We can do a better job of growing our trees if we understand what makes them tick and here is where a bit of plant physiology come in. Let’s do three mental experiments which hopefully will shed some light on the subject. In the first experiment we plant our favorite bonsai tree in lovely soil and water it enough to keep the soil moist at all times. With a bit of fertilizer and a lot of sunshine the tree will respond by growing vigorously. It might to surprise us to see that the tree has a marvelous plumbing system. There are structures at the ends of the roots, called feeder roots, whose principal job is to slurp up water as well as nutrients from the soil. The water is transported up the tree trunk to the leaves. There some of the water and carbon dioxide from the air are converted to the energy the tree needs to grow by the action of sunlight. Chlorophyll, the chemical that is responsible for the green color in leaves, makes all this happen. Much of the remaining water is then lost by the leaves and the cycle continues. As the tree gets bigger it has more leaves which in turn increases the loss of water by the tree, so it needs to grow more roots to keep the water in the tree in balance.

In the second experiment we pot our favorite tree in lovely soil but don’t water it for three weeks. We will observe that the leaves on the tree will droop and then as time goes by, turn brown and fall off. At the end of three weeks our tree is about dead and no matter how furiously we water the tree the water now does not stave off death. What, exactly, happened? The lack of water in the soil does not allow the tree to replace water lost by the leaves and the tree becomes dehydrated. The tree attempts to make up for the loss of water by dropping leaves. The feeder roots can no longer slurp up water and in addition slowly die. If the tree is watered in time some of the feeder roots can rehydrate, which maybe enough for the tree to slowly recover while it grows new roots. As the drought continues eventually all the roots will die and the tree becomes toast, literally. Once this point is reached no amount of water will revive our tree because there are no functioning roots to take up water. In the real world all this can take place in as little as a couple of weeks.

In our third experiment we pot our favorite tree into lovely bonsai soil and into a pot which does not have drain holes. We water it normally and since the pot has no drain holes the tree ends upstanding in a puddle of water. A bonsai pot with no drain holes is an extreme case for this experiment but a poorly draining soil could also have been used for the mental experiment. We see that the tree slowly drops its leaves and eventually dies. What is happening here? An autopsy reveals a surprising fact. In spite of standing in water, the tree has died of dehydration. How can this be? We live in a world of microbes and not all of them are beneficial by making bread, wine, beer and other good things in life. Standing water is an ideal breeding ground for bacteria and some of these wee beasties can cause root rot. Once the roots have rotted there is no way for the tree to replace water lost by the leaves and then the tree becomes dehydrated just as the tree is the second experiment that we did not water. In the real world this process takes place more slowly than the case where we did not water the tree. Root rot it is difficult to treat and once started saving the tree is problematic.

What does all this tell us? The most import single factor in growing our trees is to maintain conditions which lead to healthy roots. The conditions are a combination of the soil we use and how we water trees in this soil. The lovely bonsai soil mentioned above is one that drains well but still can retain moisture between watering. There are many soil compositions that have been recommended in the past. One currently popular mix is one third each of lava, pumice and akadama (an expensive form of clay pellets mined in Japan). Other mixes involve lava, fine gravel and pumice as well as about 20% of the mix being an organic ingredient such as either fir or redwood bark. Each of the various soil mixes has strong advocates, but in truth be told they all can work well. The other condition is that the soil must be kept moist so managing how much you water the tree is important. Oh yes, be sure that the pot you use for your tree has a drain hole.