“Cut thistles in May, they'll grow in a day. Cut them in June, that is too soon.
Cut them in July, then they will die.”
~An old English nursery rhyme
Canada thistle, Cirsium arvense, is one of only three plant species designated as "noxious" in Wisconsin (local governments may choose to designate additional species). It has been declared a noxious weed in 43 states as one of the most tenacious agricultural weeds. Leaves are alternate with spine-tipped lobes and the flowers are in heads with bracts that are acute at the tip, but usually do not form spines.
Canada thistle invades undisturbed areas such as prairies, savannas, glades, dunes, streambanks, sedge meadows, and forest openings, as well as croplands, pastures, lawns, gardens, roadsides, ditches, and waste sites. Once it has established it spreads quickly, forming monocultures.
Repeated pulling and mowing (minimum 3 times per growing season, when flower buds are formed, but have not yet opened) weakens roots. Late spring (May/June) burns for 3 consecutive years stimulates seed germination and kills seedlings. Later season burns are needed because early season burning can stimulate plant growth and flowering. Some gardeners report getting control of Canada thistle by chopping down the plant before it goes to seed and then pouring vinegar, salt, or a mixture of vinegar and salt water down the stem area, repeating this process until the roots are exhausted. Or foliar spray glyphosate during the early bolting phase when plants are 6-10” tall, during the bud to flower phase, or rosettes in the fall; with clopyralid or metsulfuron-methyl. Biological control methods involve stem weevil (Ceutorhynchus litura), bud weevil (Larinus planus), stem gall fly (Urophora cardui), or foliage feeder (Cassida rubiginosa).
Canada thistle continues to pop up in a planting bed surrounding a spruce tree, despite repeated attempts to weed out every thistle plant seen as last season progressed and this season began. Since seeds are believed to beviable in the soil for more than 20 years, it is likely to continue to be an ongoing process. Stocking up on heavy gloves will be a necessity to avoid the prickers.