Perennial Pepperweed (Lepidium latifolium)

Perennial pepperweed infests moist areas and the land along streams, rivers and sub-irrigated pasture. Perennial pepperweed is common in isolated areas of Montana but not widespread, so knowledge and awareness are critical in order to prevent this aggressive plant from invading further.

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PERENNIAL PEPPERWEED, OFTEN USED BY

florists in fresh and dried flower arrangements, is not just another pretty white flower growing along the roadside. This weed is quickly spreading across the West and infesting moist areas and the land along streams, rivers and sub-irrigated pasture. As perennial pepperweed becomes established in wet areas, it also encroaches onto pastures. Perennial pepperweed is common in isolated areas in Montana but not widespread across the state. Therefore, knowledge and awareness are critical in order to prevent this aggressive plant from invading further.

Identification and biology

Perennial pepperweed belongs to the mustard or Brassicaceae family. It is an erect, branching perennial forb that grows one to three feet high, but may reach heights of eight feet in wet areas. The base of the stems is semi-woody. However, perennial pepperweed starts growing as a rosette and remains in this form for several weeks before flowering stems elongate. The roots enlarge at the soil surface to form a woody crown (Figure 1A). Toothed leaves are lance-shaped and bright green to gray-green and may have a leathery texture. Basal leaves are larger than upper leaves and typically taper to an elongated stem (Figure 1B). Upper leaves are sessile, which means the leaves have no stalk and are directly attached to the stem. Flowers are white, have four petals, and occur in dense clusters of six to eight tiny blossoms near the ends of stems around mid-June (Figure 2, page 2). Perennial pepperweed is a prolific seed producer, capable of producing more than six billion seeds per acre of infestation. Nearly microscopic, reddish-brown seeds occur in flattened, elongated pods that are less than one- eighth inch in diameter and slightly hairy (Figure 3). Perennial pepperweed rosettes may re-emerge in the fall and remain green through winter.

Perennial pepperweed, or tall whitetop, should not be confused with whitetop, also called hoary cress (Lepidium draba, L. chalepense, and L. appelianum). Recent molecular-genetics data indicated that the whitetop species are within the Lepidium genus and not the Cardaria genus as previously thought. Whitetop species are usually much shorter than perennial pepperweed and the upper leaves clasp the stem, unlike perennial pepperweed. In addition, whitetop blooms much earlier than perennial pepperweed. See the MSU Extension bulletin Biology, Ecology and Management of Whitetop (EB0138) for more information.

FIGURE 1. A). Enlarged root forms woody crown. B).Basal leaves are larger than upper leaves.

FIGURE 2. Flower clusters of six to eight tiny blossoms occur near the ends of stems (photo by Matt Lavin).

FIGURE 3. Seed pods are small and have scattered hairs (photo by Matt Lavin).

In addition to seeds, perennial pepperweed spreads by creeping underground roots (rhizomes) which may grow to a length of 10 feet. New plants shoot up from the underground root and enable perennial pepperweed to form dense monocultures that block sunlight from reaching the soil, thus suppressing the growth of other plants.

Origin and distribution

Perennial pepperweed is a native of southern Europe and western Asia. It is believed to have been introduced into the United States around 1900 as a contaminant of sugar beet seeds. In the last two decades, this weed has spread rapidly throughout the West and is now found in all western states. It is also found in several midwestern and northeastern states and is listed as a noxious weed in at least 15 states. Perennial pepperweed is listed as a Priority 2A noxious weed in Montana and has been reported in 23 of 56 counties as of 2017 (Figure 4).

Perennial pepperweed is adapted to a wide range of ecological sites which allows it to have a large geographical distribution. In the Intermountain West, the plant’s distribution corresponds to river systems and riparian zones which are the primary areas of invasion in most states. However, perennial pepperweed is not limited to riparian zones and is found in waste areas, ditches, roadsides, pastures and residential areas as well. The plant can tolerate high salt concentration in soils but is not limited to these sites.

Perennial pepperweed spreads in many ways. The plant commonly travels in rivers and irrigation systems as seeds and rhizomes from eroded banks. Flood irrigation carries plant propagules into hay meadows, pastures and other irrigated lands. Perennial pepperweed is also carried in contaminated topsoil used as fill in construction and landscaping sites. Seeds are transported when they attach themselves to machinery and vehicle tires. Livestock, waterfowl, and dried flower arrangements disperse seeds long distances.

