...or is it too hot? If I plant in June or July, what extra steps need to be taken? I am in southern Ontario.

Its recommend to plant trees in the spring or fall although you certainly can plant them in the summer. You just have to make sure you water them often. The best way to do this is just to place a trickling house next a bit away from the truck and leave it running for about an hour a day. Putting the flow of water away from the tree will train the roots to go out and down. Just make sure you soak it long enough to be able to soak deep into the soil were the roots are.