Nevada’s Priority Agricultural Weeds:
Perennial Pepperweed
By: Brad Schultz, Extension Educator, University of Nevada Cooperative Extension, Winnemucca, NV
Introduction
Perennial pepperweed (Lepidium latifolium L.), also commonly called tall whitetop, is a long-lived perennial weed originally from Eurasia. Following importation as an ornamental plant, perennial pepperweed subsequently spread throughout the western states, commonly inhabiting irrigated or sub-irrigated meadows and pastures, stream banks and associated riparian areas, irrigation ditches, marshes, floodplains, shorelines, exposed lakebeds, irrigated cropland, and areas that receive abundant run-on moisture (e.g., roadsides and seasonal streams).
1A
There are many adverse effects from perennial pepperweed. A solitary plant can rapidly develop a large root system that contains many buds, with each bud capable of developing into a new shoot or plant. A single plant can expand into a large dense stand of perennial pepperweed that displaces more desired vegetation, ultimately decreasing the land’s value for livestock grazing, field crops, wildlife and fisheries, and resale. Perennial pepperweed’s root system is large but not dense; thus, it does not hold the soil together well. Sites that endure regular high-flow flood events and also have extensive stands of perennial pepperweed have an increased high risk for severe erosion. Furthermore, bank erosion from flooding typically breaks large perennial pepperweed root systems into many pieces. Root segments as short as one-inch long and one-tenth of an inch in diameter often have a bud capable of developing into a new plant. New infestations are common immediately after a flood because root segments are deposited downstream and many reside on or just below the surface of newly deposited sediment that remains moist for weeks after the flood. Moist sediment deposits are an optimal growing site for perennial pepperweed, from both root fragments and seed. Established perennial pepperweed plants also have roots that can grow deeper than most of our native plants. These deep roots can extract many salts from these depths and deposit them on the soil’s surface during leaf fall. The accumulated salts can reduce the germination and growth of better forage plants. 30 NOVEMBER/DECEMBER 2021
Figures 1a and 1b. Perennial pepperweed seedlings at 4-6 weeks of age (1a) and a one-year-old plant (1b). The seedlings have developed a tap root but have not become perennial. The yearling plant has developed a lateral root from which a new shoot has developed. Additional buds are present throughout the root system of the yearling plant
1B Plant Biology
Reproduction occurs from two sources: 1) the aforementioned buds on the roots, with as many as one bud per inch of root; and 2) from seed. The initial tap root grows downward deep into the soil (Figures 1a and 1b), and may reach a depth of almost 3 feet in less than 90 days. Lateral roots extend horizontally, often reaching tens of feet after several years. The majority of the root system is found in the top two feet of the soil, but in deep soils perennial pepperweed roots can penetrate to over 10 feet deep, often tapping into a shallow water table. Almost always, a substantial amount of the root system resides below the tillage zone of most equipment. The Progressive Rancher
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