What are the symptoms of roots being too hot?

I am concerned, b/c of switching to using granite as a substrate, that the soil for my trees might get too warm come July and August and the roots will become very unhappy with me. Last summer(my first year with granite) was not as hot here and all was well. Over the winter I read a warning from Michael Hagedorn about granite substrate holding too much heat. So...
Has anyone experienced this first hand and agree or disagree?
If agree, what would the symptoms be and are there ways to moderate the effect?

I was thinking that a 'nest' of rough sphagnum covering the entire top of the soil in each pot and dampened daily might serve effectively as an insulator. Thoughts on that?

David...I do exactly what you said re: a thick layer of sphagnum on top of the soil. Are you commited to granite? Is it because you'd be getting an upper body workout when moving them? Or helping the bonsai stay on the bench in high winds? Large size pumice works for me.

Temperature not heat is what matters, but this is thermodynamics and you want horticulture.

Get a meat thermometer and stick it in among the roots toward the edge of your pot. Make sure the soil temperature stays below 95F (35C) and all will be fine.

Thanks for the responses.

Bruce-
I am not committed to granite, but what I buy at the local feed store is cheap and relatively uniform in size, does keep the pot in place, and helps me maintain that svelte figure I have purchased 'horticultural pumice' online, but what I received has great variation in size- from almost 1/2" down to basically the size of sand. At that rate I would be sifting out and having no use for a large percentage of a product that is expensive to me. If I could find a better source, pre-sifted...

Osoyoung-
How universal is that 95F temperature? Is it the same whether succulent, deciduous, tropical or evergreen?

Precarious wrote:How universal is that 95F temperature?  Is it the same whether succulent, deciduous, tropical or evergreen?

Applies to pretty much all temperate trees.

I do wonder when the air temperature reaches 100 deg, F [ 38 deg. C ] if it makes any difference. if it is still 100 deg.F in the shade. The roots will shut down.
I would then ask what about the humidity, nothing evaporating, nothing cooling ?
Breezes to aid in evaporation?

Suggestions - first note that Bonsai is from zones 7 to zone 9/10 and the trees were local/adapted to their areas.

[b] Work with trees from your area?

Please note down here we reach 90 deg.F [ 33 deg.C ] as a maximum and only for half an hour to ten minutes, with 45 to 50 % humidity and with Easterlies.
So before anyone goes off on the hot, humid tropics bit, the tropics vary considerably. India is hitting I believe 50 to 45 deg.C presently.

For high temperatures, and high humidity, you may also need a fan to help the evaporation bit.
Unless you are doing the desert bit with high temperatures and low humidity?

Good Luck
David.

Laters.
Khaimraj

david... i would try to find a local source for small pumice and swap that out for the granite and to hell with your figure
(i feel like i may have said that before in a different thread)

Suggestions - first note that Bonsai is from zones 7 to zone 9/10 and the trees were local/adapted to their areas.

[b] Work with trees from your area?

Careful . . . pots get LOTS hotter than native soil.

However, we get a LOT hotter down here than you northerners, and my pots sometimes get almost hot to the touch. I've not seen any adverse effects on my trees other than a normal mid/late summer slowdown in growth.

Amazon temperatures from the Geography book ---- 27 deg.C [ 80 deg,F ] the humidity is what gives the feeling of a higher temperature.

So the plant shuts down, as root and top ?
Then what may be more important is a study on adaptability of trees, rather than soils holding more moisture or less moisture.

There are bound to be elders in Bonsai out there who can be reached by telephone or car and talked to, in David's area or similar.
Until.
Khaimraj