Bonsai Watering Technique

The single most difficult thing to learn in bonsai is proper watering technique. Most beginners are usually so afraid of letting their tree dry out that they wind up drowning it.

If you have read the preceding chapter on soil composition, you are aware that the soils used in bonsai cultivation are formulated to provide for proper aeration and perfect drainage. This works to the bonsai growers advantage by promoting vigorous root growth in the pot. On the other hand, its great drawback is that such soil mixtures are very granular and will dry out much more quickly than commercial potting mixes or garden soils.

This need not be an insurmountable problem and does not mean you will be required to quit you day job in order to stay home and water the trees. Putting a bonsai on an effective watering schedule is mostly a matter of common sense.

There is no set formula for how often a bonsai should be watered except to say that the objective is to keep the soil mass evenly moist… Not too soggy and not too dry and not constantly jumping back and forth between dead dry and sopping wet during the course of the day.

The rate at which the soil in any pot will dry out is affected by a number of factors:

1. Time of Year: Plants use more water during the growing season than they will use during the dormant season.

2. Temperature & Humidity: Both these factors are self evident. If its 100 degrees in the shade things will dry out a lot quicker than at 60 degrees. And if its raining… you probably won’t need to water today. If its a particularly windy or dry day you may need to water more often. Likewise those living in Arizona will need to provide more water to their bonsai than those living in Seattle.

3. Pot Size & Composition: Larger pots will retain more water and will dry out more slowly than smaller shohin sized pots, rock plantings and tray plantings. Likewise, ceramic containers tend to heat up more quickly in the summer sunshine than do plastic or mica containers or wooden boxes.

4. Soil Composition: If the soil mixture contains more organic matter it will tend to hold onto moisture longer than more granular mixes.

5. Plant Species: Some plants use water at different rates. Wisteria and Bald Cypress are thirsty specimens. Pines and Junipers prefer life a little more on the dry side.


Freshly transplanted bonsai are usually watered by submerging the pot in a container of water. Water then comes up through the drain holes in the bottom and after a few minutes will guarantee that the entire soil mass becomes moist.

How To Know

Again, the answer is mostly common sense and comes as you gain a familiarity with your trees and the conditions in your own garden. Try using the end of your finger. Insert it into the soil. The answer should come automatically and naturally.

If that’s not scientific enough, you can invest in the purchase of a moisture meter. Such devices are inexpensive and available at most garden centers. A simple “moisture meter” can also be improvised at home by inserting a toothpick or bamboo chopstick into the soil mass. The toothpick behaves somewhat like the dipstick in a car. The next day, when withdrawn, moisture at the bottom of the pot will be evident on the damp areas of the toothpick. Remember that one of the advantages of a properly prepared soil mixture is a soil mass which, although it dries out more quickly, will also dry out more uniformly … i.e., if it looks dry on the top, it’s probably also dry on the bottom and in the center.

With regard to the question of whether bonsai should be watered on an hourly, daily, weekly, or monthly basis, the answer is… You should water when the bonsai is thirsty.

If they would all dry out at the same time this would not be a problem. Unfortunately this is not the case and watering technique will have to be specifically tailored to meet particular growing conditions based on the factors outlined above. A tree planted in the ground and covered with grass or mulch will not dry out as quickly as a bonsai because sun and dry winds can attack it only from the top. Potted plants, however, are exposed to the same elements on all four sides as well as the top and bottom of the container. This means they can dry out much more quickly.

What Kind Of Water

Just about any form of H2 O you have available will be OK with your bonsai. It is not necessary to collect rain water in order to make your trees happy. We have use regular city tap water provided for more than three decades with no ill results.

One word of caution, however. If you are using water drawn from a well, you may want to have it analyzed to insure that it does not contain a high concentration of soluble salts which would build up in the pots over time and cause problems. Likewise, a professional grower in Texas once noted that the city water supply in his area was so alkaline he had to install a system which injected hydrochloric acid into his water lines in order to keep the Ph balance in his water neutral. Such situations are rare, but if you have any doubt, you would be well advised to check it out.

On the positive side, we have a student who keeps tropical fish. When cleaning his fish tanks he reserves the old water and uses it as a “bonus” fertilizer on his bonsai.

When To Water – The best time of the day to water is early in the morning just as the sun is coming up. A fresh supply of needed moisture at this point in the day will usually keep the bonsai wet enough until the same time the following day. Unfortunately, most of us will have jobs to go to and other commitments that make consistent morning waterings a difficult proposition. In such situations it is best to arrange watering times to fit your own schedule. If you water in the mornings then try to always water in the mornings. If at mid-day or in the early evening then try to always do so. The main thing is to be consistent.

If you find that some things are drying out more quickly than others, it may be advisable to bring such plants into an area where they do not receive as much full sunshine during the day.

