January and February are the hottest months of the year here at Shibui Bonsai. So far temperatures have not reached 40 C but it’s still hot enough to make watering my most pressing priority.
The nursery is watered by overhead micro sprays twice each night. I find that a single longer watering does not wet dry pots thoroughly enough, much of the water runs around the dry root zone and out the bottom of the pot. A number of times I have examined trees that have died in summer to find that although the edges of the root zone are damp the centre is still bone dry.
A short initial watering gives the thirsty roots immediate access to water to rehydrate the tree’s cells, then allows the retained moisture to gradually soak further into dry parts of the root ball over the next few hours. The second watering is absorbed more fully into the potting mix and the trees start the day with a full supply of available water.
This year I am trialling keeping both my display bonsai and pre-bonsai in full sun. Shade helps reduce the water requirements of potted plants but it also promotes longer shoots with longer internodes which makes it difficult to develop dense ramification. By keeping the trees in full sun for longer and tailoring the watering to match the increased requirements I’m hoping to achieve some improvement on branch structure and ramification. No deaths so far and only a few leaves with burnt margins. Before rushing to move trees into full sun you should be aware that moving a tree from sheltered conditions into full sun will result in badly burnt leaves. Leaves that have adapted to shade cannot cope with the stresses of heat and sun. Even pruning a tree hard will expose soft leaves that were in the interior and result in sunburn. The stress of seeing this for the first time can nearly be fatal for the owner but, provided adequate moisture is available, most trees will quickly grow new leaves that are adapted to the new conditions.
The more exposed conditions has caused the tips to wilt on my root over rock Callistemon but only on days where I have been less diligent in the watering. The wilted tips quickly revive after an initial watering.
Root over rock trees require much more water than pots without rocks. For a start the rock occupies some of the space normally taken by water holding potting mix but it also appears that many rocks are slightly porous and act as a wick, sucking moisture out of the potting mix and allowing a larger surface for evaporation.
Any trees that show signs of moisture stress such as wilting are soaked in a tub of water overnight to allow the rootball to get fully wet.
Note that trees will lose more water on hot, windy days than on hot, still days. We’re quite lucky here at Shibui Bonsai to be in a relatively sheltered area which is protected from all but the worst winds.
Water requirements not only depend on the weather conditions but also on the amount of leaf on the tree, type of potting mix and how long since it was repotted. Tree roots must continue to grow to keep the tree healthy. As the roots grow they fill the spaces in the potting mix until, after a few years, there are few spaces left for air and water to penetrate. It then becomes far more difficult to water the pot properly.
Potting mix with finer particles not only doesn’t drain well but the growing roots fill the fewer spaces in a shorter time so watering problems – both too wet as well as too dry – occur more easily.
Trimming a tree to reduce the amount of leaf it is supporting will dramatically reduce its water requirements.
Another way to help trees through hot summer days is the humidity tray. A humidity tray consists of a container that holds water filled to the top with gravel. I keep most of my smaller pots, as well as any that I’m having trouble keeping properly hydrated, sitting on humidity trays.
When the pots are watered the container fills with water which gradually evaporates during the day. The humid air surrounding the tree reduces transpiration from the leaves and the tree’s roots will have access to moisture for a longer period. Note that the pot sits on the gravel bed above the water, not sitting in water. I often find fine roots often growing out of the drain holes of the pot and into the gravel of the humidity tray. This will give the tree access to even more water and does not do any harm. Any excess roots can be cut off at the end of summer.