This morning I got to thinking about last year's efforts to keep the tomato plants cooler. Some of you might remember that last year in addition to shade cloth, I decided to hang old folded bed sheets on both sides of the line of pots to help shade them. From the Way Back Machine:



So I did more digging online and found a super duper article from U. of Kentucky about the effect of heat on the roots of container plants. Lots of great information in there. The article is geared towards nursery practices but that's OK as I use nursery pots. Having worked in a nursery for 18 years I have enough pots to do what I need to do.

It seems like I did the right thing by shading the tomato pots. Yes, there's a big effect of soil temperature on plant roots in pots and heat builds quickly in the soil. I have seen roots like the photo in the article on plants in the "dead pot pile". Now the article is geared towards a nursery mix medium so the measurements might be a little different in bagged mixes like what one would buy at the big boxes or hardware stores. I use a mix similar to a nursery mix that I buy in bulk.

The article also addresses various types and colors of pots, effects of irrigation, times of day, etc.

Here's the article. It's a bit long but well worth the read : http://www2.ca.uky.edu/agcomm/pubs/HO/HO119/HO119.pdf

One wrinkle I'm thinking about adding this summer is perhaps wetting down the sheets several times in the hot part of the day. I'm wondering if it would have a kind of "swamp cooler" effect on the pots as in evaporative cooling or if the sheets would just dry out too fast to make it worth my while. Might be worth playing with.

The quote below might explain why my tomatoes in wider, lower pots did better than in taller ones, I think. Same volume of soil just different shape of pot. I did use paper on the sunny side to reduce exposure of the black pots to the hottest sun.
For smaller ornamentals I often put a plastic potted plant into a clay pot. Even some trees if the pottery was large enough. It makes sense that they would insulate from the heat of the sun.

▷ Select container shapes that reduce the sidewall angle or decrease
the container wall surface-to container volume ratio

Its a no brainer.
Thats why you don't put a glass thermometer in the sun and temperature is measured in the shade.
The side of any container will get screaming hot from IR Light.
That's the stuff that makes heat.
Any shade will help a lot.

@GoDawgs I’m glad you’re trying the sheet watering experiment. I was deliberating doing the same on the shade cloth I’ll use this summer to see if it makes a difference. I’ll be interested to hear your results. Thanks for the article.

I lost plants last year in containers,ground temp measured 87 F
The plants were store bought and I only had them 1 month.
Planting in 2 hole cement blocks,I have one of those armored garden hoses snaked thru the holes that I pump rain water in a closed loop system.
Evaporation cooling I could try spraying the blocks but they did ok with the hose method.
The plants survived past the hurricane season here.

The calcium in the cement helps fight BER IMHO

Cement blocks were too hot in our current heat wave but what did well was a styrofoam shipping insert I saved from when I got a wood splitter 2 years ago.Sungold is doing well it a section that had no bottom so the foam is just on the sides.The other section I left intact with drainage holes.That plant was less fortunate.Another planter is an old plastic insulated cooler I drilled holes in and is portable.Super fantastic is in that and doing well.Partial shade.