Use herbicide to control problematic plants: brackish/salt marshes

Overall effectiveness category Trade-off between benefit and harms
Number of studies: 7
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How is the evidence assessed?

Effectiveness

50%
Certainty

50%
Harms

30%

Calculating overall effectiveness category

Background information and definitions

Herbicides can be applied to an entire area of vegetation, or targeted at individual problematic species (e.g. by painting onto individual plants, or shielding non-target vegetation). Herbicides could be applied once, or repeatedly to kill established vegetation or recurrent growth from the seed bank. To maximize contact with target species and minimize non-target effects, herbicides might be applied during/at the start of the dry season, as the tide is going out, or on calm rather than windy days (e.g. Tobias et al. 2016). Often, herbicide application will follow or be followed by other interventions, such as mowing, burning or physical removal of problematic plants.

Caution: In many herbicides, the active chemicals are not specific to the problematic species so can cause collateral damage to desirable species. Relying on herbicides as the only tool to manage problematic plants can lead to the development of herbicide resistance in future generations (Powles et al. 1997). Herbicides can have severe negative side effects on biodiversity, the environment and human health (Pimentel et al. 1992). Accordingly, herbicide use – particularly in or near wetlands or water bodies – is limited in many countries.

Bear in mind that the effects of herbicide might be highly dependent on the chemical used, how it is applied (e.g. season and number of applications), and local site conditions (e.g. nutrient availability, water levels, proximity of untreated invaded vegetation) (Tobias et al. 2016). Also, similarity between treated and untreated, degraded areas might not be an undesirable outcome for this action: similarity in vegetation cover after months or years could suggest, for example, that native vegetation abundance has recovered after being initially depressed by herbicide.

For this action, “vegetation” refers to overall or non-target vegetation. Studies that only report responses of target problematic plants have not been summarized.

Related actions: Use herbicide to maintain or restore disturbance; Introduce organisms to control problematic plants, including introduction of microorganisms as “bioherbicides”.

^{Pimentel D., Acquay H., Biltonen M., Rice P., Silva M., Nelson J., Lipner V., Giordano S., Horowitz A. & D’Amore M. (1992) Environmental and economic costs of pesticide use. BioScience, 42, 750–760.}

^{Powles S.B., Preston C., Bryan I.B. & Jutsum A.R. (1997) Herbicide resistance: impact and management. Advances in Agronomy, 58, 57–93.}

^{Tobias V.D., Block G. & Laca E.A. (2016) Controlling perennial pepperweed (Lepidium latifolium) in a brackish tidal marsh. Wetlands Ecology and Management, 24, 411–418.}

Study locations

Key messages

Seven studies evaluated the effects, on vegetation, of using herbicide to control problematic plants in brackish/salt marshes. Six studies were in the USA. One study was in South Africa. Two studies shared part of the same experimental set-up.

VEGETATION COMMUNITY

Relative abundance (1 study): One site comparison study of brackish marshes in the USA found that a marsh sprayed with herbicide for nine years (and burned for three) and a nearby natural marsh supported a similar relative abundance of the dominant plant species, smooth cordgrass Spartina alterniflora.
Overall richness/diversity (1 study): One site comparison study of brackish marshes in the USA reported that a marsh sprayed with herbicide for nine years (and burned for three) contained more plant species than an unburned and unsprayed marsh – but also more plant species than a nearby natural marsh.
Native/non-target richness/diversity (2 studies): One replicated, randomized, paired, controlled, before-and-after study in a pepperweed-invaded marsh in the USA found that applying herbicide did not increase the richness of non-pepperweed species over two years after intervention. The precise effect depended on the herbicide used. One study of an intertidal area in the USA simply counted the number of native salt marsh plant species that colonized after treating smooth cordgrass Spartina alterniflora stands with herbicide.

