Musk thistle: Large terminal nodding reddish purple flower head, 1 -- 2 inch diameter. Head surrounded by large spiny bract leaves. Erect stem, branched with spiny leaves. Blooms in May and June. See image under musk thistle.
Plumeless thistle: Small reddish-purple flower 1/2 to 1 inch in diameter, in small clusters. Flower surrounded by narrow bract leaves. Blooms in June through August. See image under plumeless thistle
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Canada thistle: Small, numerous, clustered pale lilac flowers, each flower less than 1-inch diameter. Flower surrounded by purplish, flat, pointed bract leaves. Leaves are wavy. Blooms in July through September. See image under Canada thistle.
Bull thistle: Oval-shaped buds produce small to medium sized purple flowers on end of branches, 1 -- 1.5 inches diameter. Flower bract leaves are slightly hairy and have a long, sharp spine. Prickly wings on stem. Underside of leaf is green. Leaf ends in a sharp, long, stout, yellow spine. Blooms in July and August. See image under bull thistle.
Field thistle: Plant resembles bull thistle. Medium sized flowers surrounded by upper leaves. Needle-like bract leaves end in colorless bristles. Underside of leaf whitish. (See image of Cirsium discolor under bull thistle). Blooms in July and August.
