Controlling Canada Thistle – Canada Thistle Identification And Control
By: Heather Rhoades
Perhaps one of the most noxious weeds in the home garden, Canada thistle (Cirsium arvense) has a reputation for being impossible to get rid of. We won’t lie to you, Canada thistle control is difficult and requires a significant amount of effort to be successful, but the effort you put into controlling Canada thistle will pay off when you have a garden that is free from this annoying weed. Let’s look at how to identify Canada thistle and how to get rid of Canada thistle.
Canada Thistle Identification
Canada thistle is a perennial weed that has soft green, deeply lobed, spear-like leaves and these leaves have sharp barbs on them. If allowed to go to flower, the flower is a purple pom-pom shape that will be produced in clusters at the top of the plant. If the flower is allowed to go to seed, the flower will become white and fluffy, much like a dandelion seed head.
How to Get Rid of Canada Thistle
When starting a Canada thistle control program, it is best to first understand what makes Canada thistle such a difficult weed to control. Canada thistle grows on an extensive root system that can go quite deep into the ground, and the plant can grow back from even a small piece of root. Because of this, there is no one and done method of Canada thistle eradication. Whether you are controlling Canada thistle with chemicals or organically, you will need to do so repeatedly.
The first step towards getting rid of Canada thistle is making your yard and garden less friendly to it. While Canada thistle will grow anywhere, it grows best in soil with low fertility and open areas. Improving your soil’s fertility will weaken the Canada thistle and help desired plants grow better and, therefore, make them better able to compete with the Canada thistle. We recommend having your soil tested at your local extension service.
Chemical Canada Thistle Control
Canada thistle can be killed with weed killers. The best time to apply these is on sunny days when the temperatures are between 65 and 85 degrees F. (18-29 C.).
Because many weed killers are non-selective, they will kill anything they touch, so it is best not to use these on windy days. If you need to treat Canada thistle where it is close to wanted plants, you might be better off using a paintbrush to paint the weed killer on the Canada thistle.
Check back weekly and reapply the weed killer as soon as you see the Canada thistle reappear.
Organic Canada Thistle Control
Controlling Canada thistle organically is done with a sharp eye and an even sharper pair of scissors. Find the base of the Canada thistle plant and simply snip it off at the base. Do not pull Canada thistle out, as this can split the root, which causes two Canada thistles to grow back.
Check the location weekly and snip off any new growth that you may see. The idea is to force the weed to use up its energy reserves by regrowing but removing the new leaves before the Canada thistle has a chance to build its energy reserves back up.
Note: Chemical control should only be used as a last resort, as organic approaches are more environmentally friendly.
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This could be Canadian Thistle but it's difficult to get a positive ID on juvenile plants, especially from a photograph alone. (Identification can be done by bringing an entire plant, as mature as possible, to our office and we will send it to the lab at no cost to you.)
Eradication of Canadian Thistle includes using chemicals. Cutting the plants back help to deplete the energy stored in the roots but is not an effective control, as you have seen. Products that control broad leaf plants (2 4 D) or non selective herbicides like glysophate (such as in Round Up) can be used for effective control. The following publication on Canadian Thistle from the USDA recommends various chemicals and the timing involved to get the best results.
Remember to read and follow all label instructions before applying pesticides.
Feel free to contact our office if you have other questions.
Let me know if I can help you further!
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Louisville KY 40243
How to Kill Star Thistle
Different types of thistle such as bull thistle, Canada thistle, yellow star thistle, musk thistle and Italian thistle are collectively called ‘star thistle’. These fast-growing, noxious pests are found in many lawns and gardens all over the United States, spreading quickly to cover an entire area with their thick growth. They are detrimental to other plants, as they compete for nutrients in the soil, and grow in almost any type of soil. Persistence is required to eradicate them from a back yard, orchard, pasture or rangeland.
Wear gloves to manually remove thistle growing in small patches or spots by pulling it off or digging it from the ground. Make sure you pull the entire root system, including the taproot, since thistle spreads by seeds or roots. Collect the thistle in a large plastic bag, knot it tightly and discard appropriately.
- Different types of thistle such as bull thistle, Canada thistle, yellow star thistle, musk thistle and Italian thistle are collectively called ‘star thistle’.
- Make sure you pull the entire root system, including the taproot, since thistle spreads by seeds or roots.
Aerate the soil with an aerator or shovel to loosen compacted soil. This ensures the herbicide or weed killer reaches deep down to prevent future growth.
Pour herbicide that contains glyphosate, or any weed-killer specifically formulated to kill the particular thistle type, over the aerated area. Follow label instructions when pouring. Use a spray bottle to spray herbicide directly over the star thistle if there is other foliage nearby, to prevent damaging them.
Grow competitive plants such as rose or crimson clover in large areas to prevent star thistle from growing there.
Release certain insects that are natural enemies of the star thistle in the area–they will attack the flower heads and reduce seed production. Different insects target different types of thistle plants, so make sure you know the specific type you want to eradicate from your lawn. For instance, the thistle-stem gallfly chews on the stem of Canadian thistle and peacock fly kills yellow star thistle. The thistle head weevil eats several species of thistle.
- Aerate the soil with an aerator or shovel to loosen compacted soil.
- Use a spray bottle to spray herbicide directly over the star thistle if there is other foliage nearby, to prevent damaging them.
