Watering Bonsai: the Ultimate Guide

Few aspects in the art of growing bonsai are as crucial to get right as the watering. Many a bonsai tree has been killed by over-watering, and as many by under-watering.

Watering bonsai is an art in itself that requires experience to get right. Expect to take some time and some trial and error to perfect watering bonsai.

Specific watering requirements will depend on the species of bonsai tree that you have, and the potting medium used, but there are some general guidelines that can be followed for most bonsai species.

Two gardeners watering bonsai trees.
Watering bonsai trees.

How to water bonsai

Correct watering bonsai is critical to their success. They are easy to over-water, but the fear of over-watering can conversely sometimes lead people to underwater their bonsai trees.

It is true that if you keep your bonsai soil saturated for a length of time, the roots will start to rot and your tree will eventually die.

However, during hot summers and given that your tree has good drainage, it’s actually difficult to over water.

Watering might seem like an easy thing to do. However, there are many aspects involved in getting the watering right, such as the requirements of the particular species you have, environmental factors like weather and temperature, the condition of the soil and even the age of the tree.

How often to water a bonsai tree

Most bonsai trees require regular watering, and most will do best if watered moderately and often. Its soil moisture should vary regularly between nearly dry and half saturated.

Bonsai soil is quite different from standard potting soil. By design, it is porous and provides very rapid drainage. So what you need is to water enough, so that the soil stays moist, but not too moist until root rot and the growth of fungus and mold occur.

If possible you should check the soil every day, and water when it is starting to dry out. Surface moss can make the pot seem moist, when the soil inside may actually be dry, so check under the edge of the moss to be sure.

How to tell when your bonsai needs watering

With experience, you will develop your own methods on how to test the soil moisture level and also develop a sense of how much water is needed each time.

For a quick check of soil moisture levels, touch the surface with your thumb. If it feels dry, it needs watering. For a better check, gently scrape back any ground cover, gravel or surface earth before checking.

A more accurate test can be accomplished easily by using a standard moisture gauge. Often looking like an ordinary thermometer, analog or digital displays will provide a much more accurate reading.

However, relying on external instruments can be misleading as they only gauge the moisture level in one part of the pot. Be sure to measure the moisture levels in multiple areas to be sure there are no dry spots.

Dry spots can occur anywhere within the pot over time, since roots may cluster in certain zones within the soil and interfere with proper irrigation. As such, if dry spots occur around the roots, it can lead to die-back of certain portions of the root system. As your bonsai tree’s roots are always growing and developing, be aware that dry zones can appear gradually over time in your pot.

Don’t be afraid to give your tree a good watering if you think it is dry, but at the same time, be careful not to keep it sodden for long periods.

What water to use

The best water to use to water your bonsai is fresh untreated water. Distilled water or water collected from flowing springs or reservoirs is ideal if available to you.

Rainwater is also a good option, however, make sure you use it fresh, do not allow it to sit and get stagnant.

You can use tap water to water your bonsai. However, because most tap water is treated with chemicals, it is best to let it stand overnight to let the chlorine and other volatile additives evaporate.

Letting the water sit for a few hours before use also allows the water temperature to reach the ambient temperature, which will avoid cold ‘shocking’ your tree.

If you use tap water regularly, ensure your tree receives rainwater occasionally, as it will help wash away the cumulative build-up of lime and other chemicals.

How to water a bonsai tree

Water daily, or as required depending on the type of soil (sandy soils require more watering), the climate, and the species. Adjust your watering accordingly, making sure that water flows to the bottom but doesn’t pool to the point of filling up the tray.

Water left standing in the tray not only makes it impossible for the pot to drain, but also encourages mold build-up.

Try to make it a habit to regularly examine the bottom tray under your bonsai pot. Draining excess water from the tray will ensure that the soil in the pot is not becoming waterlogged.

Pines and other conifers need less water than other species of bonsai. They benefit from moderate dry periods in between waterings.

Deciduous and flowering trees require and welcome more water than conifers. Look for curled leaves on deciduous trees, indicating dryness. Wilting flowers (when they should be blooming) are an indicator of too infrequent watering.

If you use a watering can to water your bonsai, then it is a good idea to choose one with a ‘fine rose’ attachment. This will reduce the weight of the water drops falling on your tree and help to prevent the soil or surface covering of your bonsai from being disturbed or washed away.

Do not use a hosepipe directly on your trees, the pressure will be much to high and could damage the tree as well as the soil. If you have to use a hose for watering, purchase a fine sprinkler attachment to reduce the force of the water.

Watering for Indoor Bonsai

Indoor bonsai trees with well-draining growing media should be watered moderately once per day, year-round.

If possible, keep your bonsai tree on a humidity tray, to help increase the humidity around your tree, so it will lose less moisture through its leaves.

Watering Outdoor Trees

Watering outdoor bonsai trees should take into account the season, temperature, sun, and winds. 

In strong sun, water directly at the base of the trunk and avoid getting the leaves wet.

Deciduous trees, such as maples and wisteria, need plenty of water in the summer. During hot spells, you may need to water as often as twice a day. While in cool wet months, once or twice a week should be sufficient.

During the tree’s dormancy period, you should keep the soil moist, but should not be watering as such.

Immersion watering

In some cases, immersion watering (watering from the bottom up) may be the best option for your bonsai tree.

This will be the case if:

  • Your tree has left for a few days without watering, and the soil has become dry and hard, or 
  • If your bonsai tree has surface moss or the compost is mounded above the rim of the pot. This can make the water run directly off the surface if you water your tree by pouring water onto the soil’s surface.

To ensure that all the soil in the bonsai pot is thoroughly moistened, try immersion watering once a month. 

The process is as follows:

  1. Fill a bucket or sink withn water, then place the pot up to the base of the tree into it. 
  2. Let the soil in the pot absorb water for up to twenty minutes, then 
  3. Carefully remove by lifting the pot up with your hands. Do not ever lift the pot by grasping the trunk and heaving; you could uproot your bonsai tree or damage the delicate root system.
  4. Leave the pot in a location where the soil can drain freely, removing any water if it pools underneath the pot.  

Controlling Humidity

The moisture level of the soil in the bonsai pot is significantly affected by external humidity levels. 

Even though the soil may be wet hours after you watered, if the weather is forecast to be dry and windy for the next couple of days, you’ll be surprised just how fast the soil will dry up.

In addition to watering, your bonsai will appreciate a regular misting or sprinkling with water, to help keep the leaves moist and the surrounding air damp.

However, avoid doing this when there is strong direct sunlight, as water droplets sitting on the leaves can act as lenses directing the sun’s rays onto the leaves, damaging them.

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