Answer: A variety of thistles are found in turf. Like most, bull thistle is a biennial. It grows over the summer months. The leaves are alternate; blades are simple and form in a rosette. The leaves are unlobed to pinnately lobed. The bladetip is pointed and the margins are toothed with spines. The root is a fleshly taproot the first year and a fibrous root system forms the second year.
The second year of growth, thistle stems elongate. The elongated stems have alternating leaves. Flowers are present from June through October on the elongated stems. The disk flowers are dark pink to purple with spined bracts. Bull thistle spreads by seeds. This explains why you keep killing the weeds and they just keep coming back; they are either reseeding, or the roots of the plant were dormant when you applied herbicides. Once the herbicide was gone, the roots simply sprouted new tops.
Individual bull thistle plants can be physically removed by cutting below the crown in early spring. Bull thistle should be removed prior to bolting and flowering to prevent seed development and distribution. Follow good turf management practices to create a dense competitive stand of turfgrass.
Thistles that are actively growing and in the rosette to flower stage of growth can be controlled with a postemergent herbicide application (such as Preen). Or, to help your lawn grow thick and lush, plus get rid of the bull thistle, try Scotts? Lawn Pro? Step? 2 Weed Control Plus Fertilizer.
Best wishes with your lawn!
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