Weed Notes: Lepidium Latifolium Tunyalee Morisawa the Nature
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Weed Notes: Lepidium latifolium TunyaLee Morisawa The Nature Conservancy Wildland Weeds Management and Research http://tncweeds.ucdavis.edu 18 June 1999 Biology: Lepidium latifolium (Family: Brassicaceae) is an extremely invasive weed commonly called perennial pepperweed, perennial peppercress, perennial peppergrass, broad-leaved peppergrass, peppergrass, slender perennial peppercress, tall whitetop or dittander. A native from North Africa to Norway and east to the Himalayan region of southern Asia, this weed has invaded every state in the U.S. except for Arizona. In the western U.S., L. latifolium was first discovered in Montana in 1935 and in California in 1936. It is believed that L. latifolium was introduced to the west as a contaminant in sugarbeet seed. L. latifolium readily invades disturbed areas and bare soils. The weed is spread by seed and rhizomes. In pastures, infestation is probably from contaminated hay or from movement of seeds in irrigation water from riparian habitats. Germination rates increase when the seeds are near the soil surface and are exposed to fluctuating temperatures. The seeds do not have a hard seed coat and so longevity in the soil may be short (several years). Therefore, reinfestation from a seed bank in the soil after population control may be low. However, new growth from rhizomes or succulent woody crowns does occur during the late winter. By mid-May the stems can reach 0.5 m in height. L. latifolium is similar to hoary cress, Cardaria draba (L.) Desv. However, L. latifolium grows vigorously both above and below ground. Also, the leaves are hairless and the edges are not serrated. Cruciform flowers are small and white, borne of erect stems. The flowers have 6 stamens with mature pods of similar shape but smaller than those of C. draba. Flowering takes place in the early summer and a large number of seeds mature by late summer or early fall. Rhizomes may grow 3-10 ft from the main plant and form new plants. Lepidum latifolium can be found in a wide range of habitats. The weed occupies habitats such as the lower edge of coniferous forest 3,300 m in elevation to saline and alkaline deltas at sea level. For example it is increasingly prevalent in saline soils and waste areas of the San Francisco Bay region. There is no survival during periods of prolonged flooding during the growing season, however, it dominates wetlands from the edge of standing water to dry upland areas. In addition to natural areas, dense colonies are formed in disturbed areas such as roadsides, rangelands, pastures, agricultural fields, and irrigation canals. A Mediterranean climate is preferred. Numerous problems are associated with L. latifolium infestations. Large colonies replace native grasses, sedges and rushes. Infestations lower protein content and digestibility of the pasture for livestock. If the area is not mowed down, the accumulation of the non-degraded stems can physically inhibit grazing. In large quantities, blindness, impaired swallowing and use of the tongue has been observed. L. latifolium may become an economic threat to alfalfa and sugarbeets. The semi-woody stems hinder nesting waterfowl and the weed prevents the regeneration of willows and cottonwoods. In addition, L. latifolium impairs scenic value and reduces biodiversity. Cultural control: L. latifolium does not survive prolonged flooding during the growing season so this may be a control option if flooding is possible. Reseeding of exposed soil after the removal of L. latifolium will help prevent reinfestation. Seeds and below ground perennial organs are commonly spread by river and irrigation systems. Rhizomes and seeds are often a contaminate of fill dirt or top soil during construction. Seeds can be carried on equipment, especially tires, dried flower arrangements, livestock, waterfowl, contaminated hay, feedstuffs and straw used in soil stabilization projects. Maintain healthy stands of desirable plants to prevent infestation. The weed grows through thick mulches including hay bales and so mulching is not recommended. Mechanical control: L. latifolium readily establishes a wide, underground root system. Forty percent of the total biomass is underground. Therefore, hand pulling or plowing may not provide adequate control. Often, burning and cultivation of the weed leads to reinfestations without competition from other species. Pieces of rhizomes can resprout (from