I think it would help immensely if the bottoms were not above the ground an inch or two. I
should have taken the bottoms out but wasn't sure about whether I might want to move them. (Also, the height now is just perfect for being able to sit on the edges and get up again without difficulty, the lower it is the more of a problem caring for things will be.) If the roots could reach the ground it would make a world of difference in a lot of ways. We don't normally get 95 degree weather so it wasn't so clear before what a problem this could be.
These are sort of big window boxes sitting on the ground so the heat penetrates right into and through ..the ones I planted too thickly don't have problems with evaporation but the size of the plants..I have some tomato plants which would apparently be happy with about a gallon of water a day each sigh. they all have masses of fruit forming though so need the fluid. One box with fewer plants, lollarossa lettuce and a couple of tomatoes, is doing fine and looking quite spectacular. But those needed water to stop the heat from cooking the roots when it was so hot.
The cardboard against the sides seemed to help with keeping the soil a little cooler but the heat did a number on most of my peas before the cardboard was put on, even with a mulch already in place.
I think that after the season is over I need to empty the boxes and knock out the bottoms and that will solve a host of problems. Maybe will simply extend the sides so the height doesn't change, though not exactly sure how stable that would be for sitting on. Also considering making sides out of the styrofoam that grapes often get shipped in and which just gets tossed, if they are covered with a scrap wood wall then that should save the styro from degrading in the sun and will prevent the soil from getting SO hot. Anyone know how to cut styrofoam so it doesn't shatter bits of itself everywhere?