FIGURE 4. Counties in Montana where perennial pepperweed has been reported. (Compiled records from EDDMapS West, Consortium of Pacific Northwest Herbaria, Intermountain Region Herbarium Network, and MSU Schutter Diagnostic Lab)

Impacts

Perennial pepperweed is a very competitive species that crowds out desirable vegetation and results in dense monocultures and a decrease in biodiversity. When established along rivers and streams, the plant interferes with the regeneration of willows and cottonwoods, reducing cover and food availability for birds. The accumulation of semi-woody stems negatively impacts nesting habitat for wildlife.

Perennial pepperweed poses a large threat to hay meadows. It is introduced through irrigation ditches and, once established, can decrease protein content and digestibility of hay. In areas that are not mowed annually, semi-woody stems can accumulate and hinder grazing. Although there is no scientific evidence, it is believed that pepperweed is toxic and could pose a threat to livestock.

Control

The best method of managing perennial pepperweed is to prevent the weed from becoming well-established. Minimizing soil disturbances from vehicles, machinery and overgrazing will reduce areas where the weed might become established. Healthy, vigorous grass stands that are properly managed can limit the establishment and growth of weed populations by using resources needed by weeds. Early detection is vital to prevent invasion. Survey your land three times a year and remove any individual plants before they become established. If you discover an infestation, containment is critical. The most effective method of containment is to spray borders of the infested areas with herbicide. Lastly, limit perennial pepperweed seed dispersal.

The weed is usually found on sites difficult to access and along waterways where control presents special challenges. Eradication of perennial pepperweed is impossible in most cases. Instead, efforts and resources should be focused on preventing its spread and using an integrated weed management program.

Integrated weed management typically uses a combination of cultural, mechanical, biological and chemical weed control techniques. Cultural control of perennial pepperweed begins with maintaining healthy, robust stands of desirable plants with proper species selection, fertilization and irrigation.

Mechanical control of perennial pepperweed is not recommended for established infestations. Digging, mowing and tilling will only encourage new plants to sprout from the root crown and creeping roots. Seedlings can be hand-pulled or tilled, but care must be taken to remove large portions of the root system. Grazing can be complicated by perennial pepperweed’s persistent semi-woody stems. Before grazing, removing old stems by mowing or burning will allow livestock to reach new growth. Questions remain regarding the possible toxicity of the plant and until they are answered, grazing infested areas should be avoided.

Biological control agents for perennial pepperweed have not been approved for release in the United States to date. Many valuable crop species like canola, mustard and cabbage belong to the same family as perennial pepperweed. In addition, eleven native Lepidium species, one of which is endangered, grow throughout the western U.S. Little effort has been taken to develop a biocontrol agent for perennial pepperweed because the risk is too great of releasing an insect that would attack a valued crop or endangered or native plant. The impact of a prospective biological control organism on agricultural crops and other closely related species must be clearly established before its use will be allowed.

Chemical control of perennial pepperweed is best achieved by using products containing the active ingredient chlorsulfuron (e.g., Telar®) or metsulfuron (e.g., Escort®). Both of these are sulfonylurea compounds. Chlorsulfuron should be applied at one ounce per acre during bud to early bloom stage on non-cropland only. One ounce per acre of metsulfuron applied when plants are actively growing, but before bud and bloom stage, will control perennial pepperweed. Both products should be applied with a nonionic surfactant (see product labels).

Unfortunately, neither chemical is registered for use in wet areas. Imazapyr (e.g., Arsenal®) is also effective against perennial pepperweed when applied to actively growing foliage. However, this chemical is non-selective and will kill all vegetation indiscriminately. Phenoxy herbicides, such as 2,4-D, will kill the shoots of perennial pepperweed, but root crowns will quickly sprout new foliage. Repeated applications for up to five years are necessary to starve the root system. To successfully manage perennial pepperweed with chemicals, competitive vegetation must be established immediately after its control to prevent reinvasion.

Conclusion

The best management of perennial pepperweed is containing its current distribution and preventing its spread into non-infested areas. While Montana currently does not have any extensive infestations of perennial pepperweed, small, local patches of the weed have been reported. Early detection and treatment of perennial pepperweed is crucial. If you think you have perennial pepperweed on your land or believe you have seen it elsewhere in Montana, contact your local Extension agent or weed district.

Note: Information in this document is provided for educational purposes only. Reference to commercial products or trade names does not imply endorsement by MSU Extension. Common chemical and trade names are used in this publication for clarity. Inclusion of a common chemical or trade name does not imply endoresement of that particular product or brand of herbicide and exclusion does not imply non-approval. This publication is not intended to replace the product label.