Spartina Invasion and Management

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Marsh masters equipped with rototillers were used to treat large continuous stands of Spartina during the restoration process.



Invasive dense-flowered cordgrass (Spartina densiflora) has infested an estimated 90% of salt marshes in Humboldt Bay and the adjacent Eel and Mad River estuaries. Cordgrass is most abundant at low to mid-marsh elevations, where it has displaced native pickleweed.  It has also been documented as spreading in the high elevation marsh, where it threatens a diverse native plant community that includes the salt marsh plants Humboldt Bay Owl’s Clover and Point Reyes Bird’s Beak, both ranked as rare, threatened, or endangered by the California Native Plant Society. Although it was previously thought that Spartina densiflora invaded only existing marsh vegetation, it is now also colonizing intertidal mudlfats, where it displaces important feeding habitat for migratory shorebirds. 


Invasive Spartina around the World

Dense-flowered cordgrass is one of 17 species of cordgrass, several of which are highly aggressive invaders that significantly alter both the physical structure and biological composition of our tidal marshes, mudflats and creeks. Over the last two hundred years, non-indigenous cordgrasses (Spartina spp.) have invaded marshes around the world, including marshes from California to British Columbia, Australia, New Zealand, China, Morocco, and Europe (UK, the Netherlands, France, Denmark, Germany, and Spain).  In many cases, these invasions have dramatically changed tidal marsh characteristics. 

Invasive Spartina Impacts Worldwide

•Hybridizes with native cordgrasses where present, such as in San Francisco Bay and in England. This can result in “genetic swamping”, where the genes of the native species are overwhelmed.
•Outcompetes indigenous plant species, including rare and endangered plant species, reducing marsh biodiversity and ecological functions. In China, invasive Spartina marshes threaten to displace coastal mangrove forests.
•Converts mudflats to vegetated marsh, eliminating important foraging habitat for shorebirds, including endangered species.
•Produces large amounts of persistent detritus. Large rafts of Spartina detritus (wrack) are deposited on the upper marsh, producing bare areas (often in native marsh) that are then colonized by Spartina.
•Changes habitat for marsh animals (vertebrates such as birds and rodents, and invertebrates such as crustaceans and gastropods) by increasing stem and root density.
•Threatens estuarine restoration projects by increasing the difficulty of establishing indigenous saltmarsh vegetation.

Other Large Scale Control Efforts on the U.S. West Coast

Large-scale efforts to control invasive Spartina are in their final stages in San Francisco Bay, California, and Willapa Bay in Washington. These efforts have primarily targeted smooth cordgrass (Spartina alterniflora), a much larger species than S. densiflora that colonizes intertidal mudflats. However, in S.F. Bay hybrids between smooth cordgrass and the native Spartina foliosa (not present in Humboldt Bay) have complicated matters due to the difficulty of detecting all hybrids. S. densiflora is also present at San Francisco Bay and has hybridized with native S. foliosa as well. However, the area infested with S.densiflora there is much smaller than at Humboldt Bay. Managers at these other estuaries have employed a mixture of mechanical and chemical control methods, including mowing, covering, and spraying with herbicides.  Biological control using a planthopper (Prokeleisa marginata) has been used in Washington on Spartina alterniflora

