by George Buehler

We in the Ohio valley are now into the most problematic season for our bonsai. The weather is getting increasingly hotter during both the day and night. We can’t depend on Mother Nature to supply the needed rain to keep our trees alive. The wind, when there is some, desiccates the leaves of our trees. During the summer months, our trees are doing what they are supposed to do – putting on leaves, lengthening roots, and expanding trunks. For each of these, they need water and plenty of it. What is the best way to water, when is the best time, how much do we add? These are all questions we will try to cover here. Let me say that I am by no means an expert in this area. What has brought about the idea for this article is that I have read a number of articles recently- both old and new – on the subject. What will be presented here is what I have gathered from these articles. Use the information at your own risk.

When to Water

Herein lays the difficulty in learning this skill. When to water your trees depends on a number of important factors. You need to know the “requirements” of your particular species of plant. You can go to various books or the internet to find out “exactly” what the water requirements of your plants are. Unfortunately, many of the bonsai books available and the articles on the net are written by authors living outside the Ohio valley. You can determine the water requirements from these written sources. However, they only give the species “requirements”. For instance, some trees like the bald cypress or weeping willow can almost live in water alone. Others like the Portulacaria only like water “once in a while” and in fact if you water too much, the tree will react in the negative by shedding its leaves and looking sickly. In general tropicals take more water than deciduous trees.

In addition to the species requirements, the type of soil that you use, the size of the pot, the ratio of the root mass to the amount of soil, the amount of sun as well as the time of day that the sun strikes the tree are all things that need to be considered when setting up a watering schedule.

In regard to the species water requirements, we will leave that up to you to determine since we can’t list every species here. When you do this research, remember that this is just a guide for the amount of water the particular type of tree thrives in best. Also remember that even if the tree requires lots of water in the summer months, this is generally not the case in the winter for outdoor trees. In general, outdoor trees should be watered every day. You should look at your trees and they should be watered when the top layer of soil is getting dry just under the surface. Several years ago, a previous editor of this newsletter explained the chopstick method of determining when the tree should be watered. This method was developed by a bonsai author and lecturer Michael Persiano. The basic idea is to insert a wooden chopstick into the soil of each tree. Every day, remove the chopstick and feel it. If the chopstick is WET – don’t water. If the chopstick is DRY – water immediately and thoroughly – you have passed the time when the tree needed to be watered. If the chopstick is JUST BARELY MOSIT – water fully – this is supposed to be the ideal time to water. If you have three or four trees, this method is practical. However, if you have a large number of trees, just feeling the chopstick on a daily basis would take a large amount of time.

Another method would be to go around and barely scratch the top soil level to see if the layer under the top is dry. If it is, then water. Still another method is to purchase a moisture meter to get a ‘scientific’ reading on the amount of moisture in the soil. I have personally never tried this since I have heard there is a large variation in the readings from time to time (the meter relies on electrical conductivity to get a reading and many things would effect the results). Even if you relied on either of these methods, it would be time consuming and most of us would rather be styling our trees than trying to determine if the trees needed watering.

Up to a point, probably the most tried and true method for outdoor trees is to water every day that it doesn’t rain. I have been doing bonsai for about 15 years now, and this is the method I use for most of my trees. As stated earlier, some trees won’t take this method too well, in that they don’t require that much water (i.e. some pines, and desert succulents). Using this method, my trees seem to thrive. I do add the caveat that I also fertilize with several types of fertilizers every two weeks which may help the trees more than the damage done in watering everyday whether the plants need it or not.

TYPE OF SOIL

If you ask 10 bonsai artists about soil, you probably will get 11 (or more) different answers. We won’t go into the types of soil here except to say that larger granular soil has more void volume and when you water, this void volume holds more water than a smaller granular soil. Therefore, if you compare the soil for a Shohin versus that for other style trees, the soil in the Shohin doesn’t hold as much water because a smaller granular soil is used. Therefore, the Shohin may need to be watered as much as twice a day. [More on Shohin in a future issue].

SIZE OF THE POT

Small pots normally need water more often than large pots. It is not only the size of the pot, it is also due to the fact that trees in larger pots have longer roots and can gather the water from a larger volume. This assumes that the size of the soil is the same and the species of tree is the same. You also have to be careful in that if a tree has been in a pot for several years without root pruning, there will be a larger amount of roots (less soil void volume) and the soil hold less water. The tree would therefore need watering more frequently to get sufficient amounts of water.

RATIO OF ROOT MASS TO VOID VOLUME

As stated above, the longer the tree has been in the pot without root pruning, the less void volume there will be in the soil and the less total water that will be held in the pot. You can prove this by weighing one of your trees that has been planted for several years. Then water thoroughly and weigh again. If you compare this to the weight of a pot with the same type soil both before and after watering, you will see that the pot – sans tree- will weigh more after watering than the pot with the well established root mass. The higher the ratio of root mass to void volume, the less weight of water pick up. As an aside, this method can be used as an indication of when a tree needs to be root pruned.

