Perennial Pepperweed

What is perennial pepperweed?

Perennial pepperweed is a tall, long-lived mustard from southeastern Europe and Asia that now infests wet ground across much of the western United States. perennial pepperweed identification is easiest in late spring, when dense white flower heads sit on the stems like a layer of foam. Before it bolts, it looks like an ordinary clump of big, stalked leaves.

The trouble is underground. Thick roots creep outward and send up new shoots, and an inch-long root piece can start a new plant. Left alone, it builds near-solid stands that crowd out other plants, lower the value of hay and pasture, and choke ditches and stream corridors. Dead stems decay slowly into thatch that degrades wildlife nesting cover, and the plant can pull salts from deep soil up to the surface, changing what else can grow there.

Perennial Pepperweed identification: what to look for

Look at the whole patch first. Pepperweed grows as a colony of many stems from one root system, often with last year's dead stalks still standing. Then compare the big stalked basal leaves with the much smaller stem leaves; that contrast plus the white flower clusters is the giveaway.

• Leaves: Rosette leaves are oval to oblong on long stalks with smooth or slightly toothed edges. Stem leaves are lance-shaped, attach directly to the stem, look waxy and gray-green, and often show a pale midvein.
• Stems: Many upright, semi-woody stems from a shared crown. They die back after seed set and decay slowly into standing thatch.
• Flowers: Tiny white flowers with four petals, crowded into dense rounded clusters at the branch tips.
• Seed pods: Flattened, round to egg-shaped pods around a tenth of an inch long, each with two reddish-brown seeds.
• Roots: Thick roots with a semi-woody crown at the surface that reach deep and run sideways, so patches widen every year.
• Smell: Crushed foliage has a sharp, horseradish-like odor.

Where it grows in the United States

Perennial pepperweed is most widespread in the western United States, lining rivers, canals and irrigation ditches and filling low pastures and hay meadows; California surveys have found it in nearly every part of that state. It is also turning up farther east. Wisconsin treats it as a prohibited invasive, and in Maine it is localized but considered severely invasive along shores, beaches, dunes and salt marshes.

It favors ground that is wet for part of the year or has a shallow water table, and it tolerates salty, alkaline soils that many plants cannot. Fine-textured floodplain soils are classic habitat, but it also grows on coarse river deposits, roadsides and residential lots. Floods and irrigation flows carry root pieces and seed downstream, so infestations often follow a channel.

Perennial Pepperweed look-alikes

A few plants get mixed up with it:

• Hoary cress (whitetop) (Lepidium draba): Another white-flowered perennial mustard, but generally under 3 feet tall, and its stem leaves clasp the stem. Perennial pepperweed is taller and its lower leaves have obvious stalks.
• Queen Anne's lace (Daucus carota): Also carries white flower clusters on tall stalks and can look similar from a distance, but its leaves are completely different from pepperweed's broad, smooth-edged leaves.

How to get rid of perennial pepperweed

Pulling, digging and mowing: Pulling or tilling handles seedlings and scattered young plants, but established clumps regrow from their roots, so dig out as much root as you can and come back for regrowth. Mowing alone does not kill it and stimulates new shoots; use it at the flower-bud stage to clear thatch before treating the regrowth. Burning old stalks in winter or early spring removes thatch but not roots. For a small patch, thick black plastic left down for two full growing seasons and extended at least 10 feet past the edge has given the best smothering results, especially after mowing and tilling. Bag pulled plants so root pieces are not left behind.

Cultural control matters most, because the right conditions keep it from coming back.

• Keep pastures and hay ground thick with vigorous sod-forming grasses or alfalfa.
• Replant treated or bare ground right away with fast-growing, site-adapted plants; perennial grasses suit pastures and natural areas.
• Sheep grazing can suppress pepperweed mixed with other plants, and spring grazing or mowing sets up a more effective follow-up treatment.
• In landscapes, once the patch is knocked down, use fabric or mulch and closely spaced plantings, and spot-treat anything that comes through.

Herbicides: Herbicides work best at the flower-bud stage and poorly on rosettes, so treat when the first buds appear. For pasture, rangeland and noncrop sites, UC IPM rates chlorsulfuron as most effective, with 2,4-D, glyphosate and imazapyr (not for rangeland or pasture) as other options, and imazamox or imazethapyr in alfalfa; Maine also mentions imazapic and metsulfuron-methyl. Most of these are not labeled for home gardens. At home, spot-treat pepperweed foliage with glyphosate, keep it off plants you want, and retreat resprouts. Expect repeat treatments for several years. Use only a product labeled for your exact site and follow the label; the label is the law. Extra rules apply near wetlands and water, and in Maine the residual products must be applied by a licensed applicator. If you hire someone to spray, make sure they hold your state’s pesticide applicator license; the rules for each state are in our pesticide applicator license guide.

Keeping it from coming back

Buy hay, straw, mulch and crop seed free of weed seed and root pieces, and walk fence lines, ditches and stream banks each spring to catch new rosettes while they are small. Clean mowers, tillers, vehicles and boots after working in an infested area.

Hold livestock that grazed an infested pasture in a small, watched pen for a few days before moving them to clean ground. After any control work, get desirable plants growing quickly; bare, wet, salty soil is exactly what pepperweed wants.

Questions we get about perennial pepperweed

Similar weeds

Hoary Cress (Whitetop)Lepidium drabaQueen Anne's Lace (Wild Carrot)Daucus carotaBuffaloburSolanum rostratum