A milkethistle inflorescence sticking through a fence in South
Davis. Thistles (Cirsium spp. and several other genera) are
annuals and herbaceous perennials/biennials in the aster family
(Asteraceae) that can grow up to four-feet or more in full sun.
Some are native, and many have economic importance, either as
crops or weeds. Much beloved by the Scots, they are typically
recognized by their flowerheads, which can be purple, rose,
yellow, or white, and their sharp spiny stems and hairy leaves.
In some places, thistles are actually a protected species!
Thistles begin to bloom locally in spring, and persist
throughout the hot summer months. They can spread by rhizome or
seeds which are attached to the tufted flower — this plant self-
seeds freely! As with most in the Aster family (incl.
dandelions), the seeds are borne on a white fuzzy 'pappus' that
helps them float away to find new places to grow. The seeds are
surprisingly large for floating on the breeze! Thistles have a
deep root system which makes them extr