What the H2O?

I have been working on bonsai for about twenty-five years now. I have been fortunate to study extensively with some of the best instructors this country has to offer. And after all of that time and study, I am still trying to figure out how to water my trees effectively. It seems to be the one skill that is so vital to the health of trees, yet remains elusive, even for the professional. Each tree has it’s own, separate watering needs, so meeting them all can be a challenge. This might seem like a strange time to talk about watering, now that we are moving into fall. But really, watering is a year round activity for all bonsai endeavors. The problem with water is keeping an even balance. Far too often we either have too much or too little on any given tree. So now is a good time to really concentrate our efforts as the needs of trees change during this fall season.

What our trees really desire is a balanced level of moisture throughout the year. They don’t really get that in nature, with swings in moisture levels from spring through summer to wet winters. But trees in nature also have a built in buffer in the ground that they grow in. That’s an advantage that we don’t have in bonsai. Our trees are out on benches and expose both top AND bottom to heat, cold, wind, and rain. Conditions can change rapidly, drying out too fast, or hold too much water in a shallow pot. Too little water and the tree starts to shut down. Growth and vigor are inhibited and wilting may occur. We may also see roots die off. Then, when we do add water, the tree is unable to uptake the water and the issue is compounded and we end up giving too much. We really stress the tree out. With too much water we start to deprive the tree of oxygen and it starts to suffocate the roots. A vicious cycle ensues.

Healthy trees use a large amount of water. Most of this is for air conditioning of the tree via transpiration through the foliage. Only a relatively small portion goes to metabolic growth. My wisterias sit in tubs of water for about six months of the year. On a hot summer day they will drain an inch of water or more out of the tub in order to stay cool, folding their leaves likes wings in order to minimize the surface exposed to the sun. So supplying enough water is critical. How are we going to do it?

First of all, good watering starts with proper potting techniques. You must set your trees up for success. It is critical to leave a portion of the rim exposed above the soil line when repotting. This edge will catch water and keep it moving through the pot instead of dribbling out onto the ground. Also, the flatter the top of the soil surface is, the more even the penetration of water to all areas of the root ball will be. You probably want it to be a little mounded though. The next thing that will help is to have the soil covered with nice green moss. This will create a buffer for the tree. It helps direct the water down into the soil when watering and it also keeps the moisture from evaporating out of our coarse soil mixes. How do you get the moss going? Mix a little grated New Zealand sphagnum moss and the green moss of your choice and sprinkle it on after repotting. You’ll have a verdant crop in no time.  

Now that your trees are set up properly in pots, get them sitting right on the benches. To water well, you need access to both sides of the pot. Hopefully you can walk on either side. I like to hit the tree from both sides, which means that at least three-quarters of the tree is getting watered twice. And that is the next important point. Make sure to water each tree at least twice each session, if it needs water. The first past just moistens the surface, kind of like remoistening a sponge. The second pass is then able to penetrate the top layer and make it down to the roots. I found that when I went back after watering and uncovered the soil that I thought was well watered, it was still bone dry. No water had made it through. Lesson learned. When a tree needs water, it is best to get it done thoroughly, letting water reach all of the roots. Then allow the tree to use that water at its own pace. That means not watering it again until it needs it. There in lies the quandary for many of us. What if you water in the morning and tree runs out of water at 2:00? Most of us have no choice. We have to go to work, so our only opportunity to water is before and after work each day during the summer. Professional nurseries often check water up to five times a day. That doesn’t mean that every tree gets water each time. Some trees like white pines that like drier conditions may only get one thorough watering a day. I have a black pine that takes a thorough drenching three times a day if it can get it. Luckily, I happen to work at home, so I can serve its thirsty desires. The point is that the more often you can check water on your trees, the more appropriately you can water.

There are some other factors that weigh in on the water usage of each tree. The aforementioned black pine is in a fairly deep pot, but not super big for the tree. What the tree has is a dense foliage mass that really churns through the water. It isn’t the size of the trunk that governs the water needs, but the foliage mass, and the proportion that it has with the pot. Obviously shallower pots are going to hold less moisture overall. Orientation is another one. My trees use water at different times and amount during the day based on when they get sun. Many of my trees do not get direct sun all day, so I must adjust my watering, even for things that are a foot apart – in shade and out. Season is also a big factor. In the spring when things are pushing, trees will have a voracious appetite for water. Then in the summer, when things have hardened off, trees may enter summer dormancy, so they may actually use less water when it get really hot. That is the challenge of staying on top of water situation. My general rule is that if it is going to be above eighty degrees, I MUST check water twice a day. When it is above ninety, I check three times. The we get to winter where we don’t have to water very much around here. But with a dense canopy on a good tree, the load is great and not much water is making it through, so keep checking unless you’ve had a hard downpour or day of rain unending. When deciduous trees lose their leaves, don’t let them dry out. They still need some moisture.

So there is an awful lot to watering. And I am still learning. I hope this will help you in some way with your trees and maximizing their health through good watering practices.

Scott E