There are a couple things that could have resulted in weak dahlias:
1. The tubers themselves could have come from a bad or diseased source.
2. There aren’t enough nutrients in the soil. Did you add any fertilizer to the soil before planting? Dahlias are heavy feeders, so having two seasons of dahlias grow in the same spot without any extra fertilizer could have simply depleted the soil’s resources. See our advice, above, about fertilizing dahlias.
There are a couple things that could have resulted in weak dahlias:
1. The tubers themselves could have come from a bad or diseased source.
2. There aren’t enough nutrients in the soil. Did you add any fertilizer to the soil before planting? Dahlias are heavy feeders, so having two seasons of dahlias grow in the same spot without any extra fertilizer could have simply depleted the soil’s resources. See our advice, above, about fertilizing dahlias.