Broadleaved pepperweed (Lepidium latifolium)

Common Names: Perennial pepperweed, tall whitetop, pepperwort, dittander, peppergrass

Description:

Broadleaved pepperweed (Lepidium latifolium) is a tough perennial in the mustard family, native to Eurasia and introduced to North America ? probably as a contaminant in imported crop seed in the early 1900s [1][2]. It spreads aggressively by deep creeping roots and by seed, forming dense, single-species stands [2][3]. MISIN tracks it because it crowds out native plants in wetlands and along waterways and even changes the soil, pumping salt up to the surface where most native plants can't grow [2].

Habit:

Broadleaved pepperweed is a semi-woody perennial that grows from about 2 to over 4 feet tall in dense clumps [3]. It starts each year as a low rosette of large leaves ? roughly 4 to 11 inches long, oval to oblong, with smooth or slightly toothed edges ? then sends up branching flower stems with smaller, lance-shaped leaves [3]. The foliage often has a waxy gray-green look [3]. In late spring and summer it produces masses of tiny white four-petaled flowers packed into dense, rounded clusters at the stem tips, giving a frothy white-topped appearance [2][3]. It is easiest to spot in early summer when flowering and standing above surrounding plants; the dead, woody stems often persist into winter [3].

At-a-glance cues:

1. Semi-woody perennial, 2 to 4+ feet tall, in dense clumps, often on wet or disturbed ground [3]
2. Large basal rosette leaves (4 to 11 inches long) with a waxy gray-green look [3]
3. Dense, rounded clusters of tiny white four-petaled flowers at the stem tips in late spring to summer [2][3]
4. Dead woody stems persist standing through winter [3]

Leaves:

1. Basal leaves: in a rosette, large (about 4 to 11 inches long), oval to oblong, with smooth to slightly toothed edges and a long stalk [3]
2. Stem leaves: alternate, smaller and lance-shaped, becoming smaller up the stem; stalked (not clasping) [3]
3. Surface: often waxy and gray-green [3]

Stems:

1. Habit: erect, semi-woody perennial, 2 to over 4 feet tall, branching toward the top [3]
2. Underground: thick, deep, creeping roots (rhizomes) that spread laterally up to about 10 feet [3]
3. Dead stems turn woody and stand through winter, building up into dense thickets [3]

Flowers:

1. Color: white, with four petals (typical of the mustard family) [2][3]
2. Size: individual flowers are tiny [2]
3. Arrangement: packed into dense, rounded clusters at the stem tips, giving a frothy white-topped look [2][3]
4. Bloom time: late spring into summer [3]

Fruit and seeds:

1. Fruit: a small, round to oval pod about 1.5 mm across, each holding a single seed [2]
2. Seed: produced in large numbers; it is a prolific seeder [3]
3. Dispersal: seeds and root fragments move easily in floodwater and moved soil, and seeds stick to tires, shoes, and animals [2]

Habitat:

Broadleaved pepperweed is native to Eurasia ? southeastern Europe through western and central Asia [1][2]. It was introduced to North America (likely as a seed contaminant) and is now a serious invader, especially across the western United States [2]. It thrives in moist and seasonally wet ground and tolerates salty soils, so it is most damaging in wetlands, marshes, riverbanks and floodplains, irrigation ditches, and wet rangeland, and it also turns up along roadsides and in other disturbed places [2].

Reproduction:

Broadleaved pepperweed spreads two ways, and the roots are the bigger problem. Its thick, creeping roots grow outward ? up to about 10 feet ? and send up new shoots, and even a root fragment as small as an inch can grow into a whole new plant [3]. Those root pieces and the plant's many seeds travel readily in floodwater and moved soil, and seeds also stick to tires, shoes, and animals, so it moves easily along waterways and on equipment [2][3].

Each year plants emerge as rosettes in early spring, flower in late spring, and set seed by midsummer [3]. For control timing: cut or pull plants before they flower to keep them from seeding and to wear down the roots, and remove as much root as possible ? but expect to repeat the effort for several years, since the roots resprout [3].

Impact and Damage:

Broadleaved pepperweed's main harm is crowding and changing the soil. It forms dense, single-species stands that shade out and displace native plants, especially in wetlands and along streams and rivers [2]. It also acts like a "salt pump": its deep roots pull salts up from far down in the soil, and the plant deposits that salt at the surface, raising soil salinity to levels that keep many native plants from sprouting [2]. Together these effects can take over wetland and riverbank habitats and reduce the variety of plants that grow there [2]. Because root fragments and seeds spread so easily in water, infestations tend to expand year after year [2][3].

Similar species:

Broadleaved pepperweed can be confused with other white-flowered mustards on disturbed or wet ground.

Hoary cress (Lepidium draba, also called whitetop) is the closest match ? another creeping-rooted mustard with dense clusters of tiny white four-petaled flowers. Tell them apart by size and leaves: broadleaved pepperweed is taller (often over 3 feet) with stalked leaves, while hoary cress stays under about 3 feet and its leaves clasp the stem without an obvious stalk [3].

Monitoring and rapid response:

Signs to watch for:

1. Tall (2 to 4+ feet), dense clumps of mustard-family plants with waxy gray-green leaves, on wet ground, riverbanks, ditches, or roadsides [3]
2. Frothy, rounded clusters of tiny white flowers at the stem tips in late spring and summer [2][3]
3. Patches that widen year after year, and dead woody stems standing through winter [3]

What you can do:

1. For a few scattered plants, pull or dig them and remove as much of the root as you can, since even small root pieces resprout [3]
2. Repeatedly cut or mow the plants before they flower, over the season and across years, to wear down the root reserves [3]
3. Smother a small patch with thick black plastic for about two growing seasons, extending the cover well beyond the patch (at least 10 feet) to catch the spreading roots [3]
4. Bag and trash flowering or seeding stems, and clean soil, seeds, and root bits off tools, boots, and equipment before leaving ? fragments and seeds spread easily in water and mud [2][3]
5. Plan for several years of repeated treatment; one pass will not clear it [3]
6. Natural area crews can map and tackle small outlying patches first, work from the edges of large stands inward, and coordinate along a whole waterway since fragments wash downstream [2][3]

When to call a professional:

1. Large or well-established stands, and anything in wetlands or along streambanks, where the deep creeping roots make hand removal impractical [2][3]
2. For chemical control options, contact a licensed applicator or your local cooperative extension office.

Report sightings to MISIN. Help track this species' spread by submitting observations through the MISIN web site or the MISIN mobile app (Android, iOS, Windows, and macOS). Photos at multiple angles paired with a precise GPS location are the most valuable data for confirming new occurrences and triggering rapid-response review.

Credits:

1. Plants of the World Online (POWO), Royal Botanic Gardens, Kew. Lepidium latifolium L. powo.science.kew.org
2. California Invasive Plant Council (Cal-IPC). Lepidium latifolium (perennial pepperweed) profile. cal-ipc.org
3. UC Statewide IPM Program (UC IPM), University of California. Perennial Pepperweed (Lepidium latifolium). ipm.ucanr.edu

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