* General:
* A stand of plants
* A patch of plants
* A bed of plants
* A grove of plants (for trees)
* A thicket of plants (for dense growth)
* Specific:
* A bouquet of flowers
* A clump of ferns
* A field of wheat
* A forest of trees
Ultimately, the best collective noun depends on the context and the specific type of plants you're referring to.