Use herbicide to control problematic plants: freshwater marshes

Overall effectiveness category Trade-off between benefit and harms
Number of studies: 17
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How is the evidence assessed?

Effectiveness

58%
Certainty

60%
Harms

35%

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

Seventeen studies evaluated the effects, on vegetation, of using herbicide to control problematic plants in freshwater marshes. Twelve studies were in the USA. Two studies were in Australia. There was one study in each of Canada, Mexico and the UK. There was overlap in the sites used in two studies. Two pairs of studies in Australia and the USA used the same general study area, but different plots or experimental set-ups.

VEGETATION COMMUNITY

Overall extent (3 studies): Two replicated, randomized, controlled, before-and-after studies in the USA found that marshes sprayed with herbicide had lower live vegetation coverage but greater dead vegetation coverage than unsprayed marshes, after 1–2 years. Overall vegetation coverage was lower in sprayed than unsprayed marshes in one study, but similar in sprayed and unsprayed marshes in the other. One study of a dune slack in the UK simply reported an increase in overall vegetation coverage between one and two years after clearing scrub (by cutting and applying herbicide).
Overall richness/diversity (6 studies): Three studies (including one replicated, randomized, paired, controlled) in ephemeral marshes/wet meadows in the USA reported that spraying invaded vegetation with herbicide (sometimes along with other interventions) typically increased total plant species richness 1–5 growing seasons later. Two replicated, randomized, paired, controlled studies (one also before-and-after) in freshwater marshes/wet meadows in the USA and Mexico found that plots treated with herbicide (sometimes along with other interventions) had similar overall plant species richness and diversity to untreated plots, after 4–8 months or three years. One study of a dune slack in the UK simply reported a small increase in total plant richness between one and two years after clearing scrub (by cutting and applying herbicide).
Characteristic plant richness/diversity (3 studies): Two before-and-after studies of floodplain marshes in the USA reported that cover of wet-prairie indicator species was higher 1–4 years after applying herbicide than before. However, one of these studies reported that the total cover of non-invasive, wetland-characteristic herbs was similar or lower 2–3 years after applying herbicide than before. One study of a dune slack in the UK simply reported an increase the number of slack-characteristic plant species present between one and two years after clearing scrub (by cutting and applying herbicide).
Native/non-target richness/diversity (3 studies): One controlled, before-and-after study in a reed-dominated freshwater marsh in the USA found that applying herbicide (along with cutting/mowing) increased non-reed species richness three years later. One replicated, controlled, before-and-after study in cattail-invaded marshes in the USA reported that marshes sprayed with herbicide contained no living native plants one year later: fewer than were present before spraying and in unsprayed marshes. One study of a dune slack in the UK simply reported an increase in native plant richness between one and two years after clearing scrub (by cutting and applying herbicide).

VEGETATION ABUNDANCE

Overall abundance (4 studies): Three replicated studies (two also randomized, paired, controlled) in freshwater marshes/wet meadows in the USA and Mexico found that applying herbicide (sometimes along with other interventions) had no clear or significant effect on overall vegetation abundance four months to three years later. Cover and density were similar to untreated plots and/or pre-treatment levels. One replicated, randomized, paired, controlled study in the USA found that wet meadows sprayed with herbicide contained less total vegetation biomass than unsprayed marshes, 2–3 growing seasons later.
Native/non-target abundance (7 studies): Four studies (including one replicated, randomized, paired, controlled, before-and-after) in marshes/wet meadows in the USA and Australia found that spraying invaded plots with herbicide (sometimes along with other interventions) did not reduce – and often increased – the abundance of native or non-target vegetation 1–3 growing seasons later. One replicated, controlled, before-and-after study in cattail-invaded marshes in the USA reported that marshes sprayed with herbicide contained no living native plants one year later: density and biomass were lower than before spraying and in unsprayed marshes. One replicated, randomized, paired, controlled study in an alligatorweed-invaded marsh in the USA found that spraying vegetation with herbicide had no significant effect on native plant biomass after 1–2 growing seasons. One study of a floodplain marsh in Australia simply reported non-target vegetation cover for up to four years after treating mimosa-invaded vegetation with herbicide (along with other interventions).
Herb abundance (4 studies): Two replicated, randomized, paired, controlled studies in wet meadows in the USA found that treating a problematic plant species with herbicide (sometimes along with physical removal) had no significant effect on cover of forbs, grass-like plants or sedges after 2–3 growing seasons. One replicated, randomized, paired, controlled study in a loosestrife-invaded marsh in Canada found that the density of sedges and grasses was not lower in herbicide-sprayed plots, than in unsprayed plots, after 2–3 years. The precise effect depended on dose of herbicide used. One study of a floodplain marsh in Australia simply reported grass/sedge cover for up to four years after treating mimosa-invaded vegetation with herbicide (along with other interventions).
Algae/phytoplankton abundance (1 study): One replicated, randomized, controlled study in a reed-invaded marsh in the USA reported that free-growing filamentous algae were more common in plots sprayed with herbicide than unsprayed plots, approximately one year later. However, spraying with herbicide had no significant effect on the density or biomass of biofilm algae.
Individual species abundance (3 studies): Three studies quantified the effect of this action on the abundance of individual plant species, other than the species being controlled. For example, one replicated, randomized, paired, controlled study in a grass-invaded marsh in Mexico found that five of five monitored native species had similar cover in herbicide-sprayed and unsprayed plots after 4–8 months. Two of the studies do not distinguish between the effects of applying herbicide and other interventions.

