I finished pruning the thornless horticultural blackberry bushes. The main one is the variety "Triple Crown", which is the best producing, best tasting and least trouble of the several varieties that I have tried (Until Ponca). Triple Crown is quite vigorous.
Each winter, I remove the old stem that produced this year. It's done and won't bear again. During the summer, I tip the new growth at about four feet tall. That produces new branches, that I prune back to about 18 inches long, in winter . The exact length doesn't seem to be too important.
Finally, I covered the old tree leaf mulch with a fresh layer of tree leaves, about 6 inches thick. I've done that for ten years. The old gummy clay (winter) and brick hard clay (summer) is now a soft loam, like forest floor. These never get fertilizer or summer water, and bear generous crops of big, delicious blackberries each summer.
I have a second bed on the south side of an old shed. They are not as old, maybe three to four years (Triple Crown from cuttings) and two years (Ponca, mail order). The shed is ancient, snd sinking into the ground. The siding is in bad shape. I dug a trench by the shed, to allow the siding to dry. I pruned the Triple Crown as noted above. Ponca is a brachytic dwarf - the internodes are quite short. So it doesn't grow nearly as tall as the others. Ponca's berries are bigger than Triple Crown's and I think the flavor is the best of all I have tried.
The one caution I have about Ponca, so far, is the berries are so heavy that the branches break off. I added a fencepost to tie supports for next year.
The brachytic dwarf character shows in this photo, with Ponca in front and Triple Crown in back.