Canada Thistle Organic Weed Control

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The Canada thistle is a broad-leafed weedy plant that invades gardens, orchards, vineyards, and roadsides. The Canada thistle is a perennial plant that grows wide and sends up new colonies as far as its tough horizontal roots can reach. Some species of thistles can also be annuals and biennials.

Botanical name: Cirsium arvense

Canada thistle
Canada thistle

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Canada thistle description and life cycle

  • Grows to 1 to 4 feet tall on ridged stems.
  • Roots grow deep and wide forming new colonies of plants.
  • Stem is erect branching at the top.
  • Oblong to lance-shaped leaves to nearly 5 inches long; alternate on the stem and deeply lobed with crinkly margins.
  • Leaves are smooth on top and smooth or hairy underneath
  • The terminal flower is 1 inch wide and bristly in shades of mauve, rose, deep lavender, or white.
  • Reproduced by seed carried on the wind.
  • Blooms mid-summer to mid-autumn.
  • Reproduces by seed and creeping roots.
  • A seedling can reproduce vegetatively in as little as 6 weeks after germination.

Canada thistle root system

Canada thistle has both deep vertical roots and wide-spreading horizontal or lateral roots. Vertical roots can grow several feet deep. Lateral roots can spread as far as 20 feet in a single season, creeping just below the soil surface.  Severed roots can produce new plants.

Canada thistle organic control

  • Remove young plants immediately.
  • Remove as much of the rootstock as possible, then return in a few weeks and dig up any new growth.
  • Till or cultivate where thistles are growing every 3 weeks for a year.
  • To weaken the root system, cut or mow new shoots as they appear; it may take several years to kill perennial thistles if roots are not removed.
  • The plant can be weakened significantly if the top is removed just before flowering when energy reserves have moved up in the plant.
  • Do not let the plant flower. Seeds are windborne and can carry long distances to start new colonies.

Canada thistle range

Northern United States and Southern Canada.

Four quick ways to control weeds

  1. Weed early. Control weeds in the first month after they germinate.
  2. Weed often. Hand weed every two weeks through the season.
  3. Weed by hand when the soil is wet (best to get roots).
  4. Use a hoe if the soil is dry. Decapitate weeds before they flower and drop seed.

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Written by Stephen Albert

Stephen Albert is a horticulturist, master gardener, and certified nurseryman who has taught at the University of California for more than 25 years. He holds graduate degrees from the University of California and the University of Iowa. His books include Vegetable Garden Grower’s Guide, Vegetable Garden Almanac & Planner, Tomato Grower’s Answer Book, and Kitchen Garden Grower’s Guide. His Vegetable Garden Grower’s Masterclass is available online. has more than 10 million visitors each year.

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