How to Manage Pests
Pests in Gardens and Landscapes
Perennial pepperweed (Lepidium latifolium), an introduced plant from southeastern Europe and Asia, is invasive throughout the western United States. It can establish in a wide range of environments and is a common problem in flood plains, irrigation structures, pasture, wetlands, riparian areas, roadsides, and residential sites. Recent surveys identify perennial pepperweed as a weed problem in nearly all of California, and both the California Department of Food and Agriculture (CDFA) and California Invasive Plant Council (Cal-IPC) list it as a noxious weed of greatest ecological concern. Populations form dense monocultures that are easily spread by root fragments and seed. Perennial pepperweed has many common names including tall whitetop, perennial peppercress, ironweed, perennial peppergrass, and broad-leaved pepperweed.
Perennial pepperweed is a member of the Brassicaceae (mustard) family. Stems range from 2 feet to over 4 feet tall. Mature plants have numerous erect, semi-woody stems that originate from large, interconnected roots. Roots are long, minimally branched, and enlarged at the soil surface forming a semi-woody crown. The foliage is glabrous and green to gray-green in color. Rosette leaves are ovate to oblong with entire to serrate margins on long petioles. Rosette leaves are about 4 to 11 inches long and 1 to 3 inches wide. Stem leaves are sessile and lanceolate, have entire to toothed margins, and become smaller toward the top of the stem. Small, white flowers form dense clusters arranged in panicles at the tip of each stem. Perennial pepperweed is often confused with hoary cress (Cardaria draba); also called whitetop. However, unlike the taller perennial pepperweed, hoary cress stems are less than 3 feet tall and have leaves that clasp the stem and lack an obvious petiole.
Perennial pepperweed is a long-lived herbaceous perennial that thrives in seasonally wet areas or areas with a high water table. Perennial pepperweed is typically found invading fine-textured, saline/sodic soils, although populations can establish and persist on coarse-textured, alluvial soils. Plants reproduce from perennial roots or seed. In early spring, new shoots emerge from root buds forming low-growing rosettes (basal leaves with no obvious stem). Plants remain in the rosette stage for several weeks before developing a flowering stem. Flowering typically begins in late spring with mature seed being produced by mid-summer. After seed production, flowering shoots die back, although rosettes can emerge again in fall and persist through winter in frost-free areas. Dead stems are slow to decay and accumulate over time, forming dense thickets that prevent growth of desirable species. Perennial pepperweed is a prolific seed producer. Laboratory tests suggest seeds germinate readily with fluctuating temperatures and adequate moisture; however, seeds do not appear to remain viable in the soil for extended periods.
Established perennial pepperweed plants develop an extensive root system capable of storing large amounts of energy. Roots can grow to soil depths greater than 10 feet and spread laterally several feet each year. Root segments also produce adventitious buds capable of generating new shoots. Perennial pepperweed's root system is the foundation of the plant's competitiveness and the major target of control efforts.
Perennial pepperweed can rapidly form large, dense stands that displace desirable vegetation. Populations easily spread along waterways and can infest entire stream corridors, riparian areas, or irrigation structures. Flooded streams often wash away roots growing along the steambank, and new infestations develop downstream. Once established, perennial pepperweed is persistent and difficult to control in crops, natural areas, and ornamental plantings. Perennial pepperweed also reduces forage quality in hay or pasture.
MANAGEMENT IN ORNAMENTAL PLANTINGS AND HOME LANDSCAPES
Prevention is the best management strategy for avoiding problems with perennial pepperweed in and around home landscapes. If perennial pepperweed is found growing in landscaped areas, immediately control the plant before it can spread. Pulling plants (try to remove as much of the root as possible) is an effective way of controlling a few scattered plants growing within landscaped areas. Use of glyphosate (Roundup and other products) is another option but care must be taken to avoid injuring nearby plants. Large populations of perennial pepperweed are unusual in landscaped areas, unless new housing or commercial developments have been built on infested land.
