Lepidium latifolium
(perennial pepperweed)

Preferred Scientific Name

• Lepidium latifolium L.

Preferred Common Name

• perennial pepperweed

Other Scientific Names

• Cardaria latifolia (L.) Spach
• Crucifera latifolia (L.) E.H.L.Krause
• Lepidium affine Ledeb
• Lepidium dioscorides Bubani
• Lepidium sativum var. latifolium DC DC
• Lepidium sibiricum Pall
• Lepidium sibiricum Schweigg
• Nasturtiastrum latifolium (L.) Gillet & Magne
• Nasturtium latifolium (L.) Kuntze

International Common Names

• English: broadleaf peppergrass; broadleaf pepperwort; dittander; giant whiteweed; ironweed; peppergrass; peppergrass mustard; perennial peppercress; perennial peppergrass; perennial pepperwort; tall whitetop; Virginia pepperweed
• Spanish: lepidio; mastuerzo montesino; piperisa
• French: grande passerage

Local Common Names

• Austria: Breitblatt-Kresse
• China: kuan ye du xing cai
• Germany: Breitblättrige Kresse
• Latvia: placialape pipirne
• Lithuania: platlapu cietkersa
• Norway: strandkarse
• Portugal: erva-pimenteira
• Sweden: bitterkrassing

• Domain: Eukaryota
•     Kingdom: Plantae
•         Phylum: Spermatophyta
•             Subphylum: Angiospermae
•                 Class: Dicotyledonae
•                     Order: Capparidales
•                         Family: Brassicaceae
•                             Genus: Lepidium
•                                 Species: Lepidium latifolium

L. latifolium was named by Linnaeus and although a number of other names have been applied since, including Cardaria latifolia, none of these are now commonly used. 

Five subspecies have been described: subsp. affine,amplexicaule, latifolium, obtusum and sibiricum. These however, are not recognised in the most recent taxonomic treatments of the species (Francis and Warwick, 2007). This is supported by a study by Gaskin et al. (2013) who found no correlation between morphological characteristics used in taxonomic keys to distinguish the subspecies latifolium, affine and obtusum and genotypic lineages or clusters.

Common names such as pepperweed and peppercress derive from the pungent, peppery taste of the foliage.

L. latifolium is a perennial herb 1-2 m high, with a creeping root system emanating from a semi-woody crown. Francis and Warwick (2007) describe the underground structures as both rhizomes and roots. Other authors quoted by Zouhar (2004) conclude otherwise, that they are all true roots. It seems likely that both types of structure can occur – short rhizomes (horizontal stems from which buds develop at the nodes) and much longer horizontal roots 10-20 cm deep, on which adventitious buds can develop at any point, especially when fragmented. Other roots can occur much more deeply, even down to 3 m (Zouhar, 2004). A number of erect stems arise from the crown, and are much branched above. Lower leaves are up to 30 cm long by 5-8 cm wide on petioles up to 10 cm long, elliptic-ovate or oblong, finely serrate on the margins and with a whitish mid-rib. Upper leaves are smaller up to 10 cm long, sessile, with entire margins, cuneate base and acute apex. Leaf surfaces may have some hairs, but are generally glabrous, leathery and glaucous.

Francis and Warwick (2007) describe the inflorescence as ‘paniculate, terminating in numerous, many-flowered, often compounded racemes; sparsely pubescent or glabrous; pedicels slender, 2–5 mm long. Sepals deciduous, oblong, suborbicular, 1–1.4 mm long by 0.8–0.9 mm wide, glabrous or pubescent, white at margin and apex. Petals milky white, obovate, 1.8–2.5 mm long by (0.8)1–1.3 mm wide, apex rounded. Stamens 6, with 4 long and 2 short filaments 0.9–1.4 mm long; anthers ovate, 0.4–0.5 mm long. Pistil 2 mm long, style nearly obsolete (scarcely visible), stigma prominent, sessile, 2–3 times broader than sepals, persistent on fruit. Fruits (silicules) 2- chambered, slightly flattened, oblong ellipsoid to oval-ellipsoid, or suborbicular, (1.6)1.8–2.4(2.7) mm long by about 1.3 mm wide, sparsely hairy with soft, crinkly hairs or glabrous, not emarginate or very minutely so. Seeds 1 per chamber, light reddish brown, flattened, wingless, finely papillate (with small swellings) with long, simple hairs; oblong-ovoid, (0.8)1–1.3 mm long by 0.7–0.9 mm wide.’ 

Pollen morphology has been described by Tang et al. (2005).

