They say that a lack of watering is the number one reason that newbies kill their bonsai trees. It is quite a surprise when you first learn about the hobby to find out that you need to water your trees *every day* and sometimes multiple times a day! It suddenly feels like more of a serious commitment than you might have been expecting. Taking a more zenlike attitude and instead learning to enjoy the time with your trees when they are being watered is just one of the delightful things you discover as you become more obsessed with bonsai.

As you’ve read elsewhere on this site, water is essential for bonsai trees. Water is essential for plants in general, including trees. It’s a key ingredient in the process of photosynthesis, along with CO₂ and sunlight, it’s a component of plant cells’ protoplasm, it’s essential for the structural support of leaves and stems (water creates ‘turgor’ ie. the water pressure which helps plant cells keep their shape), and it transports nutrients and photosynthates in the xylem and phloem sap. Water is estimated to comprise over 50% of the weight of woody plants.^ref

Surprisingly, the majority of water taken up by a tree (90% or more) is actually lost through transpiration (which means evaporation from the leaves)^ref. This is partly a by-product of having open stomata on leaves to enable the entry of CO₂, but also performs a useful function for the tree, pulling water and nutrients up from the roots by hydrostatic pressure – as the evaporating water causes a pressure differential in the xylem which pulls more water up.

What this all means is that trees need a LOT of water. They also store water for times when water is low – in this study^ref they found that Cryptomeria japonica can store 91.4 ml of water per kg of mass, distributed among leaves, sapwood and elastic tissue. For the first 2 hours of transpiration when photosynthesis started in the morning, they found that the water transpired was supplied exclusively from the tree’s leaves – it wasn’t until later in the day when stored water was low that the tree started to take up water from its roots.

OK so bonsai trees are small, they will need less than a full-sized tree of the same species, but sufficient water is necessary not just for photosynthesis but to maintain turgor in the cells, to allow the stomata to open and close, to resupply the water lost through transpiration, to bring nutrients up to its cells and sugars away from leaves, to build new cells and to avoid embolisms.

Trees in nature will spread their roots out to access water sources deep in the ground, but your bonsai doesn’t have that option. Trees in pots – such as bonsai – depend on their humans for water.

Furthermore, the water requirement of your tree (and thus how much watering is needed) will depend on several factors. In general, a tree will need more water if:

• It has a lot of foliage, since the level of foliage determines the level of photosynthesis *and* the level of transpiration, both of which require more water (but the latter being the largest driver)
• It gets a lot of sun, since sun exposure drives increased photosynthesis and transpiration (assuming foliage is present)
• The weather is hot, dry or windy – all of these increase transpiration
• Its growing medium is very open, free-draining or lacking moisture retaining components (such as bark). A more open, draining medium will lose water more quickly.
• Its pot is very shallow, as this means the water quickly drains out.
• It’s going through a growth spurt – making fruit, flowers or seed, or pushing sap up to push out embolisms
• It’s in a low-CO₂ environment – conversely if you have your bonsai tree indoors where there are lots of people, it may benefit from the increased CO₂ by reducing its water requirements^ref

When and how should bonsai trees be watered? The unscientific answer is – whenever their owner is most likely to be available and remember to do it! Convenience is important, since missing a watering could damage the trees.

But from a scientific point of view…the latest time when watering is needed is when the tree is approaching the point of running out of water. Obviously you don’t want it to actually run out for the reasons explained above. Bonsai lore is actually well-founded in this case – look at the growing medium and check how dry it is, this gives you a good indication of whether the tree needs watering.

Trees don’t need a lot of water at night, because many/most of them close their stomata which reduces transpiration – except when they are getting ready for sunrise – this article says they open their stomata up during the night in order to get water up into the leaves to be able to photosynthesis immediately that the sun comes up: they “can calculate the time of sunrise in advance”^ref This is a one-time occurrence prior to sunrise though, and a lot less than the continuous transpiration that happens during the day. According to another article trees actually do the majority of their growing overnight (that is, creating new cells), due to the increased water availability and humidity during this time (due to the lack of transpiration)^ref These points have two implications for bonsai enthusiasts – 1. if you want your tree to grow, make sure it has enough water at night but 2. it’s not going to be at its highest water usage rate overnight, so this is likely not the time when it requires watering.

At night, there is also a water gain from dew, depending on location. This article shows how much net water loss happens overnight in different geographies^ref – “in parts of the tropics and at high latitudes” dew is actually greater than nocturnal evaporation. But on average there is 8% net water loss on land overnight.

The point at which a bonsai tree is going to start running out of water will depend on all the criteria above – foliage mass, pot size, growing medium, dryness, heat and its stage of growth. In most cases this will happen at some point during the day, after the tree has been transpiring. Depending on these factors, it may require a top-up again during the day. So maybe mid-morning to noon is a good time, with a possible follow-up water later in the day if it’s excessively hot or dry.

Most bonsai enthusiasts dream of the perfect automatic watering system. Unfortunately this is quite hard to find, for a number of reasons.

Firstly, the amount of water needed for each tree varies based on pot size/growing medium/transpiration rate. The only way to achieve this is to have individually controlled watering devices for each tree. Secondly, you ideally want to avoid wasting water by watering outside the pot or when it’s not needed – this again requires individual control for each tree, plus a spray pattern which covers just the pot area and nothing else.

The final issue is that there is risk associated with relying on an automated system. This summer when I went on holidays I set up timed sprinklers and grouped my trees together for a twice daily watering. This worked great – until one of the hose connectors popped off the tap. I had quite a few losses but on reflection probably could have avoided these by setting up two independent systems. An enthusiast from Twickenham Bonsai Club which I attend has used mini soaker hose and a garden irrigation system for his holidays which he says has worked well – but it doesn’t look good enough for continual use due to soaker hose being coiled on top of every pot.

The compromise most bonsai nuts end up with is hand-watering the majority of the time and a sprinkler or similar system while they are away.

Can you use water sources other than the tap? Find out in this post.