Perennial Pepperweed (Lepidium latifolium)

plants, terrestrial, tier 3

Family: Brassicaceae
Life form: Forb

History & Introduction

Native to West Asia through Southeastern Europe, perennial pepperweed (PPW) will grow nearly anywhere within its native range including fresh, brackish, and saltwater wetlands, in and around agricultural fields, in waste places, and even on stony slopes. It can grow in elevations from sea level to above 10,000 feet. PPW was likely introduced to the US several times, possibly as a contaminant of sugar beet seeds. PPW is a noxious weed that is widespread through the western United States, from along riparian areas to roadsides. It is known to also contaimantate alfalfa fields and rangelands, which may explain its establishment on Long Island at Westmeadow beach and Flax Pond, which is adjacent to the Suffolk County owned horse farm, presently the Old Field Farm ltd. These stables were initially established in the 1930s, and originally known as the North Shore Horse Show Grounds. Feed from horses that were transported here from across the US for shows likely contained PPW seeds, or more likely root fragments, which may have been spread along the high marsh areas through the tides.

*EDDMapS Distribution – This map is incomplete and is based only on current site and county level reports made by experts, herbaria, and literature. For more information, visit www.eddmaps.org*

Identification & Biology

*Leslie J. Mehrhoff, University of Connecticut, Bugwood.org*

Form & Leaves – PPW is a perennial forb with 1 to many above ground stems. Shoots emerge late winter/early spring before most native plants. It can grow from 3 to 8 feet tall. Initially, shoots form a rosette near the soil surface, basal leaves are 4-12” long and 1-3” wide, with long petioles. As PPW grows, leaf size decreases up the stem, and upper leaves are sessile and alternate. Leaf area is the smallest when PPW begins to flower.

Flowers & Fruits – The flowers of PPW form an inflorescence that is a panicle, 5-6” wide and forms dense clusters at the tops of stems. The panicle is made up of tiny white 4-petiolate flowers that are 3mm wide. The fruit of perennial PPW is a silicle, or a silique of nearly equal length and width. PPW has the ability to produce thousands of seeds per individual. Seeds do not dehise from pods at maturity, but rather drop at irregular intervals from senesced plant stalks throughout the winter. Pods can be wind, water, and animal dispersed. In water, seeds initially sink but later form a layer of mucilage which makes them buoyant. Laboratory studies suggest that despite its ability to rapidly germinate in lab settings, the extent to which PPW spreads by seed is not well understood, as seedlings are not often found in the field or may be hard to distinguish. Seeds were viable up to one year in a lab setting, and the lack of a hard seed coat suggests that PPW seeds do not last in the seed bed after prolonged control efforts.

*Steve Dewey, Utah State University, Bugwood.org*

Roots – PPW has a creeping root system and semi-woody root crowns. The roots of PPW make up about 40% of the plant’s biomass, with roots growing as deep as 9 feet. This extensive rooting system is most likely the key to this plant’s success as it is able to exploit nutrients in soil levels that are otherwise inaccessible to other plants, and gain access to water year-round. Root and crown fragments are highly reproductive and may last for many years before resprouting. They are also buoyant and can easily spread through the tides to establish new colonies. Just as well, roots can tolerate very dry conditions and resist desiccation.

Damage

Due to its ability to uptake nutrients found deep in the soil, such as Ca and Na, it can drastically change the soil chemistry where it becomes established, decreasing plant diversity by reducing native plants’ survival, and increasing PPW’s survival and spread. By spreading clonally. Through this and it’s ability to spread clonally, it threatens the stability of the marsh ecosystem and beyond, as the extensive root system may support PPW spread into novel areas. In the west it already threatens agricultural fields as it decreases the value of land as it can be extremely difficult to control and makes for a poor forage crop.

Despite their extensive root system, PPW lacks dense root structures, providing little benefit to stabilize shorelines. Infact, as erosion takes place during flooding or high water events, exposed root fragments are easily transported by water as they are buoyant and able to travel long distances.

Control

Early detection and rapid response is the strongest management practice to fight the spread of PPW. For small populations, hand pulling and removing as much of the root as possible may be effective as plants can resprout from even very small root fragments. For established stands of PPW, an integrated approach is required. Mowing/cutting in combination with an appropriate herbicide treatment of glyphosate or chlorfusion, the latter showing the best results, though it is often not recommended for use near wetlands. Timing herbicide treatment may be crucial to controlling PPW in order to deplete carbon reserves in roots, which are lowest just before plants bolt. Maximum results have been seen when applying herbicide at the flowerbud stage in the mid to late spring.

Prolonged inundation with water has shown promising results as PPW plants seem to become stressed when roots are exposed to anoxic conditions. However, access to large quantities of water and off-target species effects are major drawbacks to this method.

Planting a competitive ecotype of a native plant, such as saltgrass or spike grass (Distichilis spicata) may also help to competitively suppress PPW.

Presently, there is some research being done to find a biocontrol for L. latifolium, however, discovery of a host-specific insect or pathogen has proven difficult for this plant. It is related to economically important cruciferous crops, as well as being within a genus that has 11 native species found within the US, one of which is endangered – Lepidium papilliferum (slickspot peppergrass), found in Ohio. Here on Long Island, the only insect we have observed consuming PPW is the harlequin beetle, Murgantia histrionica. This stinkbug family member is native to much of the Southeastern North America, though they are considered an agricultural pest feeding on a wide variety of brassicas, as well as nightshades and asparagus.

Albugo candida, a white rust fungus found growing on PPW in Nevada and California, has had some promising results in glasshouse studies. An isolate of Albugo was found only infecting PPW when also exposed to several other members of the Brassicaceae family. However, further research is needed to determine its efficacy in managing and reducing populations of PPW.