Spread a 5- to 8-inch layer of mulch over a small area that is infested with thistle. This natural method suppresses and prevents thistle regrowth after it is removed. Mulch is also beneficial as it keeps roots of other plants cool, especially during the hot summer.
Burn large areas of land to eradicate thistle and seed reserves. Although this practice is environmentally harmful, it is an effective management control method for very large areas covered with thick star thistle growth. Make sure you consult your local fire department officials for permits and regulations.
Aliases: woodbine, lady’s nightcap, wild morning glory, creeping Jenny, hedge bells, possession vine
Bindweed, a perennial cousin to the sweet potato, got no sweet comments from the 46 percent of respondents who were dealing with it in their gardens. Many bindweed-battlers noted this weed’s persistence, and one even called it a “zombie plant” because it’s so difficult to kill. Bindweed can sprout, resprout and resprout from wide, spreading roots that can reach as deep as 30 feet underground! And, if you let this weed’s flowers go to seed in your garden, watch out: The seeds can stay viable for up to 50 years. You must dig or pull all bindweed shoots when they first appear, and keep pulling. Repeated pulling will weaken the roots until — hopefully — they die out. If you let bindweed get established, you may never be able to eliminate it.
A few respondents had tried bindweed mites with some success. A Colorado gardener said free mites can be obtained from the state insectary (a program of the Department of Agriculture) and worked best in non-watered parts of the garden. A New Mexico gardener with three to five years of experience said, “This darn weed seems to ignore herbicides, and tilling just spreads the plants around. Judicious hand weeding is the only way I have found to keep this prolific weed out of my garden.” Use a fork instead of a hoe to dig up bindweed, as you’ll leave fewer fragments behind, and dig after a rain or heavy watering when the roots are less brittle.
Useful Management Techniques
Origin and Distribution
Canada thistle probably originated in southeastern Europe and the eastern Mediterranean. It has spread throughout Europe, North Africa, through central Asia to Japan, and across the northern U.S.A. and southern Canada. It is also naturalized in South Africa, New Zealand and southeastern Austr
Dormancy and Germination
The seeds are not dormant when shed from the parent plant. They germinate best at warm daytime temperatures (77
A few seeds may persist in the soil for as long as 20 years, but most disappear within the first few years.
The seeds have a special feathery structure (the pappus) that aids in wind dispersal. The seeds also commonly disperse to gardens in contaminated hay and straw used for mulch or compost.
Timing of Emergence in NYS
Seedlings emerge primarily in the spring or fall. Shoots from rootstocks begin emerging in late April or early May and emerge continuously until frost.
Optimum depth for seed emergence is 0 to 0.4 inches (1 cm) but seedlings can emerge from as deep as 2.4 inches (6 cm). Shoots can emerge from roots several feet deep.
Sensitivity to Frost
Shoots tolerate light frost but are killed by hard frost. The root system persists over the winter.
Well-developed plants are draught tolerant due to their deep root systems.
Response to Fertility
Soil Physical Requirements
Canada thistle tolerates a wide range of soil types. It does poorly, however, on wet soils, and is particularly vigorous on well drained, fine textured soils (clay to silt loam).
Response to Shade
The species is shade intolerant, but due to its extensive root reserves, shoots can often grow through competing vegetation.
Sensitivity to Disturbance
Canada thistle is highly resistant to cutting, hoeing and even to deep tillage. The root system commonly extends to depths of 3 ft (90 cm) or more and new shoots quickly reappear after the previous shoots have been removed. Carbohydrate reserves in the root system reach a minimum in June at about the initiation of flowering, and the plant is most sensitive to removal of the shoots at that time.
Time from Emergence to Flowering
Plants flower in response to long daylength, generally between late June and Aug. Thus, plants emerging from rootstocks in spring take about 8-10 weeks to flower.
Canada thistle is pollinated by insects, especially honey bees. Since male and female flowers occur on separate individuals, the species cannot self-pollinate.
Canada thistle reproduces both vegetatively and by seeds. Seed production varies from nothing to 5,000 seeds/shoot depending largely on the thoroughness of pollination. This in turn depends on the proximity of male and female plants. Seeds mainly allow the species to disperse between locations and do little to maintain local populations. Vegetative reproduction is by sprouts from the deep and rapidly spreading roots. A single plant is capable of producing 20 ft (6 m) of creeping roots in one season.
Common Natural Enemies
Goldfinches and other birds eat the seeds. Root knot nematode attacks the roots. Larvae of painted lady butterfly (Cynthia cardui ) can defoliate the plant. Larvae of Canada thistle midge (Dasypeura gibsoi) eat the seeds and larvae of Orellia rficauda attack the flowering heads. None of these organisms is sufficient to control Canada thistle alone. The weevil Ceutorhynchus litura has been released in Canada as a biocontrol agent. Larvae mine the leaf veins, stems and root collar and can greatly reduced a population, but apparently the insect is difficult to get established.
Crop Diseases Hosted
Canada thistle occasionally hosts aster yellows virus. It also hosts stem rusts of wheat, rye and barley.
Crop Damaging Insects Hosted
The species hosts bean aphid and stalk borers that attack corn and tormatoes.
Canada thistle is unpalatable to humans.
Many of these terms are defined in the Weed Ecology section.