This section is from the book "Commercial Gardening Vol2", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
This is the common "Globe Thistle", 3-5 ft. high, with coarse prickly thistle-like leaves, and roundish steely-blue flower heads in summer. Other species are banaticus (or ruthen-icus), humilis, and sphoerocephalus, the latter having silvery leaves and whitish flower heads. They are all increased by division, and from seeds sown in spring in the open or under glass. The trade is restricted to nurserymen, and is not very large.