Pink and purple thistles are in full bloom in southern Indiana. Thistle is the common name for plants, mostly from the (daisy) family, Asteraceae, that are prickly all over and whose flowers grow atop a cup-shaped bract. Thistles include plants in the tribe, Cynareae (Carueae), especially the genera Carduus, Cirsium, and Onopordum.
The thistles that grow in this part of the Ohio River valley form soft, fluffy flowers, like pom-poms, throughout the summer and into the early fall. As the flowers turn to seeds, feathery white threads form to carry the seeds away on the wind, just as dandelions do, giving rise to the phrase, “soft as the down on a thistle”.
Most thistles are biennial plants: they require two full growing seasons to complete their life cycles. In the first growing season, the thistle grows close to the ground as a cluster of leaves (rosette) and sends down a taproot into the soil. The next year, the plant sends up a stalk that will flower and produce seeds and then die.
Bull thistle (Cirsium vulgare), also known as spear thistle (it has slender-lobed, spear-like leaves), Scots or Scottish thistle, or common thistle, is prevalent in southern Indiana. It has erect, branching stalks that grow from two to six feet tall and prefers sunny, open areas. Bull thistle tolerates moist or dry soil. It grows well along roadsides, trails, and fields. Although it can encroach on cultivated land, bull thistle is not difficult to control because it only propagates by seed, unlike Canada thistle (see below). Grazing reduces the thistle in pastureland.
Medicinal and other uses for thistle
The seeds of various thistles have been used as medicine for liver diseases, cancer, and the plague, and as an antidote for Amanita mushroom poisoning and exposure to halogenated hydrocarbons. Thistle tea stimulates the appetite and curbs indigestion. Thistle can be cooked and eaten like a vegetable or ground up and fed to cattle.
Beloved to butterflies, birds (finches), and bees, certain kinds of thistles are a bane to farmers. Banned in some locations, they are in cultivated others. Milk thistle (Silybum marianum), for example, is a noxious weed in Washington, Oregon, and Texas, but in Saskatchewan, Canada, the Ministry of Agriculture gives advice on how to cultivate it, citing its use for medicinal purposes and potential for development as forage for animals.
Canada thistle outlawed in Indiana
A genus of thistle commonly known as Canada thistle (Cirsium arvense), though it is not native to Canada, has been declared a “detrimental plant” under Indiana statutes. ( See IC 15-16-8-1, et seq.) Anyone who owns real estate has a duty to destroy it by cutting or mowing or, if necessary, by plowing, cultivating, or smothering it, or using chemicals to prevent the plants from maturing. The township trustee (or county weed control board) has the power to eradicate the plants if the property owner fails to do so and can place a lien on the property to cover the cost. Knowing violators commit a Class C infraction.
The problem with Canada thistle is that it is a perennial and propagates both by seed and through a lateral root system. It grows in dense patches and crowds out other plants. The extensive root system allows it to spread quickly, as much as ten feet laterally in a single season.
Mowing regularly can control biennial thistles, but the ground must be tilled multiple times in a year and for multiple seasons to destroy the root system of Canada thistle.
What kind of thistle is it?
Canada thistle can be distinguished from other thistles by its soft seed pods and relatively small flowers (less than one inch in diameter). It grows in distinct patches easily seen early in spring and, unlike other Cirsium thistles, does not have spiny wing-like tissue growing along the stems. Because it prefers cooler summers, the range of the Canada thistle in Indiana is generally limited to the central and northern counties.
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