Cotton thistle, known botanically as onorpordum and commonly as heraldic thistle or Scotch cotton thistle, is a biennial flowering herb that is a member of the aster family. While considered an invasive and noxious species in many regions it is appreciated by some for its unique cerise pink to purple flowers and spiky foliage. Cotton thistle naturalizes and thrives in a wide array of conditions including undisturbed open lands and along the banks of fresh water bodies. It can benefit from some preparation for winter along with mulching for protection.
Don a pair of heavy gauge gardening gloves when working to winterize your cotton thistle. Leather or other thick gloves will protect your hands and wrists from scratches, scrapes and punctures that come from brushing against the thistles spine covered foliage.
Prune back any damaged, broken or diseased branching on your cotton thistle with clean sharp secateurs in the fall after the first frost. Cut back faded flower stalks as well and save the flower heads for seed or decoration if desired. In climates where cotton thistle's top foliage does not overwinter, shear off all of the green foliage above the crown of the plant. Cut up the spiny foliage into manageable pieces that are less dangerous to handle and discard them into the compost pile or trash.
Water your cotton thistle deeply several times in the fall before the first hard frost to prepare the roots and soil for the oncoming drought conditions that winter can bring. Water just around the base of the plant and not over the head of the plant to prevent frost or freeze damage or disease.
Mulch around the base of your cotton thistle or over the sheared down crown of the plant in the fall after pruning and watering. Provide a blanket of organic mulch at least 2 inches deep to insulate the plants from cold and hold needed moisture to the soil. Mulch will also keep any opportunistic and competitive weeds at bay.