During summer, thistles are a common sight throughout southwest Wisconsin in pastures, roadsides, and CRP land and a popular question from landowners is: “How do I control thistles???” First, let’s talk biology! It is important to understand that different thistle species (as well as other weeds) have different life cycles, and this information is critical to your management approach since some weeds are more difficult to control due to longevity and multiple means of spread. Here are the life cycle categories of weeds:
- Annuals: Complete life cycle in 12 months or less and only produce new plants by seeds; example: annual sowthistle. Mowing is often used for control of annual weeds to reduce seed production.
- Biennials: Require 2 years to produce seed and die; first year is “rosette” stage and requires cold temperatures to shift to reproductive growth the next year; examples: bull, musk and plumeless thistles.
- Perennials: Regrow each year from roots or crown buds and lives indefinitely; example: Canada thistle. Next, make sure you know which thistle you are controlling! Here are some key characteristics of thistles found in southwest Wisconsin:
- Leaves deeply cut, with a wrinkled gray-green surface
- Spines perpendicular to leaf surface, needle-like
- Stem appears spiny; decurrent leaves (run down along stem)
- Flask shaped flower
- Leaves smooth, with gray-green margins and white, hairless mid-rib
- Stem spiny except below the flower head
- Large flower (1 ½ to 2 inches across); pink to violet-pink, heads “nod”
- Leaves are deeply divided and hairy, especially lower surface midrib • Leaf lobes are often at an angle to midrib
- Stems spiny from base to tip • Pink flower is ¾ to 1 inch across
- Roots deep and branched (rhizomes)
- Leaves with crinkled, spiny edges
- Small pink-purple flowers, flask shaped, ¾ inch across
- Grows in patches due to rhizomes (spreads by roots)
- Male and female flowers on separate heads and plants
Key Points for Managing Thistles
Start by taking an inventory of the major weed species on your land and note the locations of infestations so that you can check on weed regrowth or species’ changes following control in the season applied as well as several times annually thereafter. Keeping track of weedy areas over time is the best way to monitor effectiveness as well as make progress in weed control.
Prior to using an herbicide, always read the product label and apply according to label directions. For pastures, be sure to observe appropriate livestock withdrawal times once herbicides are applied. More specific guidelines for bull, plumeless, musk and Canada thistles control are given below. These guidelines are not an endorsement of particular products over other similar products in the marketplace. If you have additional questions regarding thistle control options, please call your local UW Extension office.
Canada Thistle Management Because Canada thistle is a perennial species and has an extensive rhizome root system, it is much more difficult to control than the other thistles growing in southwest Wisconsin and infestations require a long term strategy for control. While mowing will temporarily set Canada thistle back, it would require mowing several times throughout the summer to expect even modest control, so in most cases, herbicides are a more effective option. For most effective use, time herbicide applications for bud to early flowering stages. There are several herbicides to choose from:
- Clopyralid (‘Stinger’) is currently the herbicide of choice since it is very effective on Canada thistle; also quite expensive
- ‘Curtail’ (‘Stinger’ + 2, 4-D) is more economical alternative that has good effectiveness • Aminopyralid (‘Milestone’) is a new herbicide formulation that is also very effective on Canada thistle. This herbicide is also available mixed with 2, 4-D (‘Forefront’).
- Dicamba + 2, 4-D (‘Weedmaster’, ‘Brash’) is very economical, but is less effective as a long term control method for Canada thistle.
- Glyphosate (‘Roundup’) is very effective against Canada thistle, but is non-selective and will harm adjacent non-target plants, so this option is recommended only when taking out a pasture or CRP and/or rotating to a crop such as corn or alfalfa.
Biennial Thistle Management
The key to long-term control of biennial thistles (bull, plumeless or musk) is preventing seed production. The best results occur when biennials are treated during their first growing season when they are in the rosette stage, any time between germination and up to “bolting” (emergence of flowers) in their second year. This provides flexible control, and fall applications are ideal because: • All plants of concern are in the rosette stage • Herbicide is more actively moved into crown & roots • A less hectic time of year, so spraying is more likely to get done Once bolting occurs, mowing to prevent seed development is the best control option as biennials become somewhat resistant to herbicides at this stage of their life cycle.
What to Use for Biennial Thistles?
Herbicide mixtures of dicamba + 2, 4-D (‘Weedmaster’, ‘Brash’) are very effective and also make the most economic sense. This combination will also control a number of other troublesome biennial weeds as well, such as burdock, wild parsnip, and wild carrot. Glyphosate may be useful in a few situations where biennial thistles have completely taken over, but use caution since glyphosate is non-selective (will kill all plants sprayed). Where appropriate, apply glyphosate to these areas using a backpack sprayer with a single nozzle and aim only for the rosette center (no need to treat entire plant)
Author: Rhonda Gildersleeve,
Iowa County Agriculture Agent
Originally Published: The Weekend Farmer, Spring/Summer 2008