Newly planted Dogwood Tree advice

You'd think I didn't already have two flowering dogwoods that I planted. The difference is, I planted them in the early spring 2 years ago. My latest - and final - dogwood, I just planted last night. While it has been milder here in the NY/NJ area, it is still August. As well, this tree is MUCH bigger (height wise) than my other two. The root ball was HUGE. I probably shouldn't admit this, but I cut about 7 inches from the bottom of the root ball. I would have been digging until I was 100 with how big this thing was and it was getting dark. While it is in the hole now, I (sigh) still have to take it out again tonight to move it back a few inches (to be in line and evenly spaced with my other two dogwoods it sits in between).

So I just wanted to find out if the "root ball cutting" I did will cause problems, and how often I should water and "feed" (and what kind of "food"). Also, I used some rooting powder. I only started using rooting powder. Didn't even know about it until recently. I didn't have it or use it with my other two dogwoods but I figured it could only (hopefully) help.