Mugwort (Artemisia vulgaris)
When the foliage is crushed it is known to release a pungent odor. One plant can produce as much as 200,000 seeds which are roughly 1mm in diameter and are spread by the wind and easily moved around on tires, mowers and construction equipment.
Yellow Nutsedge (Cyperus esculentus)
Two species, purple and yellow nutsedge (Cyperus rotundus L. #1 CYPRO and C esculentus L. # CYPES), rank among the world's worst weeds. Both are perennials, propagate vegetatively, and resist many control practices commonly used in modern agriculture.
Japanese Stiltgrass (Microstegium vimineum)
Areas infested with Japanese stiltgrass have decreased biodiversity. It thrives as a weed in lawns and gardens.
Bonus number four, any vine that climbs up your deciduous trees or Pine trees, left alone for years, smothering and slowly killing the tree.