Distinguishing Knapweeds and Star thistles (Centaurea spp.) of Interest to Canadian Weed Seed Regulation
Jennifer Neudorf, Joanne Hinke and Ruojing Wang
Canadian Food Inspection Agency – National Seed Herbarium, Seed Science and Technology Section 301- 421 Downey Road, Saskatoon, SK S7N 4L8, Canada
Importance of Knapweed and Star thistle Identifying Yellow star thistle * Images prepared by Jo Jones • Yellow (Centaurea solstitialis) and Iberian star thistle (Centaurea • Yellow star thistle is of average length and has a notably long pappus and small side notch. iberica) are native to south-eastern Europe. Both species have Fruit from the central florets is glossy with brown and white streaks, the rarely found fruit of potential to enter Canada through: contaminated seed lots and/or the outer florets is dull, dark brown with no pappus. The general shape is parallel-sided with imported hay. a rounded base.
•Diffuse (Centaurea diffusa) and Spotted knapweed (Centaurea • Yellow and Iberian star thistle fruits that lack brown streaking should be compared with the stoebe) are European species causing much damage to rangelands pale grey Maltese star thistle (C. melitensis). Maltese star thistle has a strongly tapered across Canada. These species and Yellow star thistle are currently outline, often has a greenish ring beneath the collar, wider pappus bristles, a deeper side regulated as prohibited noxious weeds in Canada. notch often shaped like a hook (some fruits have a shallow notch) and prominent side stripes.
Two fruit forms: Identifying Iberian star thistle Central • Iberian star thistle has a large, wide fruit that is usually covered in fruit reddish-brown streaks, but can occasionally be solid white, with an average-sized pappus (note: pappus may be broken or removed Outer during processing) and small side notch. fruit
• It should be compared to the similar Purple star thistle (C. calcitrapa); this species has a duller surface and no pappus. Yellow Star thistle Look-alike: Maltese star (Centaurea solstitialis) * thistle (Centaurea melitensis)
Identifying Diffuse and Spotted Knapweed
• Diffuse knapweed fruits are pale greenish-brown with a strong central stripe and faint lengthwise stripes that fade in the middle. These fruits are long and strongly tapered in outline.
• Spotted knapweed fruits are the same size, but are wider, have a long pappus and are greenish- black, broken up by strong pale stripes and a pale base. Both species have large and deep side notches. Look-alike: Purple Star Iberian Star thistle Diffuse Knapweed Spotted Knapweed thistle (Centaurea iberica) (Centaurea diffusa) (Centaurea stoebe) (Centaurea calcitrapa)