Class B Noxious Weed
- Native to southeastern Europe and the eastern Mediterranean.
- Once established it spreads quickly replacing native plants and crops.
- Poses an economic threat to the agriculture industry; the weed is an effective competitor for light, moisture and nutrients and is able to reduce crop yields.
- May be confused with bull thistle (Cirsium vulgare), a class C noxious weed, that is larger in size 3 to 7 feet tall, with larger flowers 1.5 to 2 inches, and grows as a single upright branched stem.
- Rhizomatous, herbaceous perennial that grows 1 to 4 feet tall. Complex system of roots spreading horizontally which give rise to aerial shoots.
- Leaves are alternately arranged, lance shaped to narrowly oval. Leaf edges toothed or deeply lobed with stiff yellowish spines. Upper leaf surface varies from hairy to no hairs, generally leaf undersides have hairs.
- Numerous, slender, green, freely branched stems; begin as a rosette and grow upright after 2 to 4 weeks. Stems can be slightly hairy or without hairs.
- Flower heads many, in clusters at tips of branched stems. Flowers are purple to pink, sometimes white, ½ to ¾ inches in diameter. Plants are dioecious, meaning all flowers on a plant are either male or female. Bloom June to October.
- Fruits about 1/8 inch long, somewhat flattened, brownish with an apical circle of long hairs or bristles.
- Common in pastures, crop and rangelands. It invades natural areas such as prairies and dunes that have disturbance. Also found along forest margins and stream banks.
- Canada thistle grows poorly in shaded conditions producing few flowers; it also does poorly in wet soils without much aeration.
Reproduction and Spread
- Reproduces by seed and vegetatively by creeping horizontal roots.
- Average seed production is about 1,530 seeds per year.
- Seeds are dispersed by wind and water. Seeds can remain viable in the soil for over 20 years. Studies collecting fresh seeds found germination rates of up to 95%.
- Plants have a fibrous taproot with deep, wide spreading horizontal roots. As roots travel horizontally they begin to arch towards the water table, at this point another horizontal root develops and continues the horizontal spread. Adventitious buds along the horizontal runners give rise to numerous aerial shoots. Root fragments as small as 8 mm in length can produce new shoots.
- Shoots emerge in the spring, after 2-4 weeks the rosettes experience rapid vertical growth for several weeks. Growth slows and flowering commences in early summer and continues for several months. Seeds develop 8-10 days after flowers open.
- Due to the irritating toxins in the sap, stem, leaves, and fruits, it is advisable to wear gloves and other protective gear when handling spurge laurel.
- Hand pulling small infestations is effective.
- Larger shrubs that are too big to pull can be cut below soil line.
- Watch the area for resprouts and recut as needed.
- Try using a tool like a Weed Wrench, Extractigator, or Uprooter to leverage larger plants out of the ground, along with their roots.
Aminocyclopyrachlor, aminopyralid, glyphosate, imazapyr, and triclopyr + 2,4-D applied as cut stem treatments gave promising results in initial trials. Imazapyr and triclopyr + 2,4-D applied to foliage also resulted in good control.