crop sown to suppress persistent weeds. Among the most effective smothering crops is alfalfa, which competes successfully against many weeds for growth space. Sometimes the desired crop plant can be planted so densely that it shades and “chokes out” weedy growth. Annual weeds are generally easier to smother than perennials, particularly such perennials as bindweed, Canada thistle, perennial sow thistle, and quack grass.