Introduction of L. latifolium into North America apparently dates back to the early twentieth century. Francis and Warwick (2007) indicate the 1920s as the period of introduction, corresponding with numerous records for herbarium specimens from across Massachusetts and Connecticut in the 1920s and 1930s (GBIF, 2012). However, there are earlier herbarium specimens recorded by GBIF (2012) from ‘Shaw’s Garden’ in St Louis, Missouri in 1902 and from New York Botanical Garden in 1908. It is possible that some introductions occurred via botanic gardens or nurseries, but there are suggestions that several separate introductions may have been involved, some involving contaminated Beta vulgaris (sugar beet) seed (Zouhar, 2004).

In the western USA L. latifolium was first sighted in California in sugar beet seed in 1936 (Bellue, 1936 in Howald, 2000) and was recognised as invasive in the 1980s, first in California and then in adjacent western states. The first herbarium record in Arizona dates from 1987 (Brown, 2005). Although established in eastern USA long before the west, it has until recently, been apparently less invasive there, but Orth et al. (2006) now report increasing concern that it is spreading in Massachusetts and Connecticut. 

In Canada, L. latifolium was first identified as invasive in the late 1990s in British Columbia (Francis and Warwick, 2007). 

In Mexico the earliest herbarium specimen recorded by GBIF (2012) is 1944. Vibrans (2003) reports that infestations in the Valley of Mexico are continuing to expand. 

In Australia, it is thought to have been introduced via contaminated sugar beet seed (Kloot, 1973). The earliest herbarium specimen recorded by GBIF (2012) is from 1972. 

In Norway the earliest herbarium specimen recorded by GBIF (2012) is from 1921. It is now of increasing concern as it is spreading, perhaps via soil introduced as ballast (Halvorsen and Grøstad, 1998) or in seaweed or seawater after storms (Størmer, 2011).

Soil drainage

• free
• impeded
• seasonally waterlogged

Soil reaction

• acid
• alkaline
• neutral
• very acid
• very alkaline

Soil texture

• heavy
• medium

Special soil tolerances

• infertile
• saline
• sodic

Natural Dispersal

Seeds of L latifolium are shed gradually from the pods over a period of months. Dispersal is then largely by water, whether tidal, via river flows or along the coast in seawater. The seeds sink initially in water but then develop a mucilaginous coat which causes them to re-float (Young, 1999). Dispersal of plants can also occur via root fragments. The root system is relatively sparse and does not prevent the erosion of mud in tidal situations and the resultant break-up of root systems (Young et al., 1997). In Norway, local spread is attributed to movement with seaweed or seawater after storms (Størmer, 2011). 

Vector Transmission

Carpinelli et al. (2005) show that the viability of seeds of L. latifolium may be enhanced after 96 hours in the gut of cattle and grazing animals could therefore be a means of dispersal. 

Accidental Introduction

Short-distance dispersal can result from the use of the flowers and seedheads in dry flower arrangement (Washington State Noxious Weed Control Board, 1999). Hay, feed stock, and straw used in stabilisation projects can be contaminated with seed and/or rhizomes and moving dirt or machinery that are contaminated with root fragments can initiate an invasion. Recent localised infestations of L. latifolium in the USA may have been initiated from seed or plant fragments that were contaminants in rice straw bales (ISSG, 2012). In Norway, spread is believed to have occurred via soil introduced as ballast (Halvorsen and Grøstad, 1998).

Intentional Introduction

L. latifolium is not traded commercially in horticulture and no introductions are known to have been intentional.

In northeastern California, invasions of L. latifolium moved beyond the irrigated meadows used as winter forage into intensive agricultural crops such as cereal grains and alfalfa, where infestations led to depreciation in land values for affected farms (Young et al., 1997). Zouhar (2004) also notes economic losses through reduced forage quantity and hay quality. On land used for grazing plus hay harvest, the costs of herbicide use (metsulfuron or 2,4-D) were only recouped after 4-5 years, while on land used only for grazing it would take 15 years (Eiswerth et al., 2005).

Among impacts listed by Zouhar (2004) are: altered species diversity, structure and function; displaced native species; decreased food and habitat for several wildlife species; changes in biogeochemical cycles; and increased streamside soil erosion. In summary, it has altered species diversity, structure, function, and succession in many wetland and riparian areas in the western USA.

Impact on Habitats

Blank and Young (1997) have shown that L. latifolium can act as a ‘salt pump’ which brings salt ions from deep in the soil profile and deposits them near the surface. This can favour halophytes and put other species at a disadvantage, thereby shifting plant composition and diversity. Conversely, Reynolds and Boyer (2010) recorded lower salinities under L. latifolium compared with those under Sarcocornia pacifica. L. latifolium was also shown to elevate soil solution levels of Mg⁺² and Ca⁺², thereby reducing sodium adsorption ratios that could lead to sodic soil amelioration (Blank and Young, 2002; Blank and Young, 2004). Leonard et al. (1998a; 1998b) demonstrated increased emission of mercury from contaminated soils into the atmosphere. Invasion by L. latifolium thus has the potential to alter soil properties and processes, thereby altering the trajectory of soil evolution. These effects may be exaggerated under elevated carbon dioxide conditions (Blank and Denner, 2004). 

