One may not typically think of plants as being "aggressive," let alone as "invaders," but plant life competes as fiercely for its niche in various ecosystems and habitats as any species of life. Some types of plants--kudzu, ivy, and crabgrass, to name just a few--are notoriously successful invaders. This informative overview explains why gardeners worried about weeds are not the only people who need to know about invasive plants.