Lessons about our thistle population blooming now
Thistle is at its peak this month. The flowers are lovely but it is considered a Noxious Weed in Colorado. You can cut off the head of the Musk Thistle and stop further spread, but the Canada Thistle needs to be pulled out by its roots. This is best done in spring when the ground is wet from spring run off and the spines are softer, and the roots come out of the ground easily and complete. But can still be done now, as Colorado has had an usually wet yet and it usually grows in damp areas. You can cut the seed head off and dispose of it in the trash, and throw the plant on the ground to compost, as long as the roots are not exposed to disturbed dirt where it can grow again.
Both plants — thistle and curly dock — mentioned below can grow from 2 to 5 feet tall. They often grow in wet areas and are frequently associated with overwatering or standing water in low areas. You can find them often along the paths that follow the St. Vrain River. Most thistle is considered noxious weeds. Curly Dock is not considered a noxious weed in Colorado; and in spring you can eat parts of it, like a vegetable; but full grown it can be dangerous for livestock.
The information below is provided by the Larimer County – Colorado University Extension Service.
The three thistle flowers shown below are all List B Species and are a noxious weed in the state of Colorado, and counties have plans to help stop the continued spread of these species. Contact your Extension Service if you need help in controlling large areas.
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Question:
I have 3 acres at 7200 feet elevation. I have a very bad thistle problem. Mainly Canada and musk, I think. I also have quite a few dock plants that are starting to spread. I actually like the dock, especially compared to the thistle: no thorns, tasty greens in the spring. Is it a bad idea to allow the dock to grow and hopefully outcompete the thistle? Will it take over? Is it difficult to control or eradicate later? I am reluctant to spray herbicide because there are so many other beautiful plants up here.
Comments by Larimer County Extension Service:
Before you take any action, be sure you have Canada and/or musk thistle. The link to the Colorado Department of Agriculture noxious weed program will help you with identification. Both Canada and musk thistle are on the Colorado Noxious Weed B list. Click on the weed to get information and the fact sheet with control methods. https://www.colorado.gov/pacific/agconservation/noxious-weed-species
Canada thistle (Cirsium arvense) is a perennial that reproduces by seed and an aggressive, extensive root system. Because Canada thistle is a very aggressive noxious weed, you may need to consider control with a herbicide. If you decide to use a herbicide, contact the Larimer County Weed District. They can guide you on herbicides, the best time of year to spray to not harm other plants, and you can also purchase small amounts from them.
Musk thistle (Carduus nutans), on the other hand, is a biannual and reproduces only by seed. The key to control is to prevent seed production. The management recommendations on the CDA fact sheet will guide you on management practices that do not need to include a herbicide.
Curly dock can be poisonous to sheep, cattle and horses if eaten in large quantities. The only situations I have seen curly dock become abundant is in disturbed, overgrazed areas. Curly dock can be difficult to control. If you find the curly dock is becoming a nuisance, the Larimer County Weed District recommends multiple sprayings with Rangestar.
To answer your question about competition by desirable plants, yes curly dock and other plants can help outcompete the thistle. However, you need to keep a close eye on the Canada thistle to be sure it is not spreading.
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THISTLE FOUND IN LYONS/ BOULDER AREA
Canada thistle: key points: spreads by seeds and aggressive creeping, horizontal roots called rhizomes. Canada thistle can grow 2 to 4 feet in height. The leaves are oblong, spiny, bright green, and slightly hairy on the undersurface. Unlike other noxious biennial thistles which have a solitary flower at the end of each stem, Canada thistle flowers occur in small clusters of 1 to 5 flowers. When turning to white seeds, they looks like puff balls. The 1,000 seeds per plant can last 20 years. Remove its entire root as each piece can produce a plant.
Bull thistle: key points: 1. flowers arranged in a raceme; flower is gum-drop shaped (2) base of leaves clasp the stem and extend down the stem to node below (3) top surface of leaves have stiff, rough hairs and bottom is soft. ===Flower heads removed can still be viable; and live for three years; up to 4000 seeds per plant; so disposal is necessary.
Musk thistle: key points: A biennial with a two year life cycle, first as a rosette. Musk thistle produces may produce 100 or more flowering heads; with up to 20,000 seeds per plant, only one-third being viable. Because Musk thistle reproduces solely from seed, the key for successful management is to prevent seed production.