The Humboldt County Spartina Invasion

Dense-flowered cordgrass, the invasive species of Spartina currently found in Humboldt County, is native to coastal Argentina and Brazil on the Atlantic coast of South America. The species had spread to Chile by the late 1800s, and it is believed to have arrived in Humboldt Bay around 1870 with ballast on ships carrying lumber to Chile. The species was not recognized as a non-indigenous species in California until the 1980s (Spicher and Josselyn 1985). There are few data on the spread of the species in the region prior to the 1990s. Monitoring at one site in the Mad River Slough from 1989 to 1997 tracked the species’ spread into the high marsh. Aerial photos from the 1970s and 1980s indicate that cordgrass did not yet dominate the marshes on Indian Island. The large expanse of Spartina at the mouth of Jacoby Creek established after 1970, colonizing newly deposited sediment from the creek. Ground photos document the continued spread of the species after 1990 in many sites around Humboldt Bay.
Distribution of Spartina in Humboldt Bay, Eel River, and Mad River estuaries
The first comprehensive inventory of Spartina densiflora in Humboldt Bay was conducted in 1998, when staff at Humboldt Bay National Wildlife Refuge (HBNWR) mapped stands of Spartina in two abundance classes over aerial photos in the field. This detailed mapping project (Pickart 2001; 5.5MB pdf) revealed that almost 20 years ago, Spartina was present in a total of 812 acres, or 94% of Humboldt Bay’s salt marsh. It occurred at cover values of over 70% of the marsh in 479 acres, or 55% of our salt marshes. The Mad River Slough at the north end of Humboldt Bay had the greatest proportion of salt marsh with respect to bay acres, the least severe Spartina invasion (based on both the percent of total salt marsh invaded and the proportion of salt marsh in low vs. high cover classes), and the highest densities of rare salt marsh plants. 
Between 2008 and 2010 refuge staff completed an updated inventory of all Spartina within its boundaries (Grazul and Rowland 2010; 3.26MB pdf). Spartina was present in 97% of unrestored salt marsh, but only 22% of brackish marsh. Much of the brackish marsh on the refuge is behind dikes, which limits tidal dispersal of seed to overtopping tides or failing tidegates. Mapping was expanded to all three estuaries (Humboldt Bay, Mad River, Eel River) in 2011 using more generalized cover classes and relying more on photointerpretation (Grazul and Rowland 2011). The total infestation was 1,671 acres including 35 acres of restored marsh with less than 1% cover remaining, and 185 ac of marsh currently undergoing restoration). Approximately 60% of the infestation is in Humboldt Bay and 40% in the Eel River estuary. The Mad River estuary doesn’t have the extensive tidal lands that characterize Eel River and Humboldt Bay, and currently has less than 8 acres. Based on these mapping efforts, the total area infested by Spartina densiflora in Humboldt Bay increased from 820 acres in 1999 to 1,033 acres in 2011 (not taking into account the restored areas) an increase of 26%. Clearly the invasion is still in progress.
Spartina seedbank dynamics in Humboldt Bay
Three years of research on seedbank dynamics have been completed by staff at Humboldt Bay NWR. Results to date demonstrate that Spartina densiflora has a large seedbank that persists up to 3 years, with substantial reduction in the size of the seedbank by the third year. The seedbank is much greater in areas of dense Spartina cover (lower elevation marshes) Sampling of the low marsh yielded a first year bank of viable seeds equal to over 6 million seeds/acre (Pickart 2012).  
Spartina alterniflora in Humboldt Bay
Although often overlooked in Humboldt Bay’s recent history, a Spartina alterniflora invasion was successfully thwarted in 1985, preventing potentially massive impacts to the mudflat habitats of the Bay.  After observing an increase in the size of the stand from 100 to almost 5,000 sq ft over a 3-year period, the California DFG eradicated the stand by diking the area, cutting the grass, applying salt and covering it with black plastic. 
This timely intervention prevented the massive invasions that have occurred in San Francisco and Willapa Bays. Unfortunately, a similar early detection and response strategy was not possible with the long-established Spartina densiflora. However, its continued spread in Humboldt Bay, and its availability as a source to other west coast estuaries, justifies a timely response to this invasion.

