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Bull thistle

By: Garden Gate staff
Common throughout much of North America, bull thistle is a biennial that reproduces only by seed.

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Bull thistle Cirsium vulgare

IDENTIFICATION — Common throughout much of North America, bull thistle is a biennial that reproduces only by seed. The seeds sprout in fall or spring. First-year plants are a low rosette of leaves that can grow as large as 3 ft. in diameter, although they’re usually smaller.

A second-year bull thistle is from 2 to 6 ft. tall by midsummer when it blooms. The flowers are 1 to 2 in. in diameter, deep purple to bright pink and fuzzy. There are spines along the stems, the edges of the long leaves and the midrib of each leaf. And the underside of the leaf is silvery-white.

After the blooms fade, the flower head seems to close up and a white fuzz appears at the tip. Soon, the “bud” reopens to release the seeds. Each tiny, brown, striped seed has a bit of white fluff attached that helps it drift on the wind to a new location.

FAVORITE CONDITIONS?— This is an aggressive weed with a long taproot that will grow in almost any soil. It needs a place where it can complete its two-year life cycle undisturbed, so you rarely find it in areas that are regularly cultivated, such as vegetable gardens or annual beds. Lawns, shrub and perennial borders, as well as fencerows, ditches and garden edges, are where it grows best.

CONTROL — The easiest control method is to cut the entire plant down to the ground just before it blooms. If you cut first-year rosettes or older plants too early in the season, the root can resprout and flower. Even flowering stems that are cut and lying on the ground can produce viable seeds, so burn or bury them. For serious infestations, glyphosate or 2,4-D herbicides are effective if you spray first-year rosettes in the spring or fall.

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Published: July 6, 2010
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