Impact on Biodiversity

L. latifolium is extremely competitive, forming monospecific stands that can crowd out desirable native species and a number of threatened and endangered species. Significant amounts of litter can build up in dense infestations, forming a layer impenetrable to light. This layer prevents the emergence of annual plants in these areas and may reduce competition from other species decreasing the biodiversity in an area. Reports of the impact of L. latifolium on biodiversity include that from the US Fish and Wildlife Service (2010a) noting threats to the endangered thistle Cirsium hydrophilum var. hydrophilum in California from rapid invasion of brackish tidal marsh by the weed which can readily invade both diked and tidal brackish marshes with low salinity, forming dense stands in the better-drained areas where C. hydrophilum is most likely to occur. L. latifolium is especially invasive on physically disturbed soils and where vegetation cover has been reduced, forming a continuous leaf canopy, eliminating the vegetation gaps that may be essential for seedling establishment of C.hydrophilum.

The same report indicates that Sarcocornia pacifica [Salicornia pacifica] (pickleweed) is also reduced by L. latifolium with indirect consequences for the hemi-parasitic Cordylanthus maritimus for which it is an important host; also the salt marsh harvest mouse (Reithrodontomys raviventri). California black rail (Laterallus jamaicensis coturniculus), a threatened bird of California and in the IUCN Red List of Threatened Species, and California clapper rail (Rallus longirostris obsoletus), a threatened bird of California, are also affected via loss of nesting sites in favoured native vegetation (ISSG, 2012). The US Fish and Wildlife Service (2009b) also record threats from L. latifolium to the endangered Solano grass Orcuttia mucronata [Tuctoria mucronata] in California, resulting from shading and changes in water chemistry. 

An inventory of rare and endangered plants in California indicates that L. latifolium is encroaching on several rare plant populations at Grizzly Island Wildlife Area in Suisun Marsh, including soft bird's-beak (Cordylanthus mollis ssp. mollis), Suisun thistle (Cirsium hydrophilum var. hydrophilum) and Suisun Marsh aster (Symphyotrichumlentum) (Zouhar, 2004). This author also refers to detrimental effects on nesting sites for several species of rare and endangered waterfowl. 

Blank (2002) concludes that L. latifolium is an effective competitor due to its ability to make available and take up more nitrogen than the vegetation it is replacing.

Zouhar (2004) refers to increased difficulty in the control of mosquitoes in Utah, where L. latifolium has become dominant, due to its height.

• Proved invasive outside its native range
• Has a broad native range
• Highly adaptable to different environments
• Pioneering in disturbed areas
• Long lived
• Has high reproductive potential
• Gregarious
• Has propagules that can remain viable for more than one year
• Reproduces asexually

• Altered trophic level
• Ecosystem change/ habitat alteration
• Modification of nutrient regime
• Modification of successional patterns
• Monoculture formation
• Negatively impacts agriculture
• Reduced native biodiversity
• Soil accretion
• Threat to/ loss of endangered species
• Threat to/ loss of native species

• Allelopathic
• Competition - monopolizing resources
• Competition - shading
• Competition - smothering

• Difficult to identify/detect as a commodity contaminant
• Difficult to identify/detect in the field
• Difficult/costly to control

L. latifolium may be grazed by cattle, sheep and goats but is not considered a useful forage (Zouhar, 2004). Carpinelli et al. (2005) caution that the viability of seeds is, if anything, enhanced after 96 hours in the gut of cattle, so grazing animals could be a source of further spread. 

L. latifolium has been widely used medicinally, especially as a diuretic. Wright et al. (2007) concluded that it was among the more effective herbal diuretic preparations. It has been found to have a hypotensive effect due to its diuretic action in rats. The aqueous leaf extract given in doses of 50 and 100 mg/kg through intraperitoneal and oral routes, respectively, produced significant and dose-dependent diuretic and hypotensive activities. Attempts to extrapolate the diuretic action of L. latifolium extracts from rats to man led to the recommended daily dose of 3-5 g L. latifolium extract per man per day, administered as tea, which is equivalent to 43 to 71 mg/kg body weight in a 70 kg subject (Navarro et al., 1994). L. latifolium has also been used as a folk medicine in the Canary Islands for renal lithiasis and six months of oral treatment with a suspension of L. latifolium significantly reduced prostate size and volume in castrated rats where the hyperplasia was induced by steroid treatment (Martínez Caballero et al., 2004). L. latifolium is also used in India both medicinally and as a food (Rana et al., 2012) and in the Ladakh Himalayas there is interest in it as forage (Anju Verma et al., 2008).