Humboldt Bay’s Salt Marshes

Salt marsh occupied approximately 9,000 acres around Humboldt Bay prior to Euro-American contact (Pickart 2001). Subsequent diking, draining, and conversion have left us with less than 900 acres, or 10% of the historic acreage. Similar losses to salt marshes have occurred in all estuaries in California. The rarity of this vegetation type amplifies the impacts of an invasion occurring over 90% of its total extent.
The species composition of salt marshes is strongly controlled by the influence of tides. Salt marsh vegetation can only become established at or above the Mean Lower High Water level (the average of the lower of two daily high tides). Low marsh is overwhelmingly influenced by the duration of salt water inundation. Both salinity and lack of oxygen during submersion limit plant life to only the most stress tolerant of species. In Humboldt Bay, salt marshes were classified by Eicher (1987; 37MB pdf): into three types based on their elevation, with corresponding differences in vegetation. The lowest elevation marshes, are dominated by pickleweed (Salicornia pacifica). At these elevations, pickleweed forms homogeneous stands known as Pickleweed Marsh. In our native salt marshes, pickleweed, a hardy colonizer, is the first plant to establish in a newly forming marsh. 
Although Eicher's 1987 study characterized middle elevation marsh as Spartina Marsh, Spartina has since been documented as spreading into our lowest elevation marshes and displacing Pickleweed Marsh. 
Our high elevation marshes (>7.3 ft. MLLW) are known as Mixed Marsh and contain the greatest diversity of species (over 20 species). These include pickleweed, salt grass (Distichlis spicata), jaumea (Jaumea carnosa), marsh rosemary (Limonium californica), and arrow-grass (Triglochin maritima), as well as two rare salt marsh annuals, Humboldt Bay owl’s clover (Castilleja ambigua ssp. humboldtiensis) and Pt. Reyes bird’s beak (Chloropyron maritimum ssp. palustre). Although Spartina is less vigorous on these high elevation marshes, it was found to be increasing dramatically on one high marsh (from <5% to >40% frequency  from 1989 to 1997 (Pickart 2001). In Humboldt Bay, saltmarshes occur both as islands and adjacent to the mainland. Island marshes have no freshwater input other than rainfall, so salinity is high and limiting. Mainland marshes often receive freshwater runoff that can dilute salinities, resulting jn brackish marshes at the upper edge of salt marshes. Brackish marshes around our bay are characterized by a diverse group of species including Lyngbye’s sedge (Carex lyngbyei), hardstem bulrush (Schoenoplectus acutus), salt rush (Juncus lesueurii), sea watch angelica (Angelica lucida), narrow-leaved bur-reed (Sparganium angustifolium) and others. Spartinadensiflora appears to flourish in our brackish marshes, as it does in its native range and in other countries where it has invaded.

