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Oct 19, 2020

What’s That Weed?

Exotic species:  Milk thistle (Silybum marianum)

Characteristics:  Milk thistle is a sparsely-branched, biennial thistle, growing up to 6 ft. (2 m) tall. The alternate, deeply-lobed leaves may reach 20 in. (50 cm) in length and 10 in. (25 cm) in width, and feature a distinctive white marbling pattern and spiny edges.  The plant may overwinter as a rosette.  Single purple flowers up to 2 in. (5 cm) across emerge anytime from April to October at the end of the stem, with leathery, spiny-tipped bracts underneath.  

Spread:  Milk thistle reproduces through prolific seed production; each plant may produce up to 6,000 seeds, which are viable in soil for up to nine years.  Seeds may be transported by rain/water, erosion, and animal movement.  Milk thistle can rapidly crowd out native vegetation in natural areas; left uncontrolled, it can produce four tons of vegetative material per acre, leaving little room for other plants.  It is toxic to livestock, due to its accumulation of soil nitrates.  Native to the Mediterranean region, milk thistle was introduced to the US as an ornamental/medicinal plant, and may have been brought to the Pacific Northwest in contaminated hay.

Control:  Individual plants or small infestations can be dug up before seed production; cut off the root at least an inch below soil level to prevent re-growth.  Mowing is not recommended, as it prolongs the life of the plant and may spread seeds.

Native Replacements:  Douglas aster (Aster subspicatus) is a native perennial up to 1.3 m (4’) tall, bearing pretty purple flowers throughout the summer, which will do well in habitat occupied by milk thistle.

I am one prickly character! (Milk thistle)
Lovely late-season flowers–pollinators will thank you! (Douglas aster). NB: I can be…er…enthusiastic in your garden.