Unlike most other thistles, Californian thistles (Cirsium arvense) spread mainly by increasing their root system, which expands horizontally. These roots are called rhizomes, and make the plant difficult to control by spraying. They are best controlled when the land is being cultivated.
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Natural Sciences Image Library of New Zealand
Reference:
Di2670Rbt.tif
Photograph by G. R. Roberts
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