Question

1. He likes growing flowers.
2. He likes growing a flower.
3. He likes growing the flower.
(Are they all in generic use? I mean 'flowers, a flower, and the flower'.)

Answer 1

2 & 3 is not use in generic use because it is referring to a certain flower.

Answer 2

All are grammatically correct, but 2 and 3 don't mean the same thing as 1.

Sentences 2 and 3 would be used VERY rarely because people usually grow plants that have multiple flowers on them.