How to Identify Milk Thistle That Grows in Minnesota

Thistles spread quickly and can become noxious weeds if not controlled.

Blessed milk thistle (Silybum marianum) originated in the Mediterranean region and spread along with agricultural products to colonize temperate environments throughout the world. Where it invaded, it caused enough harm to earn itself the designation of a noxious weed, subject to regulatory control. Able to produce more than 400 lbs. of seed per acre, a blessed milk thistle plant can quickly overtake a field or garden, and the toxicity of the plant threatens grazing livestock. As of 2010, blessed milk thistle is not found in Minnesota, according to the U.S. Department of Agriculture, although it thrives in the neighboring Canadian province of Ontario, making spread a possibility. Several other types of noxious thistle inhabit Minnesota that can be easily confused with blessed milk thistle.

Instructions

1. Look on the underside of the flower. The leaflike structures at the base of thistles, called the bracts, typically have bristles or sharp spines. If the thistle does not have bristles or spines on the bracts, you have a Canadian thistle (Circium arvense), a noxious species found in Minnesota. If the bracts extend into tapered spines 1 to 2 inches in length, the plant is likely a blessed milk thistle. Although not found in Minnesota as of 2010 and, therefore, not considered noxious, report your find to your local extension office to learn more about control options.

2. Inspect the leaves carefully; spines are often found on thistle leaves. If the leaves are narrow with yellowish-white spines, you have a plumeless thistle (Carduus acanthoides), considered a noxious weed in Minnesota.

3. Break the stem. If the stem leaks a milky fluid, the plant is a perennial sowthistle (Sonchus arvensis), not a milk thistle, despite this distinctive trait. The perennial sowthistle, however, is also a noxious weed in Minnesota.

4. Rub the undersides of the leaves, being careful of the spines. The musk thistle (Carduus nutans) exhibits mostly smooth leaves. The flower, if present, tends to nod. You can distinguish the bull thistle (Circium vulgare) by the bristly hairs on the tops of its leaves and the thick, wooly grayish hair on the undersides. The University of Minnesota Extension identifies both as noxious weeds that pose a serious problem in Minnesota.

Tips & Warnings

• Noxious weeds exhibit a high invasive potential and can cause serious harm, and state law requires homeowners to control them on their land. If you identify a noxious weed on your property, contact your local extension office for more information on how to remove it.

References

• University of British Columbia Botanical Garden: Botany Photo of the Day: Silybum Marianum
• Global Invasive Species Initiative: Element Stewardship Abstract for Silybum Marianum (Blessed Milk Thistle)
• U.S. Department of Agriculture: Plants Database: Silybum Marianum
• University of Minnesota Extension: The Eleven Primary Noxious Weeds of Minnesota
• Flora of North America: Silybum Marianum