VEGETATION ABUNDANCE

Native/non-target abundance (5 studies): Three replicated, randomized, paired, controlled, before-and-after studies in pepperweed-invaded marshes in the USA found that applying herbicide typically did not increase cover of non-pepperweed vegetation, in the two years following intervention. The precise effect depended on the herbicide used. Two studies on the coasts of South Africa and the USA simply quantified the abundance of native salt marsh vegetation that colonized after treating smooth cordgrass Spartina alterniflora stands with herbicide.
Individual species abundance (4 studies): Four studies quantified the effect of this action on the abundance of individual plant species, other than the species being controlled. For example, one site comparison study of brackish marshes in the USA reported that a marsh sprayed with herbicide for nine years (and burned for three) contained more smooth cordgrass Spartina alterniflora than an unburned and unsprayed marsh, and a similar amount of smooth cordgrass to a nearby natural marsh. One replicated, paired, controlled, before-and-after study in a pepperweed-invaded marsh in the USA reported that applying herbicide typically reduced cover of dominant native species over two years. The precise effect depended on the herbicide used.

VEGETATION STRUCTURE

Height (1 study): One site comparison study of brackish marshes in the USA found that in a marsh sprayed with herbicide for nine years (and burned for three), the dominant plant species (smooth cordgrass Spartina alterniflora) grew to a similar height as in a nearby natural marsh.

