Bull thistle
Cirsium vulgare
A biennial thistle forming a large spiny rosette the first year and a tall spiny flower stalk the second.
Identification
Bull thistle is a spiny biennial weed that is unmistakable for its stout spines and coarse texture. In its first year it forms a low rosette of deeply lobed, wavy-margined leaves whose upper surface is rough and prickly with stiff hairs and whose underside is cottony-white; in its second year it bolts into a branching, spiny-winged stem reaching up to about five feet, with alternate, prickly leaves running down the stem. Large flower heads borne at the branch tips are gumdrop-shaped, surrounded by sharp spine-tipped bracts, and open to purple (rarely white) flowers from summer into early fall. It grows from a fleshy taproot. Its winged, spiny stem and cottony leaf undersides help separate it from other thistles.
Symptoms & Damage
Bull thistle degrades turf and recreational areas mainly through its large, intensely spiny rosettes and stems, which make affected areas unpleasant to walk on and crowd out desirable grass beneath their coarse leaves. The first-year rosettes press down and shade surrounding turf, thinning the stand, and second-year plants tower over the canopy as conspicuous, prickly eyesores; left unchecked they set abundant wind-blown seed that spreads the infestation to new openings.
Biology
Bull thistle is a biennial that grows a leafy rosette and taproot in its first year, overwinters, and then bolts, flowers, sets seed, and dies in its second year. It reproduces entirely by seed, which is wind-dispersed on a tuft of hairs and can blow in from surrounding areas, so reinfestation often comes from off-site sources as well as the local seed bank. Flowering occurs from roughly July through September, and preventing seed set is the central lever for reducing populations over time.
Occurrence & Spread
Bull thistle establishes in open, disturbed, sunny ground and invades thin, weak, or neglected turf, pastures, roadsides, and field margins where bare soil and reduced competition let its seedlings take hold. Because its seed blows readily from nearby infestations, even well-kept turf can receive new seed, but it only establishes where the canopy is open or the stand is thin. Disturbed, low-competition sites in full sun are the classic invasion situation.
Favorable Conditions
Thin, disturbed, low-maintenance areas.
Cultural Management
In turf, regular mowing is a key cultural tool because cutting before the plant flowers prevents seed production; close or twice-a-season cutting will usually stop seed set and reduce the population over time, though mowed plants may resprout from basal buds. Maintaining a dense, competitive stand denies seedlings the open soil they need to establish. Individual rosettes can be dug out, but because the plant produces a taproot the cut or dig must go deep enough to remove the crown and prevent regrowth; doing this before flowering also stops further seeding.
Further Reading
University extension resources — open in a new tab.
Related Reports
No published reports yet for this pest.
Reports will appear here as they are peer-reviewed and published.