Two summers ago, I planted three blackberry plants in my backyard. All three summers, the plants have grown tall and wide with lots of green leaves, but they have never had flowers and never grown fruit. After the second summer, I cut down the plants to about 6 inches high in late fall. This didn't impact the growth at all - but still, no flowers and no fruit. Just lots of thorny branches (I have them staked and they lean against a neighbor's garage wall for support). They are about 7 feet high already this year. What do you suggest to ensure fruit next summer?