Removing Canada Thistle from the Home Garden

Canada thistle (Cirsium arvense) spreads aggressively by both seeds and underground rhizomes. Many control plans include the use of herbicides. When using strictly mechanical non-chemical methods, you can expect that it will take a few years to eliminate this weed completely from your garden, but persistence and patience will prevail. This article primarily focuses on using only mechanical controls, as would be most desirable in a home garden.

The first step in any weed management plan is to identify the weed you are up against. Common thistles in Pennsylvania are Canada thistle (Cirsium arvense), bull thistle (Cirsium vulgare), and swamp thistle (Cirsium muticum). Canada thistle is a perennial, whereas the others are biennial. Canada thistle and bull thistle are Class B noxious weeds in Pennsylvania. Swamp thistle is native to Pennsylvania. Canada thistle has smooth stems between the leaves and small flowers of less than 1-inch. The biennial thistles all have spiny wings along their stems. These wings are tissue that looks like a continuation of the leaf.

The National Park Service states, "Canada thistle (Cirsium arvense) and bull thistle (Cirsium vulgare) can be difficult to differentiate to the untrained eye, as they both have those notorious spiked leaves. If the thistle has rhizomes, roots that extend out from one plant to grow another plant nearby, then it is likely a Canada thistle. If it does not have rhizomes, but has a taproot and has spiny wings up the length of its stem, then it is a bull thistle. Bull thistles have also been described to be larger and "meaner-looking" than the Canada thistle."

Features of Canada thistle:

• Leaves: Alternate leaves lack petioles and are oblong-shaped, with spines along the edge, and irregular lobes.
• Stems: Stems are usually slender, grooved, and smooth, without spines.
• Flowers: Flowers are purple and pink, sometimes white. They have smaller flower heads than other thistles and spineless bracts.
• Seeds: Seeds are very small with a pappus (tuft of hairs) to aid in wind dispersal.

Canada thistle

• is a perennial weed with deep, spreading roots.
• can regrow from root fragments as small as a half-inch.
• seeds are wind-dispersed and can survive in soil for up to 20 years.

Physical Control

To eliminate thistle through manual controls, you must exhaust the root system through repeated elimination of its photosynthesis capability.

Avoid pulling or excessive root disturbance. Damaging the roots by pulling can stimulate the remaining root segments to send up new shoots from underground buds, increasing the number of plants in the area. It is the buds on the creeping roots of established Canada thistle plants that largely account for re-establishment after attempts at control. Buds on creeping roots can generate new shoots a year or more after top-growth has been destroyed.

Repeated Cutting or Mowing

• Cut every two to three weeks during the growing season.
• The goal is to deplete the root reserves by not letting them photosynthesize, allowing regrowth and carbohydrate use, but not allowing root reserves to build up again.
• Use sharp garden shears or a string trimmer close to the ground.

Smothering

Mulch thickly (5 to 6 inches)

• Use wood chips, cardboard and compost, or landscape fabric.
• This blocks sunlight and weakens the plant.

Solarization (in summer)

• Cover the area with clear plastic for four to six weeks during hot weather.
• Kills roots by heating the soil to 120 to 140°F.

Herbicide Control (if acceptable in your garden)

A more comprehensive weed management program for Canada thistle includes an herbicidal control. If you choose to include herbicide control to enhance the mechanical process, use a systemic herbicide like glyphosate. Or choose clopyralid or aminopyralid, which are often used for lawn settings.

Apply the herbicide in late summer to early fall when the plant is storing energy in the roots. Apply directly to actively growing foliage. Avoid drift.

Be sure to read the product label before application. Herbicide applications should be done with the proper equipment. Avoid contacting non-target plant material.

Prevention

• Check new soil, mulch, or compost for contamination.
• Edge your garden beds to reduce root spread from nearby areas.
• Establish dense plantings of ground covers or perennials to out-compete

Persistence is the key to removing and eliminating Canada thistle. Total eradication takes time, maybe two, three, or more years with consistent effort. Being timely and persistent will rob the root system of its energy reserves. Before long, you'll see that Canada thistle is no longer sprouting in your garden, and you can celebrate your victory.

Timing for Management of Canada thistle

(in USDA Plant Hardiness Zone 6b)

Early Spring (Mid-April to May)

Goal: Exhaust root energy early

• Identify and dig up emerging thistle rosettes before they bolt.
• Begin cutting or mowing every two to three weeks.
• Apply mulch in garden beds to suppress new shoots.
• Monitor for new shoots near previous infestations.

Summer (June to August)

Goal: Prevent flowering and seed spread, weaken roots

• Continue biweekly cutting. Never let it flower.

Late Summer to Early Fall (September to October)

Goal: Target the root system when nutrients are moving downward

• Continue cutting any regrowth.
• Apply a thick layer of mulch (5 to 6 inches) to garden beds for winter.

Winter (November to March)

Goal: Prep and rest

• No thistle activity, but you can
  • clean tools and boots to prevent seed spread.
  • inspect mulch and compost sources to avoid contamination.
  • plan crop rotations or dense plantings to out-compete weeds in spring.

Keep in Mind

• Be persistent. Thistle’s deep rhizomes take several years of consistent effort to fully exhaust.
• Don't till heavily. This can break roots and worsen the spread.
• Consider planting competitive groundcovers, like
  • Dutch white clover
  • creeping thyme
  • dense native grasses