About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

A site comparison study in 2004 of three brackish marshes in an estuary in New Jersey, USA (Hagan et al. 2007) found that spraying herbicide (along with prescribed burning) converted a marsh dominated by common reed Phragmites australis to one dominated by smooth cordgrass Spartina alterniflora, with similar cordgrass abundance and height to a natural marsh, but more plant species. After nine years of herbicide application and three years of burning, the treated marsh was statistically similar to a nearby natural marsh in terms of cordgrass dominance (treated: 78%; natural: 83% of stems were smooth cordgrass), cordgrass density (treated: 286; natural: 360 stems/m²), above-ground cordgrass biomass (treated: 457; natural: 802 g/m²) and cordgrass height (treated: 78; natural: 94 cm). However, the treated marsh contained six plant species, including common reed, whilst the natural marsh contained only three. A third, untreated marsh was still dominated by common reed (100% of stems; density: 80 stems/m²; biomass: 2,124 g/m²; height: 317 cm; no other plant species). Methods: In August 2004, vegetation was surveyed in three tidal brackish marshes. One marsh was formerly dominated by common reed, but had been sprayed with herbicide (Rodeo®) since 1996 and burned in 1996–1998. The study does not distinguish between the effects of these interventions. The second, natural marsh was dominated by smooth cordgrass. The third marsh was dominated by common reed and had not been treated. In each marsh, vegetation was clipped from six 0.25 x 0.25 m quadrats then identified, measured, dried and weighed.
Study and other actions tested
Referenced paper
Hagan S.M., Brown S.A. & Able K.W. (2007) Production of mummichog (Fundulus heteroclitus): response in marshes treated for common reed (Phragmites australis) removal. Wetlands, 27, 54-67.
A replicated, randomized, paired, controlled, before-and-after study in 2005–2007 in three brackish and salt marshes invaded by pepperweed Lepidium latifolium in California, USA (Boyer & Burdick 2010) found that plots sprayed with glyphosate herbicide typically had similar cover of native plants to unsprayed plots, over the two years following spraying. In most comparisons, total native plant cover was statistically similar in sprayed and unsprayed plots: eight of nine comparisons in year one (for which sprayed: 10–50%; unsprayed: 16–45%) and six of nine comparisons in year two (for which sprayed: 19–113%; unsprayed: 23–49%). Pepperweed cover was typically lower in sprayed than unsprayed plots: in nine of nine comparisons in year one (sprayed: 2–60%; unsprayed: 89–99%) and six of nine comparisons in year two (for which sprayed: 25–78%; unsprayed: 85–100%). Before intervention, plots destined for each treatment had statistically similar cover of native plants (10–33%) and pepperweed (90–100%). For data on the cover of other individual plant species, see original paper. Methods: In April 2005, five sets of 2 x 2 m plots were established in each of three pepperweed-invaded marshes. In each set, there was one sprayed replicate (1.25% glyphosate) and one unsprayed replicate. Treatments were randomly allocated to plots. Vegetation cover was measured before (April 2005) and quarterly for two years after (April 2007) spraying, in 1-m² quadrats. This study shared part of the experimental set-up used in (3).
Study and other actions tested
Referenced paper
Boyer K.E. & Burdick A.P. (2010) Control of Lepidium latifolium (perennial pepperweed) and recovery of native plants in tidal marshes of the San Francisco Estuary. Wetlands Ecology and Management, 18, 731-743.
A replicated, randomized, paired, controlled, before-and-after study in 2005–2007 in three brackish and salt marshes invaded by pepperweed Lepidium latifolium in California, USA (Boyer & Burdick 2010) found that plots sprayed with glyphosate herbicide typically had similar cover of native plants to unsprayed plots, over the two years following spraying. In most comparisons, total native plant cover was statistically similar in sprayed and unsprayed plots: eight of nine comparisons in year one (for which sprayed: 10–50%; unsprayed: 16–45%) and six of nine comparisons in year two (for which sprayed: 19–113%; unsprayed: 23–49%). Pepperweed cover was typically lower in sprayed than unsprayed plots: in nine of nine comparisons in year one (sprayed: 2–60%; unsprayed: 89–99%) and six of nine comparisons in year two (for which sprayed: 25–78%; unsprayed: 85–100%). Before intervention, plots destined for each treatment had statistically similar cover of native plants (10–33%) and pepperweed (90–100%). For data on the cover of other individual plant species, see original paper. Methods: In April 2005, five sets of 2 x 2 m plots were established in each of three pepperweed-invaded marshes. In each set, there was one sprayed replicate (1.25% glyphosate) and one unsprayed replicate. Treatments were randomly allocated to plots. Vegetation cover was measured before (April 2005) and quarterly for two years after (April 2007) spraying, in 1-m² quadrats. This study shared part of the experimental set-up used in (2).
Study and other actions tested
Referenced paper
Boyer K.E. & Burdick A.P. (2010) Control of Lepidium latifolium (perennial pepperweed) and recovery of native plants in tidal marshes of the San Francisco Estuary. Wetlands Ecology and Management, 18, 731-743.
A replicated, randomized, paired, controlled, before-and-after study in 2007–2009 in a brackish marsh invaded by perennial pepperweed Lepidium latifolium in California, USA (Whitcraft & Grewell 2012) found that spraying the vegetation with imazapyr herbicide reduced the richness and cover of non-target vegetation over two years, but that spraying the vegetation with 2,4D had no significant effect on these metrics. After two years, imazapyr-treated plots contained only 0.5 non-pepperweed plant species/0.25 m² (vs 2,4D: 2.0 species/0.25 m²; untreated: 2.1 species/0.25 m²) and only 7% cover of plants other than pepperweed (vs 2,4D: 70%; untreated: 66%). Imazapyr-treated plots had only 1% cover of pepperweed (vs 2,4D: 26%; untreated: 31%), and above-ground pepperweed biomass was only 7 g/m² (vs 2,4D: 29 g/m²; untreated: 40 g/m²). The pattern of results was similar after one year, although not the values of some metrics (e.g. only 3–34% cover of plants other than pepperweed). Before intervention, plots destined for each treatment had similar non-pepperweed richness (2.0–2.6 species/plot), non-pepperweed cover (30–35%), pepperweed cover (27–39%) and pepperweed biomass (87–110 g/m²). Methods: Thirty-six plots were established (in six blocks of six) in a degraded, historically tidal, brackish marsh. In 2007 and 2008, twelve plots (two plots/block) received each of three treatments: spraying with dyed imazapyr (Habitat®), spraying with dyed 2,4D (Weedar®) or no herbicide (spraying with dyed water only). Vegetation was surveyed in April before (2007) and after (2008, 2009) intervention. Plant species and cover were recorded in three 0.25-m² quadrats/plot. Pepperweed was cut from three 0.125-m² quadrats/plot then dried and weighed.
