Spartina, commonly called cordgrass, is a perennial grass which typically grows on the upper edges of tidal mudflats and in salt marshes. One species of cordgrass, S. foliosa, is native to the west coast of the United States. It is found from Baja California, Mexico north to Bodega Bay. Historically, extensive native Spartina salt marshes existed throughout San Francisco Bay and its estuaries.
Over the years, four species of non-native, invasive Spartina have been introduced into San Francisco Bay; S. alterniflora, S. anglica, S. patens, and S. densiflora. Three of these occur in Marin. Two species, S. alterniflora and S. densiflora, have successfully hybridized with the native Spartina to create extremely vigorous hybrids.
The areas in Marin with the largest populations of invasive Spartina are:
- The Corte Madera Creek Complex, including Creekside Park, Piper Park, Corte Madera Ecological Reserve and areas surrounding the Larkspur Landing Ferry Terminal. S. densiflora is the species found most abundantly in Corte Madera Creek; however two other species do occur in Creekside Park.
- Blackie’s Creek in Tiburon. The channel and marsh at the mouth of Blackie’s is mainly infested with hybrid S. alterniflora, although S. densiflora occurs there as well.
- Several infestations of hybrid S. alterniflora in the San Rafael Canal area including Tiscornia Marsh, Loch Lomond Marina, the cove by Marin Yacht Club, and the cove off Beach Drive
Invasive cordgrasses are a threat to the San Francisco Bay estuary because they can clog flood channels, displace native vegetation, ruin tidal marsh restoration efforts, significantly raise mudflat elevation, and reduce habitat for fish, shellfish, shorebirds and migratory waterfowl. The habitat of two Federally-listed Endangered Species, the California clapper rail and salt marsh harvest mouse, have been heavily impacted by hybrid Spartina.
In 2005 imazapyr was approved in California for use in estuaries for controlling noxious weeds like Spartina. Imazapyr is a systemic broad-spectrum herbicide that is applied to the foliage. It is then absorbed and translocated throughout the plant and down into the belowground roots and rhizomes. Because three of the four Spartina species spread rapidly via rhizomes, the translocation of the herbicide into the rhizomes effectively prevents further spreading of the clone once the aboveground portion of the plant has died.
Because the application of herbicide is highly effective with very low environmental impact compared to non-chemical control methods, it is the preferred control option on about 95% of the Spartina treatment sites. S. densiflora is a bunch grass that can be successfully dug when the environmental impacts of manual removal present the lowest impacts to the marsh ecosystem. Visit Use of Imazapyr Herbicide to Control Invasive Cordgrass (Spartina spp.) in the San Francisco Estuary for more information on the use of Imazapyr.