Heat stress or leaf burn as I call my trouble has been my biggest hurdle in the 4 years I have been doing this. I got hooked on the hornbeams that are a cold climate tree. They strugle in our heat. I was constantly searching for the best solution to shield them from the heat. I made some mistakes. a couple of years ago I made a shade house that was in a large area behind the shed. It was 70% shade cloth and had very limited ventilation I set this up with a misting watering system. This was the worst year I had. I lost branches and one of my hornbeams.
I spent the next year researching and talking to anyone I could for advice on this. This is what I have found.
Area is very important. Many of the part shade trees like maple and Hornbeam don't even need direct light. If you can find a spot that is always in the shade but has sky above, you will have a great start. My area is a back corner, which is not optimal for ventilation but is the best I have. It is important to have some understanding of air flow around your garden. You want good ventilation but you don't want it exposed to wind currents. I have erected a triangle pergola with bench that gives me sun until about 10am then a range shade from two neighbors trees. The outer most part of my bench is were I put the sun lovers.
Micro climate is very important. An article in Bonsai Europe #66 was very informative in this. It states that misting is of little use as the local humidity will be slightly increased but only for a minute. Appaerently placing water trays or even ponds near the trees will have only a marginal effect on a still summers day but no effect as soon as there is wind.
But have you ever noticed how fresh and cool it is when standing near lush vegitation. This is apparently because the leaves are constantly transpiring. This is much greater even when compared to the surface of a pond. So surrounding your bonsai with plants that have large or soft leaves will dramatically influence the micro climate of your growing area. Hosta are a great choice as they have soft lush leaves are easy to grow and die back in the winter frost to have their bulbs burst out again in spring. They can also make great accent plantings.
Like Pup I think shade cloth is best. I Find lathe houses interesting and I will have to experiment with them in the future.
The best trick with shade cloth as Pup states is to not have it up at the beginning of the growing season. I put mine up when the temps start to hit the 35*. This allows the leaves to harden up some. Remember though I have part shade from trees as well. Remove the shade cloth in Autumn as the temps again drop down past 35*(Autumn colour is increased from sunny days and cold nights)
As heat stress comes from the roots not being able to supply water to transpire through the leaves fast enough then it seems obvious that the better your root system then the better it will be able to cope. So I work at getting the best root system possible. I do this by the best soil I can manage. This is free draining a soil that can never be overwatered. This allows me an almost a scientific approach to watering. There is no need to check each morning like I had in the past which need watering. I can tell when the trees will need watering by the weather conditions of the day. When in the 30's about a third of my trees need watering twice a day. When it gets close to 40 or over I tend to water all three times a day. The joy of this system is I can water all trees multiple times a day with little concern of over watering. Also I can instruct anyone I have recruited on nesacary watering even though I can not see the trees.
Also to encourage root growth I fertilise weekly with full strenght fertiliser as long as I have growth and use seasol to get as much growth as possible from the roots.
Lat season and this have been a great improvement although not yet perfect. In the far corner of my growing area I can feel the coolness from the shade and lush growth under my benches. We have got to 40* this year(not as hot as some) but I still have new growth comming out on my hornbeams.
This is the overall shot of my area showing the shade trees.
bench long.jpg
Her is the inside corner of my bench area. I went mad with hostas last year but they are hidden behind all the other plants/trees growing under my bench. I cut these back before Christmas but I will let the grow wild until the worst of the temps are over. You may also notice the 50% shade cloth (sail). After having trouble finding someone to sew a custom one up my Wife and I spent a day sewing rope to the outside edges so it hooks onto eyelets on the pergola and can be removed easily.
bench corner.jpg
I also have been game to leave some sun lovers out in the full sun with no protection at all. I did pull these in on our hottest days recently as I was testing a auto watering system and I decided to be safe than sorry.
bench pond.jpg
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It's too bad your in such a hurry cause the stories I could tell you, Bushels and baskets of stories, hole crates full of stories. But if you can spare a moment I will tell you one story.
There are a few small green bits of trunk and branch left, so we'll see.
good luck to everyone this summer
