Once your blackberry plants grow new shoots that develop some leaves and are a foot or two tall, that will prove out the viability of the clump. It would then be fine to prune out some of the old original canes and leave a couple to bloom and fruit. Blackberries in good soil and properly sited are vigorous plants and can tolerate a lot of literature ignoring. If for some reason the old canes do not bloom and the new canes get pretty tall and leafy, then cut out all the old canes.

Lisa, Blackberries produce most of their fruit on two-year-old wood. In other words, each nice strong green cane that grows needs to suffer some winter weather before it will produce most of its fruit its second year. Then after that, it dies or is close to death. I think this is why you are seeing some of your canes falling off. They have fruited over the past couple years and are now dead or not worth keeping. At the same time, you have a lot of brand new bright green canes coming up. Those will not produce much fruit this year and should be left alone to grow until they are about 4 or 5 feet high. So basically, here's what you need to do: Each winter (or I guess you could do this right now if some of your canes are falling off), identify the canes that look totally dead and full of branches and ready to fall off, and cut them off at the ground. These ones have already fruited and thus must be three years old and are no good to you anymore. If they have fresh flowers, you could keep them this year but I'd whack everything that wasn't vigorous and green next winter. With the canes you have left, they should be greenish-brown instead of a dull gray. Thin the greenish ones out so that there is one every ~4 inches or something like that. It's not good to have them so close together that you cannot reach all the fruit. A lot of growers say that each plant that you originally put in the ground is reall only intended to grow maybe 5 or 6 canes at one time, so if your original plant is growing 10 or more canes, then you have way too many! Pick out the weakest ones and cut them off at the ground, and leave all the strong green ones alone. Each summer, when the green ones get to be around shoulder height, cut the tops off to promote more fruiting branches for next year. If they don't get to be shoulder height, you can just leave them alone. You'll also want to tie them up so they don't fall over. Each year you should have about a 50/50 mix of brand new green canes that don't produce fruit, and older brownish canes that produce all of your fruit and then die in the winter. So then when winter comes, you cut the dead ones off, and new green ones grow to take their place next spring while the green ones from last year make your fruit. Make sense now?

Unless you are in the Texas Highlands, I would keep those in the pots, and in mostly shade, and water them enough just so the pots have a little heft; make sure the water is penetrating to the roots and not just running down the sides and out the holes. Plant out maybe 1 October. Prune off dead areas anytime.

There's lots of great information on the NC State University's co-op site. I also recommend stella otto's book about berries, the backyard berry gardener or some such. The ones i grow fruit on second year canes(there's a new primocane bberry), so it's similar to june bearing raspberries as far as pruning. In a nutshell, you prune out the canes once they've borne. The new canes that will fruit the next year should already be waist to head high at that point. There's lots of ways to go about accomplishing it. One of the more interesting is to have 2 fences on either side of the row and tie the canes to alternate ones each year so you're pruning all the canes from one fence each year after they bear for the summer. Good luck with your new venture!