I have successfully planted young small trees in the summer. You have to shade them. IF you plant in the fall, make sure you water them. They can dry out.
Up in NY, if it doesn't rain after about 5 or 6 days, I water them. Mulching really helps with weeds n moisture retention. When I water I usually put some miracle grow in the water. MAybe 1/2 advertised strength.
If possible, get some agriform tablets. Also, try not to plant them level to the ground. Plant them a little hilly. Sometimes loose soil in the hard existing soil can keep them too wet. It is very common for landscaping contractors to place th shrub / tree on the ground and build a hill of soil around the planting.
My spring planting rows of trees start in the fall. I rototill a straight line where the trees will be, amend the soil with some lime and fertilizer, add some clover seed, and then roll it back it with my truck tire. They say not to mess with the soil, but this method has time for the soil to settle down and I compact it back in. I also roundup about 5ft on each side and clover it. I do mostly bareroot. I also save cardboard squares about 18 inches and use them as a mulch base. Keeps the weeds out. I put some sort of mulch over the top, sometimes composted leaf matter, sometime landscaping mulch.
Pat the soil in good, you wont want any air pockets in the rootball. I soften a bit of the soil on the base, pat the sides as I go up. When about 2/3's full of soil, I water it, put a bit more soil to level it out, then water again. I poke a hole about 2 or 3 inches below the base of the rootball and put a couple agriform tablets in there.
