Many thistles in the western United States are native species that generally go unnoticed as weeds. However, thistle species introduced from Europe, Africa, and Asia can be very aggressive opportunists. They often invade overused or otherwise disturbed land.

The plants spread rapidly, out-compete established and introduced plant species for nutrients, and can render pastures, rangeland, and forests nearly unusable. Thistles are especially troublesome following cool wet summers and falls, when seed production and seedling establishment are high. An integrated weed control
program that combines chemical, cultural (such as grass competition), mechanical and biological methods is most likely to be successful.

There are two main types of thistles: Biennial (live 2 seasons) and Perennial (live several seasons). Biennial Noxious Thistles: Biennial thistles such as Scotch, plumeless, and bull thistle are not as difficult to control as the perennial thistles, but spread rapidly by seeds that are produced in great numbers, ranging from 8400 seeds per plant with plumeless thistle to 120,000 seeds per plant from musk thistle. Biennial thistle seed generally germinates in the summer and fall, and the plant over-winters as a rosette. The following spring the plant resumes vegetative growth, bolts, flowers and sets seed. Plants die after setting seed, completing the life cycle. Biennial thistles reproduce only from seed, so the key to a successful management program is to control the plants before they flower or go to seed. Perennial thistles such as Canada thistle are generally more difficult to control than the biennial thistles because they spread by both root and seed. Top growth control is not enough; for effective control of a perennial thistle, one must also deplete the root system of nutrients.

Keys to controlling all thistles include:

• Establish a three- to five-year management program using several integrated methods.
• Control small patches before they spread.
• Reseed disturbed areas immediately with desired species.

For additional information, contact Pitkin County Land Management at 920-5214 or the City of Aspen Parks Department at 429-2026