With longer summer days and temperatures in excess of 40C several days in a row the pressure is really on watering and watering properly. With the dry weather I start to notice the areas on the benches that don’t get full coverage from the watering system as a few trees start to wilt and leaves brown off. Sensitive trees are moved to areas that recieve a full share of water while drought tolerant species can manage better in the ‘rain shadow’ areas.
The hand watered bonsai in the display areas are mostly still doing well despite being in full sun the whole day. I had to move a few that were more tender and just couldn’t take the intense sun. Wisteria was the first to wilt and develop burnt leaves and after a series of days above 40C my crabapple and Japanese maples also had burnt leaves and have been moved to places under the shade cloth. I still have the ‘waterswept’ callistemon, tridents, Chinese elms, shimpaku and a zelkova in full sun and showing very little ill effects. These trees are watered by hand every morning and evening to ensure the water penetrates right through the rootball.
I am still convinced that as much sun as possible is best for bonsai. Trees in full sun develop shorter internodes and smaller leaves and look better proportioned than those grown in shade. the trick is to water properly to keep up with the increased demand for moisture. After several cooler summers I have had to put the shade cloth over the nursery to help the developing trees keep growing and to give a little shelter to the more tender species. The shade cloth is the lightest I could obtain at 30% shade to try to keep growth more compact.
Different species cope with dry conditions differently. Many with stiff growth like pines rarely show signs of stress until it is too late. The tips on trees with new growth usually wilt when the roots are getting dry giving you a cue that water is needed urgently. In extreme cases trees can shut off supply to the leaves to conserve dwindling internal moisture reserves. The leaves will turn brown and even drop off but all is not lost. If the roots are rehydrated in time the tree can often recover and grow a new set of leaves. Whenever I see signs of moisture stress I water immediately and, if possible, submerge the whole pot into a tub of water. Simple watering often does not penetrate into the centre of a dry rootball and the tree will go into the following day already partly dry. Soaking for several hours allows the water to soak right through the dry root ball and allows the tree to soak up water and rehydrate properly.
I find that trees planted with rocks and those that have not been repotted for some time dry out far quicker than other bonsai so pay more attention to these in hot weather.
There is a persistent myth that watering during sunny days will burn leaves. There are many successful bonsi growers who mist their trees several times on hot days to cool the trees and keep them hydrated. Water absorbs large amounts of heat as it evaporates which cools the surrounding air. In high humidity the pores on the leaves close and there is less transpiration from the tree which conserves the available soil moisture so watering on hot days makes good sense if it is needed. Just take care when watering during a hot day. The water in a hose lying in the sun can be close to boiling point and will scald plants so direct the initial flow away from the plants until cooler water comes through.