What is bull thistle?

Bull thistle is a European biennial that has spread across Pennsylvania’s fields, roadsides and yards. Good bull thistle identification saves you work, because this thistle grows from a single taproot and reproduces only from seed. Unlike Canada thistle, it doesn’t creep underground, so you can actually get rid of it.

In its first year it forms a flat, spiny rosette that stays green through winter. In its second summer it shoots up a tall, branching stem covered in spines, flowers, drops seed and dies.

Pennsylvania lists bull thistle as a Class B noxious weed. That reflects how common and troublesome it is in pastures and hay fields.

Bull thistle identification: what to look for

The quickest test is to carefully run a gloved finger over the top of a leaf. Bull thistle leaves are rough and bristly on top, almost like sandpaper. Most look-alikes are smooth there.

• Rosette: deeply lobed leaves with a sharp spine at the tip of each lobe, flat to the ground in the first year.
• Leaf surfaces: rough, prickly hairs on top and a soft, woolly underside.
• Stem: second-year stems have spiny wings running down them where the leaves attach.
• Flowers: pink to reddish-purple heads about 1½ to 2 inches across, sitting on a spiny, egg-shaped base, blooming from early summer into fall.
• Seeds: each seed rides a fluffy white parachute on the wind.

Where it grows in Pennsylvania

Bull thistle loves sunny, disturbed ground. Around central PA it’s common in pastures, old farm fields, roadsides, logging clearings and construction sites. It moves into yards wherever there’s bare soil for the wind-blown seed to land on, such as freshly graded areas, new beds and thin lawns.

Overgrazed pastures and neglected fields can produce a lot of seed, so homes next to open land tend to see more of it.

It rarely survives in a lawn that’s mowed regularly, because the flowering stalk gets cut down before it can bloom. Where you’ll usually find it in a yard is the unmowed edges: along fences, behind sheds, in a rock pile or on a steep bank the mower can’t reach.

Bull thistle look-alikes

Canada thistle is the one you most want to tell apart, because it’s much harder to control. It forms patches from creeping roots, has leaves that are smooth on top, and carries clusters of small lavender flowers.

Musk thistle has bigger, drooping flower heads with wide, purplish bracts. Spiny sowthistle and prickly lettuce have yellow flowers and milky sap when you break a stem, which bull thistle doesn’t have.

How to get rid of bull thistle

For a handful of plants in a yard or bed, digging is the right answer. Because bull thistle has no creeping roots, removing the top of the taproot usually ends it.

• Dig rosettes: wear thick leather gloves and use a sharp spade or a long weeding tool. Cut the taproot a couple of inches below the soil line. Fall and early spring are ideal, when rosettes are easy to see and the soil is soft.
• Cut bolting plants: if a plant has already sent up its flower stalk, cut it off at the base before the flowers open. If you’re late and flowers are already open, bag the heads, because they can still ripen seed after cutting.
• Don’t let it seed: one plant can scatter a lot of seed across your yard.

Cultural control. Regular mowing at a healthy height keeps lawn thistles from ever flowering. Thick turf and mulched beds leave little bare soil for seed to land in, so overseed thin spots in early fall and keep beds covered.

Chemical control. In lawns, selective broadleaf herbicides containing ingredients such as 2,4-D, dicamba, triclopyr or clopyralid work best on rosettes in fall or spring while they’re actively growing. Penn State also lists glyphosate as a non-selective spot treatment in beds, where it will harm any plant it touches. Read and follow the product label, which is the law, and make sure the product is labeled for your site, whether that’s lawn, landscape beds or pasture. Applying herbicides for hire in Pennsylvania requires a PA Department of Agriculture pesticide applicator certification, so large pastures or fields are a job for a licensed applicator.

Keeping it from coming back

Since bull thistle comes back only from seed, the patch shrinks fast once you stop new plants from flowering. Walk your yard in fall and spring and dig any rosettes while they’re small.

Seed will keep blowing in from neighboring fields, so your best long-term defense is ground that’s already covered: dense lawn, healthy plantings and a good layer of mulch. After any grading or excavation, seed or mulch bare soil quickly before thistles find it.

Questions we get about bull thistle

Similar weeds

Canada ThistleCirsium arvenseMusk ThistleCarduus nutansSpiny SowthistleSonchus asper