Asked April 21, 2015, 11:37 AM EDT
Chester County Pennsylvania
Thistle is a very difficult weed to
eliminate from your lawn or garden once it has taken hold. Eliminating
thistle for good my take several years because thistles are either
biennial or perennial (see below for thistle identification).
Biennials, such as musk thistle and tall thistle, germinate in summer and fall, spend the winter as rosettes, then produce many flower heads the next spring. Canada thistle, a perennial, spreads by way of its roots.
Steps to eliminate thistle:
Pull up as many of the existing thistle plants as you can, both mature plants and seedlings, making sure to get the whole root network. This is especially important with Canada thistle. Since mature plants have root systems that can extend for 10 feet, you will have better luck pulling up immature plants. Note: if even a tiny piece of the root survives, the whole cycle can start again.
Mow your lawn short to reduce thistle seed reserve, and keep it that way. It's very important to catch thistle at its early bud growth stage, before it can spread new seeds. Don't let thistle get to the stage in which the stems elongate and begin to produce flowers. Thistles in your garden can be sliced off with a hoe; cut them as close to the ground as possible.
Apply herbicides to kill thistle, especially in spring and fall, before thistles can flower and seed. Use glyphosate (eg., RoundUp) for your garden, and use a broad-leaf herbicide containing 2,4-D or MCPP for your lawn. Since glyphosate kills all plants, you must keep application specific. Use a sponge to apply, or cut thistle plants and then use an eye-dropper to put a drop or two into the stem.
Put down mulch liberally. It conserves moisture, enriches the soil and prevents germination of new thistle.
Prevent new thistle invasion by keeping lawns thick; re-seed disturbed areas. Keep gardens healthy and pest-free, and pull any new thistle plants as soon as they appear.
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. 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.
Musk thistle (Carduus nutans) - 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.
Plumeless thistle (Carduus acanthoides) - 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.
Flodman thistle (Cirsium flodmani) 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.
Please let me know if you would like additional information.
-Mary OSU Extension Master Gardener Volunteer Thank you for allowing us to help you with your gardening questions. State Master Gardener Volunteer website - http://mastergardener.osu.edu For more OSUE FactSheets go to http://ohioline.osu.edu