It's pointless to talk about maximum temperatures for woody plants without also considering the humidity. Japanese maple, Acer palmatum foliage can burn from excessive transpiration even in the high 80's F in strong dry winds. On the other hand, I start cuttings of them in the greenhouse with the humidity at 80 to 90% with the daytime temperature held at 95F and above. I don't start seeing defoliation on the cuttings until it gets higher than about 102F. Below this temperature, they are as happy as clams, and they don't even have a root system.
So, the solution is to treat them as cuttings. Give them 50% shade, perferably with shade cloth to avoid any direct sunlight. Next, loosely tent with with poly, but allow some air circulation, mist under the tent when it starts to get dry. Keep them OUTSIDE, unless you have a greehouse.
An even more bizarre examples: I helped my neighbor start some grape cuttings one year in his greenhouse. On one of the hottest days of the summer (of course) the power goes off and we have to ventilation and no mist. His helper comes running to me and asks me if he should throw open the doors to cool it off. I tell him absolutely not! As long as the greenhouse is closed, the humidity stays very high. The power was off for a couple hours. Later, I read the max/min thermometer. It got up to 135F, but very few of the plants were damaged.
Brent
EvergreenGardenworks.com
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