However, there are other cultivated plants that are commonly used in landscapes and turf areas that are weeds in certain situations. Tall fescue in a fine Kentucky bluegrass lawn is a weed. Goldenrod was introduced as an herbaceous perennial flower for colonial gardens. Purple loosestrife is a beautiful flower for today's gardens, but has become a horribly invasive weed in wetlands across the country. Cleome is an annual flower that produces an abundance of viable seed that germinates and becomes a weed in future plantings.
Weeds are first classified botanically, and then by their life cycles.
These are the leaves that are produced by the germinating seed. All of the turf grasses and weed grasses are monocots. All of the broadleaved weeds are dicots. Sedges are in a group that is distinct from monocots and dicots. The significance of classifying weeds into these groups is that their physical structure and biochemical reactions are different enough that herbicides have been developed that can selectively control one group without injuring the others.