Introduced from Japan in the 19th century, Japanese knotweed is now an invasive non-native plant of many riverbanks, waste grounds and roadside verges, where it prevents native species from…

The tightly packed, thistle-like purple flower heads of common knapweed bloom on all kinds of grasslands. Also regularly called black knapweed, this plant attracts clouds of butterflies.

The ragged-edged, purple flower heads of Greater knapweed bloom on sunny chalk grasslands and clifftops, and along woodland rides. They attract clouds of butterflies.

On first glance, the meadow thistle looks a bit like a knapweed - it's not as prickly as other thistles and only carries one pinky-purple flower head. It can be found in damp meadows and…