Centaurea solstitialis (Yellow Starthistle)

Plant Info
Also known as: St. Barbaby's Thistle, Yellow Knapweed
Genus:Centaurea
Family:Asteraceae (Aster)
Life cycle:annual
Origin:Eurasia
Status:
  • Invasive - ERADICATE!
Habitat:sun; dry disturbed soil; fields, pastures, roadsides, open woods
Bloom season:July - October
Plant height:6 to 60 inches
Wetland Indicator Status:none
MN county distribution (click map to enlarge):Minnesota county distribution map
National distribution (click map to enlarge):National distribution map

Pick an image for a larger view. See the glossary for icon descriptions.

Detailed Information

Flower: Flower shape: 7+petals Flower shape: indistinct Cluster type: panicle

[photo of flower] Yellow flower heads, single at branch tips, ½ to ¾ inch across with numerous disk flowers.

[photo of spiny, hairy bracts] The bracts around the base of the flower head are light green, variably covered in cobwebby hairs and may become smooth. At the tip of each bract are multiple spreading spines, a few short and needle-like plus a large, sharply pointed, yellowish spine in the center nearly 1 inch long.

Leaves and stems: Leaf attachment: alternate Leaf attachment: basal Leaf type: lobed Leaf type: simple

[photo of basal rosette] Leaves are basal and alternate, the basal and lower leaves are up to 6 inches long, often lobed in narrow sections on the lower half or so of the leaf, and tapering to a short stalk. All leaves are toothless and covered in woolly hairs giving a gray-green cast.

[photo of upper leaves and winged stem] Leaves become much smaller, linear-oblong and undivided as they ascend the stem with the leaf bases extending down the stem, forming “wings”. The wings are often wavy and may be broad with jagged or smooth edges. Stems are covered in woolly hairs and usually much branched from the base as well as in the upper plant.

Fruit: Fruit type: seed with plume

Fruit is a dry seed 2 to 3 mm long, those in the center with a tuft of white hairs to carry them off in the wind, those in the perimeter ring lacking the hairs.

Notes:

Yellow Starthistle is not known to be in Minnesota, though was once reported to be in Clay County and never confirmed. If it was there it did not persist. It is a serious pest plant especially in the western US, where it infests agricultural fields, pastures and roadsides. It is drought tolerant with a long taproot, which gives it an advantage over other annuals with shallower root systems, allowing it to outcompete native plants. According to one source, seed output can be as high as 30,000 seeds per square meter, nearly all of which are viable and can survive dormant in the soil for several years. The seeds with hairs are transported to new locations by wind and critters, while the hairless seeds fall near the mother plant, increasing the local density and further crowding out other vegetation. It's a very bad plant, and is on the Eradicate list for Minnesota. If you see it, notify the MN Dept. of Agriculture and don't hesitate to kill it. The yellow thistle-like flowers, long, sharp spines on the bracts, gray-green hairy foliage and winged stems are a unique combination and not likely to be confused with any other species.

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More photos

Photos by Peter M. Dziuk taken in Chico, California. Centaurea solstitialis plant By Harry Rose from South West Rocks, Australia, via Wikimedia Commons. Centaurea solstitialis infestation By Steve Dewey, Utah State University, via Bugwood.org.

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