> Patents on food crops, even genetically engineered ones, are evil

Eh, I'd say it's fair to give them a short window of patent protection. Say 20 years, akin to pharmaceuticals.

The only exception should be if the protected variety has a monopoly. Then it immediately loses patent protection.

20 years is the max length of nearly all patents. So you really aren't giving them any concession there.

Also, all patents are monopoly. That is actually what a patent provides, it provides a limited government sanctioned monopoly over the invention.