Invasive Spartina Impacts in Humboldt County 



Outcompetes indigenous plant species   
Cordgrass displaces pickleweed (Salicornia pacifica) in low and mid-elevation marsh. In restored marsh where S. densiflora has been removed, pickleweed colonizes rapidly and creates a closed canopy in a few years, with salt grass (Distichlis spicata), increasing at a slower rate and in locally higher topography. Where freshwater influences are present through streams or seeps entering the marsh, Spartina outcompetes seaside arrow-grass, sea milkwort (Glaux maritima), western grasswort (Lilaeopsis occidentalis) and other native brackish species. In the high marsh, Spartina threatens a diverse plant community that includes the rare Humboldt Bay owl’s clover and Point Reyes bird’s beak. It is unknown what the ultimate extent of the cordgrass invasion could be if it is not controlled. In Spain, which has also undergone a major invasion by dense-flowered cordgrass, the species has spread beyond intertidal environments to occupy extensive areas of terrestrial habitats (Bortolus 2006). In Humboldt County, Spartina densiflora can be found not only in salt marshes, but in brackish marshes, along slightly brackish river channels, on sandy substrates in dune wetlands, on rocky substrates along dikes, and colonizing bare mudflats. 
Alters invertebrate assemblages
Studies show that cordgrass invasion has a dramatic impact on both terrestrial and intertidal invertebrates. Mitchell 2010 (489K pdf), Mitchell 2012 (2016K pdf) compared invaded with restored marsh, measuring both abundance and diversity of above-ground invertebrates. Two years after Spartina removal was initiated, restored marsh had dramatically higher abundance and diversity (as measured at the order level) of invertebrates than the invaded site. Although Spartina plants are taller and offer more refuge from high tides, they are structurally less complex than native vegetation and reduce the opportunity for diversity of invertebrate consumer strategies. The invaded marsh favored mosquitoes, a trend also observed at invaded salt marshes in Spain. Recent research (Mitchell 2012) measured a trophic shift in terrestrial salt marsh invertebrates, resulting in increased cellulose consumers in invaded marshes. The presence of emergent vegetation at high tides shifted arachnid predation from active to ambush tactics. S. densiflora encouraged dense populations of the invasive snail Myosotella mytosis, with greater proportion of the native, threatened snail Littorina subrotundata found in restored marshes. Higher abundance of the Talitrid amphipod, Orchestia, an important prey for higher organisms was found in restored marshes.
Alters estuarine productivity 
Recent research indicates that Spartina densiflora has a negative impact on estuarine productivity (Lagarde 2012; 700K pdf). S. densiflora dominated marshes showed decreases in net ecosystem productivity compared with native marshes during the spring, with no effect during summer and fall. These results suggest that the turnover or herbivory rates of the native autotrophs (vascular plants and algae) are higher than those of S. densiflora. Monitoring of past restoration has shown that nonvascular plant diversity and abundance increases after removal of S. densiflora (Augyte and Pickart 2013).
Threatens estuarine restoration projects
Cordgrass colonizes areas opened to tidal influence, making it much more difficult for restoration projects in the region to establish indigenous salt marsh vegetation. Cordgrass is usually the first species to colonize in bare areas created when new tidal flow kills off existing fresh or brackish vegetation. Many people in the region question the value of tidal marsh restoration projects because of the threat of cordgrass colonization. 
Source for cordgrass invasions of other areas
In addition to its impacts locally, cordgrass in the Humboldt Bay region threatens to colonize other west coast estuaries via ocean dispersal of its seeds, as demonstrated by the preliminary results of a drift card (88K pdf) study carried out by Portland State University. Drift cards from Humboldt Bay in 2004 and 2005 were found within a month of their release in numerous locations along the Oregon Coast, as well as in southwest Washington. 
Spartina densiflora control and salt marsh restoration in Humboldt Bay
Spartina control efforts in Humboldt Bay began with an experiment carried out from 2004-2005 at Humboldt Bay National Wildlife Refuge and funded by the State Coastal Conservancy (Pickart 2005; 3MB pdf). Spartina on a mid- to high-elevation marsh in the Mad River Slough was mowed repeatedly in square-meter plots using a weed-whip. After two years, all Spartina plants were killed (compared with minor Spartina cover reduction in control plots), and native plants had recolonized in many areas. No Spartina recruited from seed during this time. 
Following this success, a 25-acre mainland salt marsh was restored, with combined funding from the Conservancy and the FWS, beginning in Fall 2006. Metal-bladed brushcutters were substituted for weedwhips, and a methodology evolved that eventually consisted of cutting above ground biomass into small enough pieces to avoid forming dense floating wrack and “grinding” the shallow rhizomes. Depending on how deeply the treatment is applied, mortality can be achieved in a few visits. However, in the first year following treatment, seedling flushes may occurr on resulting bare areas on the mainland marsh where freshwater input is abundant, necessitating follow-up treatments using flaming or brushcutting. Seedlings can emerge both from the persistent seedbank or by seeds newly dispersed from nearby areas. The latter possibility emphasizes the need for a regional approach to eradication, in which seed production is entirely halted. Research has confirmed that the seedbank persists for at least 3 years. 
The pilot restoration begun in 2006 resulted in a highly functioning salt marsh with increased diversity of algae, vascular plants, and invertebrates. Revegetation was explored, but determined to be unnecessary. Native marsh vegetation achieved full recovery, although removal of newly established seedlings will be required until regional eradication cuts off seeds sources (Pickart 2012; 5.5MB pdf). Restoration resulted in the dramatic recovery of two rare salt marsh plants, Humboldt Bay owl's clover (Castilleja ambigua ssp. humboldtiensis) and Point Reyes' birds-beak (Chloropyron maritimum ssp. palustre) (Pickart 2012, Eicher and Pickart 2012). Both are hemiparasites, and grow on high marsh and/or at the brackish interface of salt marsh and dunes. The brackish marshes had been highly invaded by Spartina, which is much taller and blocks more light thean the native hosts of these rare species. Following Spartina removal, the population of Humboldt Bay owl's clover increased from a mean of 1,657 (1989-2001) to approximately 100,000 (2007-2012).
Following the success of the pilot projects, in 2010 HBNWR was awarded a $1 million competitive allocation to carry out removal of all existing Spartina within refuge boundaries. The refuge developed partnerships with the Humboldt Bay Harbor, Recreation, and Conservation District, the California Conservation Corps, CalFire, the Northcoast Resource Center, Friends of the Dunes, and others to carry out this challenge. Beginning in July 2010, crews were deployed throughout the refuge over a total of more than 300 acres of salt and brackish marsh. In addition to the baseline mapping discussed above, permanent monitoring plots were established throughout the site to measure progress. Work is concentrated in summer months due to favorable tides and weather, but continues through the winter. As of January 2013, approximately 75% of the Spartina has been treated on the refuge. The primary control method is the use of brushcutters, but manual removal (especially on rocky substrates like dikes) and heavy equipment have also been employed. The latter has been used to remove dense stands on the refuge that can be reached from the dike or are otherwise accessible. 
Of the remaining Spartina, a large portion is on the Eureka Slough Unit, a highly invaded medium elevation marsh. Given the labor-intensiveness of this site, for mechanical removal, experimental mowing followed by rototilling using an amphibious tractor (Marshmaster) was carried out. This method was very efficient and is planned for the remainder of the Eureka Slough in spring 2013.

