If you’re dealing with whitetop (hoary cress) effective management is essential to controlling and eventually eradicating infestations.

This invasive, deep-rooted perennial poses a serious threat to both private and public lands across Routt County. Whitetop spreads aggressively by both seed and underground roots (rhizomes), making it difficult to control and often requiring herbicide treatments.

Once established, it forms dense monocultures that crowd out native vegetation, reducing biodiversity and degrading wildlife habitat. It’s especially problematic in pastures, rangelands, roadsides, and along waterways, where it can outcompete forage species, leading to reduced productivity for livestock and altered soil health.

The best time for management is typically early spring—mid-May to early June—when the plant is in the bud stage and actively growing. However, with warmer conditions arriving early this year, earlier intervention may be necessary. 

Hoary cress is often mistaken for field pennycress, as both belong to the mustard family and share similar white flowers. However, there are key differences. Field pennycress has darker green, less hairy leaves, and features whorled flowers that grow up the stem in a spiral pattern.

Whitetop, on the other hand, has heart-shaped seed capsules, and its oval, blue-green leaves clasp the stems and are covered in fine white hairs. While both are considered weeds, only hoary cress is listed as a noxious weed by the Colorado Department of Agriculture and Routt County.

Despite their similarities, the two species differ in several important ways, and accurate identification is critical before taking any control measures.

The most effective approach is integrated management, which involves mowing before the plants are flowering and then applying a systemic herbicide to target the plant while it’s directing energy into its roots. Since whitetop spreads via rhizomes, disturbing the roots through activities like digging or tilling can actually exacerbate the infestation by encouraging new growth.

An essential part of whitetop eradication is managing the plant before it goes to seed—often indicated by its flowers changing from bright white to a creamy color—as a single plant can produce up to 5,000 seeds.

Maintaining healthy native plant communities is one of the most effective ways to prevent hoary cress from becoming established. Because it rapidly invades disturbed areas, proper revegetation is essential for controlling this and other noxious weed species.

The most effective way to manage whitetop and other weeds is through early detection and rapid response. It’s crucial to exercise caution when transporting topsoil or nursery stock, as these are the main pathways for the spread of this invasive weed.

Whether you’re a landowner, land manager, or just passionate about healthy landscapes, this is your chance to learn about invasive weeds like whitetop, how to identify them, and what you can do to control them.

Dependent on weather, this event will take place on May 31 in Steamboat Springs, and June 8 in Oak Creek. Please visit https://www.co.routt.co.us/216/Weed-Program for more information and event updates.

Questions?  Please visit our website or contact the Routt County Weed Program at weeds@co.routt.co.us