If you find that small containers are drying out too quickly, try nestling several of them in a tray filled with sand, turface or sphagnum moss. The sand and moss will provide more moisture around the sides and bottoms of the smaller pots, preventing them from drying out as quickly. Thirsty plants such as wisteria and bald cypress can be placed in trays of water which cover the pots drainage holes and allow the roots to soak up the moisture they need. As stated before, you need to become familiar with the tree’s individual water and growing requirements.


When Not To Water

Watering in most locations during the hotter summer months is usually a daily event. In winter, when plants are “packed away” for protection and in a dormant condition, watering may only be necessary every few weeks. However, for those living in the southern regions where summertime temperatures can rise into the plus 90 degree range for two or three months at a time, some special consideration may need to be given to watering routines.

Plants don’t need water in the winter because they are dormant (asleep) and not using it. At temperatures rise out of the 80 plus degree range for prolonged periods, many temperate plants will begin to shut down their respiratory systems in an effort to conserve moisture. i.e.- They stop growing and stop using water. This is called summer dormancy.

Bonsai  growers who are unaware of this reason that because they are hot and dry their bonsai must want a drink as well. This can lead to excess moisture in the container and encourage root rot. If you find yourself in such a situation avoid dumping unneeded water on the plant and instead, mist the leaves, bark and container. Fogging attachments to fit your garden hose are easy to obtain and will help your bonsai gain a little relief from the hot summer sunshine without the danger of drowning the roots.

How To Water

There are only two ways to provide water to a bonsai… bottom watering and top watering. Bottom watering is by far the safest and most effective way to water your tree. Place the bonsai in a pan or sink of room temperature water up to the edges of the pot. Water will come in through the drainage holes in the bottom of the container and move up towards the top of the pot guaranteeing total saturation of the root mass.

After a minute or so, the pot can be removed from the water and excess moisture will escape from the pot through the drainage holes. Tip the pot at a 45 degree angle if you wish to quicken the time it takes to drain the container.

If you have only a few bonsai, this is the best way to water them. It is also the recommended method for the initial watering of a freshly transplanted bonsai and for the application of water soluble fertilizers which we will discuss in the next chapter.

For most of us, the shear number of plants and the varying sizes of containers make bottom watering impractical. Top watering with a garden hose then becomes the best answer. You may use a watering can or a garden hose either of which are fitted with a fine rose so that the water stream does not wash soil out of the container. Give each pot one or two passes with the hose.

Water should stand on the surface of a bonsai pot for no more than a few seconds before sinking in. If it does not, it is an indication that: the soil is too dense and should be changed for a better draining mixture; or, that the tree is becoming root bound and should probably be transplanted in the near future. One final caution. The water inside of a garden hose which has been laying in the summer sunshine can be scalding hot. Test the hose output with your fingers before testing it on your bonsai.


Watering the trees is work, but it can also be the bonsai equivalent of “quality time with the kids.” I enjoy watering my bonsai even though the task takes over an hour each day. True it is less “fun” when the temperatures are plus 90, but the task gives me the daily opportunity to visit with my trees and inspect each one to see how its doing. Try to develop a positive attitude about watering and to appreciate the more zen aspects of it.

If daily personal watering is not possible for your schedule then you may want to check into an automatic watering system for your bonsai. There are a large variety on the market, all of which do a reasonably good job. However, care must be taken when the system is installed to insure that it delivers the correct amount of moisture to each plant. One more thing… There is a saying in the bonsai community. “Automatic watering systems will work perfectly until the day you leave on vacation.”

They do occasionally malfunction. That’s why its a good idea to have a friend, neighbor or family member who owes you a favor or two. If someone can check to make sure the system is functioning correctly while you are away it will contribute greatly to your peace of mind.

Most people find moss an attractive addition to any bonsai composition. If you do use moss make certain that you do not use so much that it prevents you from observing the moisture content of the soil. An old Japanese rule says: “Moss may touch only three sides of the container”.

This means that you will always have some area of soil showing so that you can gauge its moisture content. Also remember that dry moss will repel water like a ducks back. This could create a condition on the surface of the container which would make it difficult to get water into the roots.

Watering Indoor Bonsai

For those who wish to grow their tropical and semitropical bonsai indoors, the factor of humidity must be considered. We like our homes dry and warm… particularly during the winter months. Comfort for us does not mean comfort for our bonsai. If you were to keep the humidity in your home at a level which would make the bonsai happy, you would have moss growing on the walls. Try keeping a misting bottle of room temperature water nearby your bonsai. Then, as you pass by, several times a day, provide the leaves with a light spritzing.

Another “trick” which will help to increase humidity around an indoor bonsai is to sit the tree in a tray which has been filled with gravel. Water may then be poured into the tray and as it evaporates will help to increase the humidity levels around the bonsai. Make certain to set the bonsai on top of the rocks and do not allow the water to come into contact with the pot’s drainage holes.


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