Watering is the single most important topic in bonsai. Many trees will survive being chopped in half or even if we forget fertilizer for a full year. No tree will survive forgetting to water them.

Trees need a balance of water and air in their root system. If you picture fully submerging a tree’s roots underwater, all of the air will be displaced by water. If you then picture letting a tree fully dry out, there will effectively be no water in the root system, and the space that was inhabited by water will be taken up by air instead. Our job in bonsai is to keep a balance between the two.

Underwatering a bonsai can effectively just kill it outright. If a tree goes fully dry, one occurrence could be the end of the tree. In general, conifers can go drier than deciduous or tropicals, but none can go fully dry.

Overwatering a tree causes three main issues. The first is a lack of air. If the roots stay full of water, there is no vacant space in the system for air. Air is pulled into the root system from the top of the soil and through the large hole(s) in the bottom of a bonsai pot as the tree begins to dry out.

The second issue caused by overwatering is root rot. When roots stay wet all the time, they begin to rot. Some trees are quite resistant to this, and don’t mind being wet, but other trees begin to rot quite quickly. Junipers in particular like to be on the drier side, while maples don’t mind being a bit wetter.

The third issue caused by overwatering is pests. When trees lose health, pests such as mites or aphids can quickly sense this and start infesting a tree. It’s not a sure thing, but if you see bugs, you can usually assume that the tree is being overwatered or is unhealthy for some other reason.

It’s important to note that poor soil conditions can cause or exacerbate these same issues that are caused by poor watering practices. Most commonly, poor soil conditions would be caused by either using a poor soil mix (such as potting soil) or by improper repotting technique.

Potting soil will turn to “muck” very quickly and can create an environment where oxygen cannot reach the roots and can also cause roots to stay wet too long or too short.

Improper repotting techniques that would cause watering issues would mainly be caused by leaving too much space around the roots or not repotting often enough. Leaving too much space around the roots can cause situations where the areas in the soil with no roots present can dry very quickly and the areas with roots dry much slower. This can cause difficulties in growing new roots. Not repotting often enough can cause the same issues as using a potting soil would cause.

The way that we describe when to water in bonsai is “when the roots begin to become dry”. The easiest way to check this is to stick your finger down into the soil, and if it feels dry, water the tree. If it’s wet, you should wait a bit. “A bit” comes down to the species, the size of the pot, the humidity, temperature, etc. While learning this process, check your tree about 3-4 times per day. The key to this is that you’re not watering when the top surface gets dry, but when about one inch down from the surface goes dry.