Wood chips also work but no nitrogen. I would always wood chip the tree pots thickly. In the heat of the summer i water them everyday. I also fertilize a lot more then i use to. Usually a basic liquid fert…comes in those clear plastic bags.
I use rainwater when i can but i do use house water which comes from a well and the temp is about 53F even in mid summer…so it probably cools those pots down too.