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Picture: An equipment operator maneuvers a Marshmaster to cut S. densiflora at ground level on the salt marsh.

Spartina densiflora eradication and climate change

Given the expected sea level rise associated with climate change, the investment in Spartina removal and salt marsh restoration has been questioned by some. Two issues related to Spartina and sea level rise are addressed below.
Q: Couldn't Spartina help us adapt to climate change by trapping sediment and raising the elevation of the marsh?
A: While Spartina densiflora does trap more sediment than many native marsh species, it cannot trap sediment rapidly enough to significantly improve the ability of our marshes to keep up with sea level rise. Spartina alterniflora, a close relative of S. densiflora, is much more effective at trapping sediment and raising mudflat or marsh elevations, and has been introduced in China and other locations to reclaim land and protect against flooding (Wan et al. 2009). Other approaches to climate change adaptation are available, such as restoring marshes on the bay side of levees and other infrastructure, and these approaches can protect against sea level rise without sacrificing biodiversity.
Q: Why should we restore salt marshes just to watch them drown a few decades later?
A: With proper planning and management (e.g.beneficial reuse of sediment from dredging), it may be possible to prevent the loss of marshes to sea level rise. Even in locations where sea level rise will convert existing marshes to mudflats, significant parts of Humboldt Bay provide opportunities for salt marshes to migrate inland and persist in new locations. Species that can colonize disturbed areas rapidly will be the first to dominate new areas as sea level rises. If Spartina is allowed to remain in our region, it will therefore become even more dominant as marshes migrate. As it becomes more dominant, its negative ecological impacts will increase. Furthermore, sea level rise and climate change will result in many unexpected impacts to marshes, and a diverse marsh community will be more resilient to these changes. A Spartina monoculture will have less of the morphological and genetic diversity that would enable it to adapt to changing conditions. Spartina eradication is therefore critical to achieve before sea level rise accelerates.
Next Up: Regional Eradication
Complete eradication of Spartina from the West Coast of California, Oregon, Washington, and British Columbia has been set as a goal by the West Coast Governors' Agreement and the Pacific Coast Collaborative. Both scientists and politicians have recognized that without a regional approach to eradication, Spartina will continue to re-infest and spread along the West Coast, with colossal ecological and economic implications. In our area, the State Coastal Conservancy is spearheading regional eradication in our three estuaries. A Regional Plan and Programmatic Environmental Impact Report (PEIR) were recently completed and are available for review on the California State Coastal Conservancy website. The Plan and PEIR, funded by the Conservancy and Pacific States Marine Fisheries Commission, cover the Mad River estuary, Humboldt Bay, and the Eel River delta. The goals of the plan are to guide tidal marsh restoration through Spartina eradication and to prevent future infestations. The Plan describes the regional coordination, costs, outreach requirements, and permitting strategy that will be needed to achieve these goals. Potential Spartina treatment methods, including mechanical (e.g. brush cutters) and chemical (herbicide) methods, are described. The Plan's PEIR evaluates the impacts of these eradication methods, and identifies mitigation measures. Individual eradication projects are expected to "tieir off" this programmatic impacts evaluation, but may need additional California Environmental Quality Act documentation than what is provided in the PEIR. Implemetation of the Plan is expected to begin in 2013, after permitting and fundraising are completed. For more information or to be included on their mailing list, contact Joel Gerwein at (510) 286-4170 or email
An educational brochure has been developed to summarize the impacts of Spartina and issues surrounding restoration of salt marshes. Volunteer events have been held annually, including the Spartina ShinDIG in 2009 and “People for Pickleweed” events in 2010, 2011and 2012. These workdays have been sponsored by HBNWR, Friends of the Dunes, and Friends of HBNWR. Volunteers are a great source of labor in areas that are best treated using shovels and manual labor, check the Announcements board for upcoming events and presentations.

