Musk thistle (stalk and flower head)
Canada thistle (plant, root, young shoot)
Flodman thistle (leaves and flower head)
Thistles are often troublesome weeds in Minnesota gardens and lawns. The first step to proper thistle control is their identification. Biennial thistles can be controlled by digging and cultivation; this is more difficult with perennial thistles because they spread by creeping underground stems (rhizomes). The most effective way to remove perennial thistles is through the use of herbicides. Broadleaf herbicides containing 2,4-D and MCPP can control thistles in lawns. In gardens, it may be best to spot treat thistles with a non-selective herbicide containing glyphosate, such as Round-up.
Herbicides must be applied when weeds are actively growing and air temperatures are roughly 60° to 85° F. The best times to control weeds are in the fall (September through mid-October) or spring (late April through mid-June). Always read and follow all pesticide label directions carefully.
Biennial thistles form a low-growing rosette of leaves the first year, followed by a taller, flower and seed-bearing stem the second year. They live only two years. It's best to eliminate them the first year so they never have a chance to bloom and produce seeds.
Bull thistle (Cirsium vulgare) reproduces by seed and can be found throughout Minnesota. The stem is heavily branched and may be 2 to 4 feet tall. Leaves are green on both upper and lower leaf surfaces (the lower surface somewhat paler than the upper surface) with yellow tipped spines. Leaf bases run down the stem giving it a winged appearance. Compact rose to reddish purple flower head, 1 to 2 inches in diameter, bloom from June through September. (See Figure 1.)
Musk thistle (Carduus nutans) is found in the southern half of Minnesota. The 3 to 6 foot tall stem is erect with spiny wings. Leaves are alternate, coarsely toothed, very spiny, and extend down the stem. Both stem and leaves are densely covered with short hairs. Purple to lavender flower heads up to 2 inches in diameter are held on long nearly naked stems. (See Figure 2.)
Plumeless thistle (Carduus acanthoides) may be found in most areas in Minnesota, except in the northeast 'Arrowhead'. The stem (3 to 6 feet tall) is erect. Leaves are deeply divided with alternate lobes ending in white to yellowish spines. Leaves at the base of the plant and lower stem are large and decrease in size as they progress up the stem. Leaf hairs are scattered on the upper surface, but are denser on the lower surface especially along the mid-vein. Single or loosely clustered, reddish-purple flowers are globe-shaped, 0.5 to 1 inch in diameter, with spiny wings to the base. Flowers bloom in late May through early July.
Tall thistle (Cirsium altissimum) is similar to the bull thistle, but it can reach a height of 10 ft. Lancolate leaves are green on the upper surface and white and wooly on the lower surface. Rose-purple flowers are 1 inch in diameter.
Perennial thistles come back each year from roots that survive the winter. They bloom and set seed yearly.
Canada thistle (Cirsium arvense) reproduces both by seed and vegetatively by rhizomes. Roots can extend horizontally and vertically several feet. Stems are 2 to 5 feet tall, branching only at the top. Leaves are somewhat lobed and crinkled at the edges with spiny margins. Numerous, compact, lavender disk flowers, 3/4 inch or less in diameter, are surrounded by bracts. (See Figure 3.)
Flodman thistle (Cirsium flodmani) is found in the western half of Minnesota. It is deep-rooted and reproduces by seed or rhizomes. Stems are 2 to 3 feet tall covered with white felt. Leaves are greenish-gray on the upper side and white with matted hairs on the lower side. Reddish-purple flowers are borne on heads surrounded by small, prickly leaves at the base. (See Figure 4.)
Reference and Illustrations: Weeds of the North Central States. North Central Regional Research Publication No. 281., pp 199-202.