Perennial Pepperweed (Lepidium latifolium L.)

Perennial pepperweed is a native of southern Europe and western Asia. The species was probably introduced to the United States through contaminated beet seed imported from Europe around 1900. Since its introduction, the plant has invaded thousands of acres across the West. It occurs in all far-western states except Arizona. In Washington, scattered populations are found throughout the state, with heavy infestations in central Washington. Perennial pepperweed is a Class B noxious weed in Washington. Control (prevention) of all seed production) is required in designated areas.

An aggressive plant, perennial pepperweed tends to establish and rapidly colonize pastures, riparian habitats, and waste places in wetter areas. The plant can also be a problem in roadside, rangeland, and field crop situations. It often reaches fields via irrigation ditches from riparian areas. In riparian zones, the weed interferes with the regeneration of willow and cottonwood species. Accumulations of perennial pepperweed's semi-woody stems degrade nesting habitat for wildlife. This extremely competitive plant may also completely displace more desirable species, which poses a particular threat to natural areas and hay meadows. Perennial pepperweed lowers the digestibility and protein content of hay, and the accumulation of old pepperweed stems inhibits grazing. Livestock avoid eating this weed if other forage is available.

IDENTIFICATION: Perennial pepperweed normally grows 1 to 3 feet tall, but may reach up to 6 feet. The plant has many stems that emerge from a somewhat woody root crown. The alternative, waxy leaves may have smooth or toothed margins and a prominent, whitish midvein. Basal leaves are lance-shaped and up to 12 inches long; they are attached by a long stalk that can be almost as long as the leaf blade. Stem leaves are smaller, with shorter stalks.

Perennial pepperweed's delicate, milky white flowers, which are sometimes collected for dry flower arrangements, grow in dense, rounded clusters at branch tips. Dense infestations appear brilliantly white in mid-summer. A member of the mustard family (Brassicaceae), each flower has 4 sepals and 4 petals. Plants flower from June through September, producing small (1/16 inch), round or egg-shaped fruits. Each fruit contains 2 reddish-brown seeds.

BIOLOGY AND ECOLOGY: Each perennial pepperweed flower produces a small round fruit, or silicle, which does not open at maturity. Instead, seed falls at irregular intervals throughout the winter. Abundant seed production is followed by a high germination rate, with seeds germinating from February to March. A wide and fluctuating temperature range produces the highest germination rates. Constant cold temperatures produce a low germination rate. There is no heavy seed coat, and no dormancy requirement for the seeds. Seed banks may not be a concern in controlled sites, if buried seeds do not have the wide temperature fluctuations.

In addition to profuse seed production, perennial pepperweed has an extensive, creeping root system. Dense colonies establish when shoots emerge in late winter and early spring from the branching, underground root system.

Adjacent lands are susceptible to infestations of perennial pepperweed from the ample seed source. Other methods of spread include the movement of contaminated agricultural products and the transportation of root fragments on earthmoving and tillage equipment.

Perennial pepperweed is adapted to a wide range of habitats. In the Intermountain West, it occurs along river systems from higher elevation coniferous forests to saline or alkaline deltas. The plant is very tolerant of salty soils, but it is not restricted to these habitats.

CONTROL: Because of its extensive root system and waxy leaves, perennial pepperweed can be very difficult to control. Therefore, prevention of new infestations is critical. Clean equipment before moving it from infested sites. Do not allow perennial pepperweed plants to go to seed. Hand pull or dig plants in small infestations. Dig out as much of the root as possible. Even very small sections of root can contain buds that will sprout to produce new plants, so continue to monitor sites and remove all regrowth and seedlings. Establishing competitive vegetation aids in controlling perennial pepperweed. Make certain that seed used for revegetation work is not contaminated with perennial pepperweed.

Herbicides may be useful for controlling infestations. However, great care must be used, since many infestations occur close to open water. For chemical control recommendations, refer to the Pacific Northwest Weed Control Handbook, an annually revised publication available from Washington State University Cooperative Extension.

Control of well-established infestations requires a carefully planned and integrated approach. Contact your local county noxious weed control board, weed district, or Washington State University Cooperative Extension office for assistance developing control strategies for specific sites.

By Lisa Lantz and Bridget Simon, Washington State Noxious Weed Control Board.

Reference material and information are available from the Washington State Noxious Weed Control Board in Kent, Washington.

Published in June 1998.