Bushes:
* A thicket of bushes
* A clump of bushes
* A grove of bushes
* A stand of bushes
* A copse of bushes (if they are small trees)
Corn:
* A field of corn
* A stand of corn
* A crop of corn
* A patch of corn
* A tassel of corn (for the flowering tops)
* An ear of corn (for the mature corn)
It's important to note that these are just a few suggestions, and the best collective noun to use will depend on the specific context.