them being exposed to extremes they would normally not encounter in nature.

Healthy Roots

Taking care of your bonsai tree’s roots is an important part of keeping your bonsai healthy – its roots are the tree’s way to take in the water and nutrients it needs to survive, and they stabilise the tree in the ground (or pot) (among other roles). Roots should be in a suitable growing medium which allows them to access oxygen, water and nutrients, and to grow more roots.

There needs to be enough root cells – and associated water and nutrient absorption – to feed the requirements of the trunk and leaves above. Since there isn’t very far for roots to go in a small bonsai pot, it helps to develop a dense root ball.

The tree should be planted in a pot which is chemically inert and has some volume for the roots to expand – once the tree starts to become rootbound (or before it does) – the tree should be repotted. It should be well secured into the pot so that the roots are not strained or broken by trunk movement.

Finally there should be a good community of mycorrhizal fungi and beneficial bacteria among the roots and in the growing medium.

Damage & Repair

Sometimes a bonsai tree becomes physically damaged, stressed or nutrient deficient. There is only so much that can be done about a physical wound. Addressing nutrient deficiency is a topic for a future post….

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