Scottish thistle, native to and beloved in Scotland, is a thorny biennial plant with tall stems and purple flowers on top. People unfamiliar with the flower often mistake them for weeds, and in the United States, they are often classified as an invasive weed. To keep Scottish thistle in check, keep your plant count low. They grow best in hardiness zones 5 to 8, the areas where temperatures do not drop below negative 20 degrees F (See Resources).
Plant the Scottish thistle plant or seeds in early spring in well-drained soil where it will get part to full sun. Once established, the thistle can thrive in dry, gravelly soil and full, harsh sun.
Water the plant, keeping it moist until it becomes established. Once it starts growing vigorously, decrease the amount of watering, allowing the soil to dry in between and eventually tapering off completely. When firmly established, the thistle can survive and thrive on natural rainwater.
Cut spent flowers carefully with clippers once or twice per week during the summer, putting on thick gloves before touching the plant. Put the flowers in a container to be thrown away or burned later.
Cut down the plant after it has finished flowering, as it will only flower once in the second year of a biennial cycle. Cut it down in sections, taking care not to drop any part of the plant onto the ground. Replace the old plant with a new one.