Why peat moss? They don't need acid soil last time I checked. I would dig the hole as big as you want to dig and mix the native soil with compost. Keep in mind that 'Apache' gets BIG. It's one of those blackberries that sends out canes thicker than my thumb! and they can go at least 12 feet. It doesn't seem to sucker too bad, all the babies tend to be within 3 feet of the mama plant. Nice big berries.

Trianglejohn, thanks for getting back to me. For some reason, I thought blackberries were like blueberries and would also need peat moss. I'll pass on it and will just amend with compost. Thanks for letting me know about their size, I wasn't aware. I will bear that in mind when I choose a planting site and space them.

With most of the cane crops (blackberries and raspberries) you will want to be able to mow or weed around them on all sides. After they get established they like to send runners out to take over the world. These can sometimes be quite far from the original plant. I have all of mine planted in a row with a simple wire "fence" running down the middle (t-posts with clothes line cords). I wrap or tie the canes to the lines as they grow and prune any that go off in a weird direction or want to creep along the ground. You'll get better berries if you limit how long the canes get. For some types 6 feet in the limit with 'Apache' I think you could go 8 feet for the really thick canes. If let them stay long the berries tend to stay small and some of them never fill out. If you ever let the canes reach out and touch the ground they will root and make a baby plant and become a problem - so keep the canes up and secure to make your life simple.