The "beating back" thing does not work with salmonberry. Perhaps if you catch it when its truly young... These roots have amazing energy stored in them. I have used cardboard. Both these plants punch right up through it. I have my own answers, but I am hoping for philosophies or experiences for others. Its might be a very unique aspect to my own property, being old farmland in a river valley with just the right circumstances, and tons of birds dropping the seeds literally everywhere. Any area which is not constantly groomed will have these plants covering it within a year, and then it takes a lot of work with a machets, loppers, and some stiff gloves to get anywhere near it. Truly a jungle, and made of extremely nasty thorns. Another sad note is that while it seems variable regionally, the salmonberry here ranges in flavor from awful to wholly insipid. I am sure that with enough sugar, a tolerable jam could be made, but I don't bother. The blackberries, on the other hand, are delicious and I don't mind dedicating some pretty huge areas to their growth. But as above, the land is useless for other plants, animals, and people if its not kept somewhat in check.