Invasive Plants of California's Wildland

Smooth cordgrass (Spartina alterniflora) is a perennial, spreading grass from one foot tall in spring to six or eight feet tall in fall. It grows naturally only in intertidal estuarine habitats, and is often found in large, nearly monospecific stands in coastal or bayside marshes. Its large, round stems are hollow in cross section. Leaves are hairless, and leaf tips are sharply pointed. Young, healthy green shoots and leaf sheaths are often streaked with red or purple just below the sediment surface. This species is easily confused with the closely related native California cordgrass (S. foliosa), which is usually less than four and a half feet tall in fall and lacks red pigment in green, healthy shoot tissues (it may have red pigment on decaying tissues). Smooth cordgrass is highly variable in size, depending on growing conditions, and may hybridize with native California cordgrass.

In California smooth cordgrass currently is found in Marin County and south San Francisco Bay, where it is rapidly spreading across open intertidal mud flats. Smooth cordgrass also invades established salt marsh communities, where it is likely to be found in the company of pickleweed (Salicornia) and California cordgrass (Spartina foliosa). In Washington smooth cordgrass is rapidly invading Willapa Bay and Puget Sound (Padilla Bay). Smooth cordgrass may be expected in other Pacific Coast estuaries in the future. A small patch was found in Humboldt Bay in the early 1980s, but it has since been eradicated (Daehler and Strong 1996). Smooth cordgrass does not grow on wave-swept Pacific Coast beaches and does not invade freshwater marshes that lack saltwater influence.

Smooth cordgrass is native to the Atlantic and Gulf Coast marshes of North America. It is a dominant component of Atlantic Coast salt marshes, where it forms extensive monospecific stands (Adam 1990). Introductions to San Francisco Bay and Padilla Bay, Washington, were associated with salt marsh restoration and erosion control projects (Spicher and Josselyn 1985, Daehler and Strong 1996 and 1997a). The introduction in Willapa Bay, Washington, appears to have been in association with oyster shipments during the nineteenth century (Sayce 1988).

Vegetative fragments may break off from established plants on eroding banks of tidal sloughs. Dredging an area infested with smooth cordgrass can promote the spread of vegetative fragments. A viable vegetative fragment must contain either root or rhizome material and can be transported with tides. Seeds can float and may also be transported with tides.

Open intertidal mudflats are characteristic of Pacific Coast estuaries and provide important feeding grounds for many shorebird species (Daehler and Strong, 1996). Smooth cordgrass transforms open intertidal habitats into monospecific stands of tall grass, reducing shorebird feeding areas. The dense growth of smooth cordgrass also traps and holds sediments and can clog flood control and navigation channels and alter hydrology. The spread of smooth cordgrass threatens productive oystering grounds in Willapa Bay, Washington. Oystering grounds in other estuaries, such as Tomales Bay, California, would likely also be threatened if smooth cordgrass were introduced there. Smooth cordgrass can invade and replace native California cordgrass stands (Callaway and Josselyn 1992, Baron 1994), and the genetic integrity of native California cordgrass is threatened by hybridization with smooth cordgrass (Daehler and Strong, 1997b).

Smooth cordgrass grows most rapidly from April-September. Winter dieback of large flowering stems begins around October, and large, dead culms are usually washed away by tides in winter. Young, green shoots remain on most plants throughout the winter in California. During the early stages of mudflat invasion, plants grow as isolated, circular patches (clones). Clones spread laterally by vegetative shoots, often three andone-thirdfeet(>1 m) per year (Callaway and Josselyn 1992). Over time, circular patches fuse together, and mudflats are transformed into meadows of smooth cordgrass.
Smooth cordgrass can become reproductive as early as the first year, but more often plants first

(click on photos to view larger image)

flower after two to three years. Flowering occurs in late July-September. Copious inflorescences but few seeds are produced in most plants (Daehler and Strong 1994). Most clones require cross-pollination for good seed set, and cross-pollination (by wind) is rare in isolated patches (Daehler 1996a). A few plants with high self-fertility have consistently high seed output (Daehler 1996a). Ripe seeds fall from inflorescences from October-January. Seeds trapped in mud or wrack germinate in February-May. Most seedlings do not survive the first winter because of winter storms and burial by

Smooth cordgrass can grow on very soft,
sometimes hip-deep mud. Some patches may be inaccessible by foot and must be
approached b

Manual/mechanical: Hand pulling is the simplest option for small propagules (one to a few plants <0.5 m diameter). On soft substrates, plants can be removed by gently pulling rhizomes and roots from the mud. It is necessary to feel around in the mud for any rhizomes that might have broken off and to remove them as well. On harder substrates, a shovel may be needed, and some rhizomes are likely to be missed. All plant material should be carried to an area well above the high tide mark, where it will dry out and die. The site should be marked with stakes and revisited after a few months to be sure the entire propagule was removed.

Solarization: If the infestation is small (one to a few patches 1-10 m diameter), burial of the plants under geotextile fabric or a black plastic tarp can be a means of eradication. Stems can be mowed with a weed whacker or similar device and then covered with 100 percent shadecloth (geotextile fabric) or heavy-duty black plastic. Covering is best begun in spring. The area covered should exceed the plant diameter by at least one meter, and the covering must be well anchored to the mudflat with sandbags or boards and deep stakes. Completely covered patches may die within four months, but mortality is assured only by waiting one year or more before removing the cover.

Mowing: This technique has sometimes been successful in killing smooth cordgrass outside California (Wijte and Gallagher 1992, Aberle 1993), especially if all shoots are mown to mud level in late fall. In areas where plants grow year round, such as California, mowing eight or more times a year may be required to kill smooth cordgrass. Occasional mowing in spring or summer will not kill the plants and may increase shoot density within patches (Daehler, pers. observation).

Biocontrol of smooth cordgrass using host-specific insects might be possible in Willapa Bay, where plants appear to have reduced resistance to herbivory (Daehler and Strong 1997b). However, no control agents have been USDA approved for release in Pacific Coast estuaries. In California biocontrol is probably not feasible because most suitable agents would be likely to attack endemic California cordgrass as well as smooth cordgrass.

Large infestations (many patches > 3 ft or >10 m diameter) are best controlled with herbicide. Glyphosate (as Rodeo®) is currently the only herbicide approved for use in estuarine wetlands, and it must be applied by a licensed pesticide applicator. For hand spraying, applications of 2 to 5 percent glyphosate along with a surfactant are recommended. Surfactants that have been used with some success include X-77 and LI-700. To maximize chances of killing smooth cordgrass, Rodeo® must be applied at low tide, when most plants are exposed to the herbicide for eight or more hours. Rodeo® may be applied by spraying or wicking. Chamberlain (1995) found spring applications to be more effective than fall applications. Multiple applications may be required, especially if some leaves are missed during the first application, if exposure time is less than eight hours, if leaves are muddy, or if plants are growing low in the intertidal zone. To date there have been no successful eradications of large infestations of smooth cordgrass, although attempts are in progress in Willapa Bay and San Francisco Bay (Alameda County).