ok, after three days of 43 C in the fraser valley (Chilliwack BC, Canada) with humidity pushing it over 50 C each day, here are the unfortunate results along with the positive results of the japanese maple tree damage on our property. We have never experienced these types of temps in the 37 years on our property. Our full moon and orange dream did experience burnt tops / tips ( these are both north facing, totally protected, very minimal morning sun). They have never been damaged in any other year. Ozakauki and Iijama sunago, both full sun, and both with burnt leaves and tips ( again, never before has this happened) Surprisingly our autumn moon maple, full sun, no signs of damage at all. All other green leaf and verigated cultivators including seiryu, shishigashira, butterfly, bonfire, viridis, waterfall, atrolineare , kasagyama, all full sun, no damage at all. Every red leaf cultivator (too many to list) on the property, all full sun, no damage at all. I guess we had a good run during all our years in the nursery business as we never experienced a heat bomb of this magnitude.
Hi! If those temperatures are punctual and occasional, and the plant has humid soil, I don't think it will affect it much. Now, what are the minimum temperatures you have in the area? It is not the same that there is a maximum temperature of 40ºC and a minimum of 20ºC, than a maximum of 40ºC and a minimum of 10ºC. Some years ago in my area (Mediterranean climate) we were having a summer with maximums of 30-33ºC and minimums above 20ºC. The maples were having a hard time, but they had an even worse time when we reached 37ºC one day. That day the minimum temperature was 26ºC, I think. The maples all ended up with the leaves badly affected. The use of natural biostimulants helps a lot to these plants to better overcome the summer if it works for you. Best regards.
It only takes a few hours below a plant's minimum temperature tolerance to damage it. What varies is how conditions the previous growing season affect a plant's full cold tolerance being present during a fall or winter cold snap. In that if circumstances that interfere with complete hardening off occur then a plant may be less cold resistant than it would be otherwise. Note that this situation is failure of the plant's full ingrained hardiness to be realized - there is never a method or practice available that will make a plant more hardy than it already can be on its own. Regarding Japanese maples specifically there was a containerized tree and shrub root hardiness study using a set of different kinds including at least one variety of Acer palmatum that found that this last was not root hardy terribly far below freezing when left sitting on top of the ground in an unprotected container. With woody plants in general the first to die from cold are the youngest roots closest to the pot walls. The next to go is the older roots back farther inside the container. And the last to fail is the plant top visible on the outside. This means that there will a tendency to assume various plants have not been damaged by cold exposure because the appearance of the top afterward did not involve signals like scorching or dieback. When in fact a significant proportion of the root system may have been lost. With this perhaps manifesting the following growing season as slower top growth - something that may not be recognized as a signal that the plant was damaged previously.
During our nursery operating years we would get the occasional -20C in winter with strong arctic outflows. Potted and container trees always did fine. Our normal summer temps are between 20 - 30 C with the odd heat wave of 34 - 37C, but very rare. Of course temps like we had this past week have never been encountered. Interesting how the trees reacted overall. I will observe and see how they recover. I don't expect any to have long term effects other then scorched leaves.
I think that drought is worse than heat. Maybe if they can be kept in a very humid atmosphere, and in a shaded place, they will have more chance to stay healthy. But of course, this is for trees that you can move, potted trees. And this is just a guess.