To control perennial pepperweed infestations before landscaping newly developed areas, start by applying glyphosate to the entire infestation and wait a couple of weeks to allow the herbicide to translocate to the roots. After the initial herbicide treatment, carry out any necessary soil preparation activities and wait an additional 2 to 6 months before seeding or transplanting desired vegetation to allow any remaining pepperweed to resprout so it can be treated again. Once perennial pepperweed ceases to resprout, apply landscape fabric or mulch and transplant closely spaced herbaceous plants to prevent reintroduction. If resprouts grow through or around mulch barriers, hand pull or spot treat them with glyphosate. Turf is also an excellent ground cover for preventing reintroduction of perennial pepperweed. The frequent mowing schedule and common herbicides (2,4-D) used for turf management are not conducive to perennial pepperweed establishment.
MANAGEMENT IN PASTURES, RANGELAND, RIGHTS-OF-WAY, AND CROPS
Established perennial pepperweed populations are difficult to control and require multiple years of intensive management. Suppressing the extensive root system is critical for successful control. A management program should include prevention, monitoring, and treatment of small satellite populations before plants develop extensive roots. If large populations exist, focus management on containing the infestation and preventing further spread to surrounding areas.
Prevention is the foundation of any weed management program. Techniques that prevent perennial pepperweed establishment save time and resources in the future. Perennial pepperweed root fragments or seeds have been found in straw, hay bales, mulch, and crop seed, so ensure that these items are free of weed seed and propagules before applying them to an area. Periodic surveys of property lines, roadsides, waterways, and riparian corridors help detect new infestations before they become well established. If construction or soil disturbance occur in infested areas, make sure root fragments and seed are not transported to other sites. Always clean vehicles, machinery, and clothing after visiting infested areas. If livestock graze perennial pepperweed, hold animals in closely monitored paddocks for several days to allow seed to past through their digestive system before transporting the livestock to new areas.
Establishing and maintaining competitive perennial vegetation can dramatically slow the introduction and spread of perennial pepperweed. Vigorous sod-forming grasses, alfalfa, or cropping systems with annual tillage help prevent perennial pepperweed introduction and establishment in agricultural areas. Closely spaced plantings of herbaceous perennials, shade trees, and/or fabric or plastic mulches can help prevent introduction in ornamentals.
Hand pulling and tillage
Seedlings are easily controlled by hand-pulling or tillage, but these techniques do not control established plants because shoots quickly resprout from vast root reserves. Root segments as small as 1 inch are capable of producing new shoots.
Mowing and burning
Mowing and burning are not effective at reducing perennial pepperweed stands, but they are helpful at removing accumulated thatch. Perennial pepperweed thatch burns best in winter or spring under dry conditions before initiation of spring growth. Mowing breaks old stems into small fragments and helps prevent shading of favorable species. Mowing also stimulates perennial pepperweed plants to resprout and produce new growth. Combining mowing with herbicides has been shown to be an effective control strategy. For best results, mow plants at the bolting or flower bud stage and apply herbicides to resprouting shoots once they have reached the flower bud stage.
Establishing desirable vegetation in disturbed areas is crucial to managing perennial pepperweed and preventing future weed problems. Because perennial pepperweed is very competitive, seed or transplant desirable vegetation after dense perennial pepperweed stands are controlled. Choose vigorous, fast-growing plant species that are adapted to the site. Perennial grasses are a good choice for natural areas and pastures. Grasses are tolerant to some selective herbicides used for perennial pepperweed control and over time form thick sod that prevent future weed establishment. In pastures, promote grass expansion and vigor with fertilization and grazing management.
Several postemergent herbicides control perennial pepperweed, but repeat applications are usually necessary for several years to treat resprouting shoots and seedlings. Extended control with herbicides is greatly enhanced by establishing competitive vegetation at the site. In areas with a dense buildup of thatch, mow or burn old shoots before applying herbicides.
Herbicide application timing is critical. Herbicides work best when applied at the flower bud stage and worst at the rosette or early bolting stage. Because plant phenology differs between location and year, regularly observe infested areas in spring and begin applying herbicides when flower buds appear. If herbicide cannot be applied at the flower bud stage, mow plants and apply the herbicide to regrowth. With seedlings, apply herbicides as soon as possible to prevent plants from producing new lateral shoots from the root. Herbicide choice depends on label restrictions, land use objectives, and cost. See Table 1 for a summary of effective herbicide choices.
UC Weed Research and Information Center — http://wric.ucdavis.edu/
Authors: R. G. Wilson, UC Cooperative Extension, Lassen County; J. M. DiTomaso, Vegetable Crops/Weed Science, UC
Davis; and M. J. Renz, Extension Plant Sciences, New Mexico State University
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