I planted about 8 thornless blackberry plants about 4+ years ago and I'm only getting about 10 blackberries per year. The plants are very healthy and grow great. The new growth is about 10 feet long now. I have tried pruning, not pruning, less water, more water, taking the old canes, leaving all the canes. I'm perplexed. Should I spray it all with Roundup and start again with a different variety? I've done a little research and it sounds like the Triple Crown Thornless are the best fit for the area. I live in Monroe, Utah. Do you have any ideas?
Sevier County Utah blackberries
My guess is that your problem is related to winter and/or spring cold injury. Nearly every thorn-less blackberry only fruits on second-year canes. Outside of the Wasatch Front and Dixie, getting blackberries to fruit consistently is very difficult in Utah. The canes are killed outright during the winter, or the flower buds are damaged either in the winter or by spring frosts.
If your problem is the latter (damaged flower buds, but not cane death), you might try the thornless cultivar 'Chester' which tends to bloom again on secondary flower buds if the first ones are injured. If you are having significant cane dieback, you could try 'Illini Hardy' which is the blackberry with the best winter hardiness. Unfortunately, it is obnoxiously thorny and can be weedy.