Study and other actions tested
Referenced paper
Whitcraft C.R. & Grewell B.J. (2012) Evaluation of perennial pepperweed (Lepidium latifolium) management in a seasonal wetland in the San Francisco Estuary prior to restoration of tidal hydrology. Wetlands Ecology and Management, 20, 35-45.
A before-and-after study in 2009–2015 in an estuary in South Africa (Riddin et al. 2016) reported that within three years of spraying invasive smooth cordgrass Spartina alterniflora with herbicide, native salt marsh plants had colonized. Herbicide application began in 2011. In October 2014, the first seedlings of native salt marsh plants appeared. In November 2015, 49 of 60 former cordgrass patches contained native salt marsh plants with up to 95% total cover. The total area of smooth cordgrass in the estuary was 10,221 m² in 2011, then only 10 m² in 2015. The above-ground biomass of smooth cordgrass within patches was 933 g/m² in 2009, then only 240 g/m² in 2015. Methods: From 2011, smooth cordgrass in the Great Brak Estuary was sprayed with herbicide. Intense treatments began in January 2013, with 2–3 applications each summer of glyphosate (10 kg/ha) and 0.5% imazapyr (100 g/L). Before 2014, herbicide was broadcast over cordgrass patches. From 2014, herbicide was applied to individual cordgrass plants. Between 2009 and 2015, cordgrass patches were mapped. Vegetation was also surveyed in living (pre-treatment) or dead (post-treatment) cordgrass patches (details not clearly reported for all surveys). Biomass was dried before weighing.
Study and other actions tested
Referenced paper
Riddin T., van W.E. & Adams J. (2016) The rise and fall of an invasive estuarine grass. South African Journal of Botany, 107, 74-79.
A replicated, paired, controlled, before-and-after study in 2007–2009 in a brackish marsh invaded by perennial pepperweed Lepidium latifolium in California, USA (Tobias et al. 2016) reported that spraying the vegetation with herbicide typically reduced native plant cover. Results summarized for this study are not based on assessments of statistical significance. Over two years, cover of the two dominant native species (Pacific pickleweed Sarcocornia pacifica and alkali heath Frankenia salinia) declined in sprayed plots in 8 of 12 cases (from 5–85% before intervention to 0–45% after two years) but was stable or increased in unsprayed plots in six of six cases (before: 12–67%; after: 24–70%). Cover increased in only one of the remaining cases in sprayed plots (from 17% to 45%). The size and direction of the effect of on native cover depended on the species, herbicide composition and location within the marsh (see original paper). The number of pepperweed stems decreased in plots treated with herbicide (from 21–36 stems/m² before intervention to <1 stem/m² after two years) compared to an increase in untreated plots (from 27 stems/m² to 32 stems/m²). Methods: Thirty-six 16-m² plots were established in a pepperweed-invaded brackish marsh. In May 2007 and 2008, twenty-one plots were sprayed with herbicide: 10 with imazapyr (Habitat®) and 11 with mixed imazapyr and glyphosate (Rodeo®). Fifteen plots were not sprayed, but pepperweed flowerheads were removed. In May 2007–2009, vegetation was surveyed in the central 1 m² of each plot.
Study and other actions tested
Referenced paper
Tobias V.D., Block G. & Laca E.A. (2016) Controlling perennial pepperweed (Lepidium latifolium) in a brackish tidal marsh. Wetlands Ecology and Management, 24, 411-418.
One study in 2003–2015 of an intertidal area invaded by smooth cordgrass Spartina alterniflora in Washington, USA (Patten et al. 2017) reported that after treating smooth cordgrass with herbicide, native salt marsh vegetation developed. One year after herbicide treatment began, three salt marsh plant species were present: glasswort Salicornia pacifica, Canadian sandspurry Spergularia canadensis and arrowgrass Triglochin maritimum (see original paper for frequency and cover data). After 12 years, three additional species were present. Saltgrass Distichlis spicata was the most abundant species (both frequency and cover) at high elevations, next to an existing salt marsh. Total native species cover reached 100% at these high elevations. Methods: Smooth cordgrass was controlled in a 300-ha cordgrass meadow that had developed on intertidal mudflats. Between 2003 and 2005, the meadow was sprayed with herbicide (1.7 kg/ha imazapyr). In subsequent years, remaining cordgrass plants were spot-treated (2% glyphosate, 0.75% imazapyr). Vegetation was surveyed between 2004 and 2015: approximately 300 quadrats/year, along sixteen 600-m transects extending seawards from the edge of an existing salt marsh.
Study and other actions tested
Referenced paper
Patten K., O’Casey C. & Metzger C. (2017) Large-scale chemical control of smooth cordgrass (Spartina alterniflora) in Willapa Bay, WA: towards eradication and ecological restoration. Invasive Plant Science and Management, 10, 284-292.

Please cite as:

Taylor N.G., Grillas P., Smith R.K. & Sutherland W.J. (2021) Marsh and Swamp Conservation: Global Evidence for the Effects of Interventions to Conserve Marsh and Swamp Vegetation. Conservation Evidence Series Synopses. University of Cambridge, Cambridge, UK.

Where has this evidence come from?

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This Action forms part of the Action Synopsis:

Marsh and Swamp Conservation

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