What is the primary characteristic of Russian Knapweed?

Russian Knapweed is recognized primarily for its distinct characteristics, particularly its dense growth habit and extensive root system. It is a perennial that can grow up to three feet tall and typically features narrow, spiny leaves, as well as purple or pink flowers that bloom in clusters. Its ability to spread quickly and form large stands allows it to outcompete native vegetation, making it a significant concern in agriculture and natural ecosystems.

While some of the other options also include aggressive plants, they do not specifically match the defining traits of Russian Knapweed. Bull Thistle, for example, is known for its thorny leaves and large purple flowers but does not share the same invasive behaviors as Russian Knapweed. Jointed Goatgrass is a grassy weed, and Coast Fiddleneck is known for its distinctive flowering pattern rather than the invasive growth habit attributed to Russian Knapweed. Moreover, Mayweed Chamomile is a flowering plant that can also be problematic in some areas, but it does not exhibit the same root structure or competitive characteristics as Russian Knapweed. Hence, the selection reflects the unique and problematic traits of Russian Knapweed most accurately.