Your goal of growing in pots in 100 degree climate with only infrequent watering is not likely to be a success. A soil mix that retains water that well is probably going to lack oxygen and foster root pathogens and rot. Calistoga Al

I grew two, the Sweet Bay magnolia and another one that I forgot the name of, in large plastic pots this summer, and although they dried out a lot, they did fine. When I got them this spring, they were six inch twigs, but now they are both over two feet high, and bushy. I watered them a lot, but the bottom of the pots were cracked, so water just ran out most of the time. I think that as long as you don't let it wilt, and keep them out of the hot afternoon sun, it will grow great.Put them on the north side of your house, or under a large tree. It helps to keep a tray underneath to hold extra water that will wick up into the soil, too.