pieces as small as 1/10th of an inch in thickness) leading to increases in the level of infestation. Cultivation can bring up seeds to the soil surface where they may germinate more readily. Mowing is successful only when repeated several times per year and over multiple years. Mowing at the bolting stage (when minimum energy is stored in the roots) followed by a herbicide application (glyphosate at 3.33 kg/ha) applied at the flower bud stage may help reduce infestations. It is essential to stop seeding (over 6 billion seeds can be produced per acre) but increasingly important that broken root pieces are not spread to clean areas. Biological control: Due to safety considerations, biological control is currently not a viable option for control. Eleven species of perennial Lepidium are native to the western US and one species is on the endangered list. Important agricultural crops such a broccoli, radish and other mustard relatives are in the same family and could potentially be impacted. Chemical control: The best control of L. latifolium seems to be from the use of herbicides. Herbicides may be most effective when applied during the flowering stage but some research indicates flower bud stage. A spring spray of metsulfuron (Escort) or chlorsulfuron (Telar) combined with a surfactant (X-77 0.25% v/v) both provided moderate to good control dependent on dosage. Metsufuron provided 89 to 94% control after 3 months using 2.4, 4.2, and 6.0 ml ai/ha (0.2, 0.35 and 0.5 oz ai/acre). Chlorsulfuron provided 96 and 99% control using 8.4 or 12 ml ai/ha (0.7 or 1.0 oz ai/acre). The herbicides were applied with a backpack sprayer and a 11003LP flat fan nozzle delivering 24 gpa at 15 psi. The waxy leaf surface of mature leaves and coverage problems due to height can prevent good control. Also, chlorsulfuron can not be used in wetlands. 2,4-D was shown to provide poor control (47 to 50% mortality) in some research studies, however, it is listed as a control measure for use in water (Weedar-64). Triclopyr provides satisfactory control for six months or longer at the rate of 4.48 kg/ha. L. latifolium can be controlled by glyphosate at 700-1200 ml/decare (97.7% control) However, glyphosate may cause only top-kill leading to plants that resprout and reinfest treated areas. Research currently being conducted suggest that glyphosate applied after mowing may significantly increase control (see mechanical control). Since glyphosate is nonselective, bare ground leads to the rapid reinfestation of L. latifolium. One benefit is that glyphosate (Rodeo) can be used in aquatic sites. However, it is recommended that glyphosate is applied only at low tides and if water is 10.7-15.2 m (35-50 feet) from the application site. Windspeeds should be less than 5 mph to reduce drift and if native plants are present, the use of wick applicators is recommended. present, the use of wick applicators is recommended. References used: 1. Bellue, M.K. 1936. Lepidium latifolium L., a new perennial peppergrass. Calif. Dept. of Agri. Bull. 25:359. 2. Corliss, J. 1993. Tall whitetops crowding out the natives. Agric. Res. May:16. 3. Geronimo, J. 1978. Response of several herbaceous weed species to triclopyr and 3,6-dichloropicolinic acid. Abstracts 1978 Meeting of the Weed Science Society of America. p. 43. 4. Kloot, P.M. 1973. Perennial peppercress...a warning. J. Agric. S. Australia. 76:72-73. 5. Miller, G.K., J.A. Young and R.A. Evans. 1986. Germination of seeds of perennial pepperweed (Lepidium latifolium). Weed Sci. 34:252-255. 6. Miller, R.F., T.J. Svejcar, J.A. Rose and M.L. McInnis. 1994. Plant development, water relations, and carbon allocation of heart-podded hoarycress. Agronomy J. 86:487-491. 7. Renz, Mark J. 1999. Personal communication. 8. Robbins, W.W., M.K. Bellue and W.S. Ball. 1951. Weeds of California. Calif. Dept. Agr., Sacramento, California. 9. Sebastian, J.S., K.G. Beck and D.E. Hanson. 1989. Perennial pepperweed control with metsulfuron, chlorsulfuron, and 2,4-D on Colorado rangeland. Res. Prog. Rep. Western Soc. Weed Sci., pp. 7-8. 10. Sexsmith, J.J. 1964. Morphological and herbicide susceptibility differences among strains of hoary cress. Weeds 12:19-22. 11. Trumbo, J. 1994. Perennial pepperweed: a threat to wildland areas. CalEPPC Newsletter 2:4-5. 12. www.extension.unr.edu/tallwhtop.htm 13. Young, J.A., C.E. Turner and L.F. James. 1995. Perennial pepperweed. Rangelands 17:121-123. 14. Young, J.A. and C. Turner. 1995. Lepidium latifolium L. in California. CalEPPC Newsletter 2:4-5. .