Knapweed

Centaurea

A Silvery Checkerspot visiting spotted knapweed

Imagine a genus with 500+ species in it… Then imagine how intimidating it is to name a flower in this group to species! So in the interest of avoiding an error, I’ll stop at the genus level on this one. The folks at Wikipedia report that all the members of the genus Centaurea share these characteristics:

  • they are in the family Asteraceae
  • the leaves at the bottom of the plant are usually divided and become smaller and entire toward the top
  • across the genus, the flowers of plants in this genus can vary in color:  red, pink, blue or yellow
  • the disk flowers (center of the flower) may be darker than the ray flowers along the margins of the flower
  • the flowers erupt from a basket-like cluster of scaly bracts
  • many species are allelopathic, which gives them a competitive advantage; consequently many are considered invasive weeds
  • common names include: knapweed, centaury, centory, starthistles, basketflower, cornflower
  • all are ‘copious nectar producers’ that are attractive to pollinators like butterflies and bees
  • some species, like cornflower (or bachelor’s buttons) are cultivated as beloved ornamentals in gardens

I believe that at least some of the flowers in this gallery are Spotted Knapweed, or star thistle, which is an introduced species that is listed as an invasive plant in much of North America. This biennial or short-lived perennial thrives on disturbed sites. The name “spotted” is derived from the spots formed by black margins on the flower bract tips. The flowers of spotted knapweed are pink to purple and have a cone of bracts below the flowerhead that looks a bit like a basket.  I’ve included some leaf images here as well, so if there’s an expert out there that can key these flowers to species, I’d be grateful.

The plants here were photographed in July and August, growing abundantly in abandoned hayfields near Blacksburg, Virginia.

2 thoughts on “Knapweed”

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s