Alfalfa

Alfalfa, Medicago sativa, is a common cultivated forage crop. It's nice in rotation since, as being in Fabaceae, it fixes nitrogen, helping rejuvenate the soil. The problem?

The seeds stay in the seed bank and spread around. If you're trying to do a prairie restoration in an old field, alfalfa will be a common pest; prairies tend to be nitrogen-deficient, so the alfalfa will have a comparative advantage.

At least it's pretty.