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Picture: Local volunteers dig Spartina to aide with saltmarsh restoration. In addition they learn the importance of habitat improvement projects and the hard work it takes to restore native habitat.
More technical outreach has consisted of annual to biennial symposia, beginning with the 2008 Summit and continuing with the 2010 and 2011 Symposia. Technical reports have also been completed and can be accesses through the References section below.


 Augyte, S. and A. Pickart. 2013. (900K pdf) Algal response to removal of invasive cordgrass Spartina densiflora in a saltmarsh in Humboldt Bay, CA. Unpublished report, U.S. Fish and Wildlife Service, Humboldt Bay National Wildlife Refuge, Arcata, California.
Bortolus, A. 2006. The austral cordgrass Spartina densifloraBrong.: its taxonomy, biogeography and natural history. 
Eicher, A.L. 1987. (3MB pdf) Salt marsh vascular plant distribution in relation to tidal elevation, Humboldt Bay, California. M.A. Thesis, Humboldt State University, Arcata, California.
Eicher, A.L. and A.J. Pickart 2011. (1538K pdf) Impacts of mechanical Spartina treatments on rare plants in Humboldt Bay salt marshes. Unpublished report, U.S. 
Fish and Wildlfie Service, Arcata, California and California State Coastal Conservancy, Oakland, California
Falenski, H.D. 2007. (1.4MB pdf) Spartina densiflora, an invasive species in the marshes of Humboldt Bay. M.S. Thesis, Humboldt State University, Arcata, California. 
Grazul, Z.I. and P.D. Rowland. 2010. (3.3MB pdf) The distribution of Spartina densiflora in Humboldt Bay National Wildlife Refuge: Baseline mapping, 2010. Unpublished report, U.S. Fish and Wildlife Service, Humboldt Bay National Wildlife Refuge, Arcata, California.
Grazul, Z.I. and P.D. Rowland. 2011. (4057K pdf) The distribution of Spartina densiflora in the Humboldt Bay region: Baseline mapping. Unpublished report, U.S. Fish & Wildlife Service, Humboldt Bay National Wildlife Refuge, Arcata, California
Kittelson, P.M., and M. Boyd. 1997. Mechanisms of expansion for an introduced species of cordgrass, Spartina densiflora, in Humboldt Bay, California. Estuaries 20:770-778. 
Mitchell , M.L. 2010. (489K pdf) A description of terrestrial invertebrate assemblages and a comparison of sampling methods in pre and post-restoration Humboldt Bay salt marsh: A pilot study. Unpublished report, U.S. Fish and Wildlife Service, Humboldt Bay National Wildlife Refuge, Arcata, California.
Mitchell, M.L. 2012. (2016K pdf) A Comparison of Terrestrial Invertebrate Communitites in Spartina-Invaded and Restored Humboldt Bay Salt Marshes. M.S. Thesis, Humboldt State University, Arcata, California. 
Patten, K. and Casey, C.O. (3MB pdf) Shorebird use of Spartina-affected tidelands- Can we achieve functional habitat post-control?  Washington State University 
Pickart, A. 2001. (5.5MB pdf) The distribution of Spartina densiflora and two rare salt marsh plants in Humboldt Bay 1998-1999. Unpublished report, U.S. Fish and Wildlife Service, Humboldt Bay National Wildlife Refuge, Arcata, California.
Pickart, A. 2005. (3.0MB pdf) Control of invasive Spartina densiflora in a high elevation salt marsh, Mad River Slough, Humboldt Bay National Wildlife Refuge. Unpublished report, U.S. Fish and Wildlife Service, Arcata, California. 
Pickart, A. 2008. (1.2MB pdf) Spartina eradication, Humboldt Bay National Wildlife Refuge. Status Report to the State Coastal Conservancy. U.S. Fish and Wildlife Service Grant Agreement No. 06-017. 11 pp.
Pickart, A. 2012. Spartina densiflora invasion ecology and the restoration of native salt marshes, Huumboldt Bay, California. unpublished report, U.S. Fish & Wildlife Service, Arcata, California
Spicher, D., and M. Josselyn. 1985. Spartina (Gramineae) in Northern California: distribution and taxonomic notes. Madroño 32:158-167. 
Wan, S., P. Quin, J. Liu, and H. Zhou. 2009. The positive and negative effects of exotic Spartina