Chemical Control of Weeds and Genetic Off-Types in Smooth Cordgrass (Spartina alterniflora) Production Ponds

Major efforts to reduce rates of land loss are needed to preserve Louisiana's coastal wetlands. Louisiana has 40% of the wetlands found in the lower 48 states (Bourne 2000). However, Louisiana has the highest wetland loss rate in the continental United States; from 1956 to 2006 Louisiana's wetland loss ranged from 1,640 to 10,170 ha per year (Barras et al. 2008). The financial impact of Louisiana's wetland loss is estimated to be $37 billion, which includes impacts to fisheries, wildlife habitat, flood control, and hurricane protection (Bourne 2000).

To reduce coastal erosion, smooth cordgrass (Spartina alterniflora) is commonly used in restoration projects throughout Louisiana. Smooth cordgrass is the most prevalent plant of Louisiana's coastal wetlands (Godfrey and Wooten 1979, Broome et al. 1986). It rapidly produces dense growth along intertidal areas (Bush and Houck 2008), accumulates sediment and organic matter to build land (Redfield 1972, Nepf 1999), and reduces wave energy, scouring, and flooding of inland communities (Redfield 1972, Christiansen et al. 2000). Sediments trapped by smooth cordgrass contain nutrients that benefit plants, fish, and crustaceans that comprise economically valuable fisheries of this region (Broome et al. 1986, Mitsch and Gosselink 2007). Smooth cordgrass also provides food and habitat for economically and ecologically valuable migratory birds traveling along the Mississippi and Central Flyways (Broome et al. 1986).

All smooth cordgrass varieties used for coastal restoration projects in Louisiana are clonal. To provide plant material for restoration projects, smooth cordgrass varieties are clonally propagated from rhizome segments that are attached to stems. Clonal propagation is extremely labor intensive and little science-based information exists on efficient and economical production of clonal smooth cordgrass varieties. Weed management is an essential component of efficient large-scale plant production, particularly in freshwater smooth cordgrass production systems where weed populations can rapidly exceed smooth cordgrass plants. Freshwater weeds, especially cattail (Typha spp.) and ducksalad (Heteranthera limosa), compete for valuable water and nutrient resources, which reduces yields in many commercial crop production systems. Control of smooth cordgrass seedlings, which are a source of genetic off-types, is also essential in clonal production fields because seedlings are a source of genetic contamination that compromises the genetic identity of clonal production fields. The ability to maintain clonal smooth cordgrass production ponds for many years is essential for the economical and efficient production of clonal smooth cordgrass varieties necessary for large-scale restoration projects.

Table 1.

Average herbicide injury for three developmental stages of smooth cordgrass 14 days after treatment (DAT) and 42 DAT in greenhouse studies, Baton Rouge, Louisiana, 2010. Means within and across columns and rows followed by the same letter were not statistically different according to the t-test on difference of least square means at p ≤ 0.05.

The objective of this study was to determine the effects of 10 herbicides on three developmental stages of smooth cordgrass. We harvested seed from 13 smooth cordgrass clones in November 2007 and 2009 from a seed production nursery in Baton Rouge and stored them at 2 ± 1°C in aqueous solution. In September 2009, we produced 13 month smooth cordgrass plants by planting seeds harvested in 2007 into pots (10 cm × 10 cm × 8 cm) containing 1:1 washed sand and pine bark compost media. We placed pots in 5 cm of water in greenhouses maintained at 25 ± 15°C (natural light conditions). To produce 1 and 8 month seedlings, seed harvested in 2009 was planted as described above in September and February 2010, respectively.

We selected 10 herbicides (Table 1) based upon their activity in aquatic environments, presence of a current herbicide label in aquatic environments, and specific activity on grass or broadleaf weeds, to investigate their effect on three developmental stages of smooth cordgrass plants: established plants, large seedlings and small seedlings. The established plants were 13 months old, averaged 31000 mg in weight, were 100 cm in height, and had...