TREE PLACEMENT

Trees that are in full sun all day long will require more water than trees that are exposed to only several hours a day of direct sunlight. This is again a generalization and would have to be a direct comparison between two trees of the same species. As an experiment, I have placed a tree in full sunlight for the majority of the day, weighed it and then watered it and reweighed. I did the same when the tree was placed where it would only receive a couple hours of direct sunlight. The indication was that the tree in direct sun all day took more water. The relative humidity plays a part in the results but were neglected in this test.

TIME OF DAY TO WATER

It has also been suggested (J.R. Cody – Journal of the American Bonsai Society – Spring 2004) that watering at certain times cools the growing media and leads to better root growth. The experiment was that the temperature of the root mass was measured, compared to the ambient temperature and then watered at various times of the day. Cody concluded that when the tree was watered around noon, the temperature of the pot and root mass was cooled down and this cooler temperature was more beneficial to root growth. He also concluded that smaller pots had greater swings in temperature during the heat of summer and watering during the hottest part of the day was more beneficial. It should be noted that when watering in the AM or midday, the water evaporates during the warmth of the day and may in fact dry out so that the plant suffers more than when it is watered in the early evening.

HOW TO WATER

What is the best way to water? I can’t find a consensus from the various sources I have read. The most common – two sources, if that is a consensus – is as follows: First wet the surface so it will take water; then go back and water thoroughly. Water until it starts coming out through the drainage holes. As long as the soil drains well, it is hard to over water. Also several sources say to spray the foliage both on top and underneath in addition to watering. This will both wash off any “crud” on the leaves and also help get rid of some pests. I spray most of my trees once a week using a 1/4 inch self storing hose that I obtained from a garden supply catalog. It has a flow rate of about 2 gallons/minute at full opening but can be controlled to a very small stream. Using the full stream washes off the leaves but doesn’t damage them. Do not water the leaves in the hot sun because they might burn. Nor should you water late at night since leaving the leaves wet at night may encourage diseases.

Let’s look at the most common methods of watering.

Dunk method – As the name implies, you simply lower the pot into a container of water until the top of the pot reaches the level of the water. Then let the water come up through the bottom drain holes until the surface of the soil is completely wet. Many people use this method; however, I find that it is difficult to control holding the pot level with the top of the water without causing the water to come over the top of the pot. If you put only the proper level of water in the container, you can then control the level on the pot. If you are like me, you don’t have one size pot, therefore, the level of water would have to be adjusted for different pot heights. Also, some of my pots are rather heavy and large and the container of water would have to be very large.

Sprinkling can method – You simply fill up a sprinkling can and pour it onto the soil. There are several types of watering cans listed in bonsai catalogs that are made out of metal and have long spouts. Probably a plastic type obtained at a garden center would also work well with a little patience. I would recommend that the head – sometimes called a rose – have a large number of small holes which would give a gentle spray. If you use this type device, watering should be done in early morning so that the leaves dry out before the sun hits the leaves. If done in the evening, it should be done early so the leaves dry before dark.

Drip Irrigation method – As your collection grows, you might want to investigate an automatic watering system. This is the method that I have used for the last three years. Don’t take this as a recommendation to try it. It is fraught with situations that have to be overcome. It works on a regular schedule and waters whether the plants need it or not. This may be good or bad depending on the article you read or the person you talk to. I went to this method because of my travel schedule. DripWorks, a mail order company offers a number of different heads suited to bonsai. With time, experimentation, and money you can get a system set up to handle different size pots and different water requirements. If you go to this method, you will still need to examine your trees on a regular basis. MOST importantly, make sure you have a back up in case the system doesn’t come on properly. When we have to be away, I employ a neighbor boy to daily check to make sure the system came on and to check the trees that are not on the system (i.e. the ones that don’t need daily watering). More on this type system in a future article.

FINAL THOUGHTS

Lee Squires has offered an excellent tip on watering. Lee states that "around July 1, he starts using a combination of Roots and AquaGro in his watering schedule. Roots is a liquid chelated iron fertilizer with plant hormones, Vitamin B1 and growth inducers and stress reducers. Roots also helps the roots to absorb water and nutrients much easier. The chelating part helps make iron available for the plant and ,therefore, keeps the plant green and avoids chlorosis (yellowing) AquaGro is a wetting agent and it helps break the bond on top of the soil that makes water soaking in.. Water soaks in very quickly to all parts of the pot leaving no dry spots in the pot. Mix and use these together in a watering can. They are great. I use them before I go on vacation and this reduces heat stress and inadequate watering caused by an unfamiliar person watering your trees. In other words, all of the water goes into the pot and doesn't run off. I have advocated this for years.”

Whatever method of watering you use, the trees should be checked regularly to look for other problems. This is a must. If your collection is small the immersion or sprinkling can method is suitable. As you get more trees or your pots become large, you may want to settle on the sprinkling can or the garden hose equipped with a fine nozzle. You have to know’ your trees to decide on which method you use. Keeping detailed records on when trees were root pruned or when they suffered from other problems will help your decision – as in knowing the root mass. Whatever system you use, make sure you watch your trees as we go into the hottest part of the year.