Sunflowers belong to the Asteraceae or Compositae family, which includes many weedlike, invasive flowers. Sunflowers are an attractive garden plant. Commercial farmers grow it for its edible nuts and oils and amateur gardeners celebrate it for its bright colours. Within the sunflower plant family are many kinds of weeds, and sunflowers themselves may exhibit some weedlike characteristics.
The common dandelion (Taraxacum officinale) is in the sunflower plant family. Known as one of the most difficult weeds to eradicate, dandelion seeds heavily and spreads quickly without cultivation. Marsh sowthistle (Sonchus arvensis), which resembles dandelion, is also an invasive weed in the sunflower family. Sunflowers share many characteristics with these weeds. All three produce yellow flower heads at the top of long, slender stems, but sunflowers are much larger than both dandelion and sowthistle.
Several flowers in the Asteraceae family look like sunflowers. Heart-leaved arnica (Arnica cordifolia) is an invasive but pretty flowering weed with bright yellow blossoms. The prairie coneflower (Ratibida columnifera) looks like a drooping sunflower. Prairie coneflowers have yellow petals that grow around a central brown disk with a drooping, limp form. The blanket flower (Gaillardia aristata) and the golden aster (Heterotheca villosa) also resemble sunflowers but both are invasive.
Nuttall's sunflower (Helianthus nuttallii) is an invasive sunflower that resembles more familiar garden varieties of the plant. Nuttall's sunflower grows without cultivation and self-seeds itself, spreading to new areas as it grows. Alpine sunflower (Tetraneuris grandiflora) is also an invasive sunflower you don't want in the garden. Both flowers have vivid yellow blossoms and yellow centres.
Sunflowers do display some weedlike characteristics, and some varieties of the plant are classified as noxious, or invasive. Some sunflowers have creeping roots that are capable of producing new sunflowers, which makes the flowers difficult to control and contain in garden environments. Sunflowers produce seeds that are also capable of becoming new plants, and if left to their own devices the sunflowers may produce an entire field of flowers that compete with existing vegetation.