Knapweed and Lesser knapweed

Knapweed noun - Any of various plants of the genus Centaurea having purple thistlelike flowers.

Lesser knapweed and knapweed are semantically related. In some cases you can replace phrase "Lesser knapweed" with "Knapweed", this terms are similar.

Lesser knapweed noun - A weedy perennial with tough wiry stems and purple flowers; native to Europe but widely naturalized.

Knapweed and lesser knapweed are semantically related. You can use "Knapweed" instead a noun phrase "Lesser knapweed".