I purchased thornless blackberry bushes from the local (Wayne County, Pennsylvania) Conservation Department about 8 years ago. They look vigorous and are in good, loamy soil and have good drainage with full sun. They have never produced more than a handful of berries. I prune them back in the spring and they grow back vigorously, but again no berries. They don't look diseased, but they don't produce.
Do I need another variety? Should I dig them up and start from scratch? Suggestions?