Tall Whitetop- aka 'Perennial Pepperweed' (Lepidium latifolium)

The first project of the Weed Partnership is the tall whitetop management program. Tall whitetop made it to the top of the priority list because of the pernicious nature of the weed, the potential for management in the Las Vegas Wash system, and the threat to downstream Lake Mead and the Lower Colorado River system. The start of on the ground control activities began Fall 2002.

FACTS ABOUT TALL WHITETOP Native of Eurasia Accidentally introduced in the U.S. in the early 19th century Listed as a state noxious weed Perennial mustard Grows to 8 feet tall Vegetative propagation Low habitat value Increases soil erosion Forms impenetrable thickets

Tall whitetop was first identified in the Wash near Vegas Valley Drive in 1995 and further down the channel in March 2001. The majority of the infestation has remained in the upper (western) Wash area. The fact that this weed has only recently invaded this area means that there still may be an opportunity to control this noxious weed.

If left untreated, tall whitetop will continue to spread into the Lake Mead Recreational Area and its associated washes, tributaries and springs, resulting in declines in extent and quality of riparian and wetlands wildlife habitat in Southern Nevada and along the Colorado River system. Controlling tall whitetop in the Wash will significantly reduce the upstream seed source of this noxious weed.

Tall whitetop (Lepidium latifolium) is an aggressive invasive species that has infested tens of thousands of acres in the western United States. Found in most counties in Nevada, it infests the Carson River, Walker River, Humboldt River basin east to Elko, and several locations in Ely and Pahrump (Donaldson & Johnson, 1999). Tall whitetop has begun to infest areas along waterways in southern Nevada as well. It has been identified in the Las Vegas Wash and its associated tributaries, Lake Mead National Recreation Area, and the Muddy River. Tall whitetop is listed on the State of Nevada's Noxious Weed List and is regulated by law (Masters & Shelley, 2001). In riparian and wetland areas, tall whitetop interferes with the regeneration of important plant species such as cottonwoods and willows and reduces the cover and food available for birds, especially nesting waterfowl (Donaldson & Johnson, 1999; Renz, 2000). It is an extremely competitive plant, reaching 7-8 feet in wet areas (Donaldson & Johnson, 1999), and crowds out native species and quickly forms dense monotypic stands. This loss of plant diversity degrades habitat quality and causes a corresponding loss of animal diversity (Donaldson & Johnson, 1999; Renz, 2000).

Tall whitetop can increase erosion because the roots are narrow and break easily, which causes soil destabilization and increased pollution of waterways with silt and debris (Eiswerth, Johnson, Lewis & Hughes, 2001). It can also affect ecosystems by acting as a "salt pump", taking salt ions from deep in the soil and depositing them near the surface. This can give an advantage to halophytes (salt-loving plants), shifting plant composition and diversity (Renz, 2000). A copy of the Weed Wanted Poster is available courtesy of University of Nevada, Reno.

Treatments have occurred biannually each fall and spring since 2002. By controlling tall whitetop in the upstream sources in the Las Vegas Valley Watershed, we are preventing the Las Vegas Wash, Lake Mead and the lower Colorado River System from further degradation due to this species that has been observed elsewhere in the state.