VEGETATION STRUCTURE

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 replicated, randomized, paired, controlled study in 1991–1994 in a floating freshwater marsh invaded by purple loosestrife Lythrum salicaria in Ontario, Canada (Gabor et al. 1996) found that the effect of spraying herbicide on the density of sedges, grasses and purple loosestrife depended on the dose. After 2–3 years, the density of sedges Carex spp. did not significantly differ between sprayed plots (68–322 stems/m²) and unsprayed plots (150–199 stems/m²). Grass density did not significantly differ between plots sprayed with low-medium herbicide doses (sprayed: 80–177 stems/m²) and unsprayed plots (47–65 stems/m²). However, it was significantly greater (vs no spraying) in plots sprayed with high herbicide doses (124–161 stems/m²). Purple loosestrife density did not significantly differ between plots sprayed with low herbicide doses (27–41 stems/m²) and unsprayed plots (36–56 stems/m²). However, it was significantly lower (vs no spraying) in plots sprayed with medium-high herbicide doses (1–15 stems/m²). Methods: In summer 1991, twelve 30-m² plots were established, in three blocks of four, in a loosestrife-invaded marsh. Three plots (one random plot/block) were sprayed with each dose of triclopyr amine: low (4 kg/m²), medium (8 kg/m²) or high (12 kg/m²). The other three plots were not sprayed. In July and August 1993 and 1994, plant stems were counted in three 1-m² quadrats/plot.
Study and other actions tested
Referenced paper
Gabor T.S., Haagsma T. & Murkin H.R. (1996) Wetland plant responses to varying degrees of purple loosestrife removal in southeastern Ontario, Canada. Wetlands, 16, 95-98.
A replicated, randomized, controlled, before-and-after study in 1990–1993 of 17 freshwater marshes dominated by cattails Typha spp. in North Dakota, USA (Linz et al. 1996) found that marshes sprayed with herbicide had lower overall and live vegetation coverage than unsprayed marshes, but greater coverage of dead vegetation. After 1–2 years, coverage of emergent vegetation was significantly lower in sprayed marshes (70% of marsh area) than in unsprayed marshes (88% of marsh area). Sprayed marshes also had lower coverage of live vegetation (sprayed: 29%; unsprayed: 70%) but greater coverage of dead vegetation (sprayed: 40%; unsprayed: 17%). Before intervention, marshes destined for each treatment had statistically similar coverage of emergent vegetation (sprayed: 84–90%; unsprayed: 89%; live and dead not separated). Methods: In July 1990 and 1991, glyphosate herbicide (Rodeo®) was sprayed on to a total of 12 cattail-dominated marshes (5.8 L/ha across 50–90% of each marsh). Five similar marshes were left unsprayed. Emergent vegetation coverage was estimated from aerial photographs of each marsh, taken before (June 1990) and 1–2 years after (August 1991–1993) intervention. This study used a subset of the marshes in (3).
Study and other actions tested
Referenced paper
Linz G.M., Blixt D.C., Bergman D.L. & Bleier W.J. (1996) Response of ducks to glyphosate-induced habitat alterations in wetlands. Wetlands, 16, 38-44.
A replicated, randomized, controlled, before-and-after study in 1990–1993 of 23 freshwater marshes dominated by cattails Typha spp. in North Dakota, USA (Linz et al. 1996) found that marshes sprayed with herbicide had similar overall vegetation coverage to unsprayed marshes, but less live vegetation and more dead vegetation. After 1–2 years, coverage of emergent vegetation did not significantly differ between sprayed marshes (61–81% of marsh area) and unsprayed marshes (76–85% of marsh area). However, sprayed marshes had lower coverage of live vegetation (sprayed: 14–39%; unsprayed: 61–69%) including cattails (sprayed: 31%; unsprayed: 65%), and greater coverage of dead vegetation (sprayed: 25–58%; unsprayed: 15–16%). Before intervention, marshes destined for each treatment had statistically similar coverage of emergent vegetation (sprayed: 70–91%; unsprayed: 87%; data not reported for live, dead and cattail coverage). Methods: In July 1990 and 1991, glyphosate herbicide (Rodeo®) was sprayed on to a total of 16 cattail-dominated marshes (5.8 L/ha across 50–90% of each marsh). Seven similar marshes were left unsprayed. Emergent vegetation coverage was estimated from aerial photographs of each marsh, taken before (June 1990) and 1–2 years after (August 1991–1993) intervention. Some of the marshes in this study were also used in (2).
Study and other actions tested
Referenced paper
Linz G.M., Blixt D.C., Bergman D.L. & Bleier W.J. (1996) Responses of red-winged blackbirds, yellow-headed blackbirds and marsh wrens to glyphosate-induced alterations in cattail density. Journal of Field Ornithology, 67, 167-176.
A controlled, before-and-after study in 1995–1998 in a freshwater marsh dominated by common reed Phragmites australis in Connecticut, USA (Farnsworth & Meyerson 1999) found that applying herbicide to the vegetation (along with cutting/mowing) increased the evenness of the plant community and the abundance and richness of non-reed species. After three years, treated plots contained a more even plant community, less dominated by one or two species, than an untreated plot (data reported as a coefficient of variation; see original paper for data on individual species abundance). Treated plots also had greater plant species richness (excluding common reed; treated: 5–7 species/m²; untreated: 3 species/m²) and contained a greater density of non-reed stems (treated: 78–97 stems/m²; untreated: 15 stems/m²). Common reed was less abundant in treated plots, in terms of stem density (treated: 6–19 stems/m²; untreated: 36 stems/m²) and frequency (treated: 45–64%; untreated: 98% of surveyed quadrats contained common reed). Before intervention, all plots had relatively similar plant species richness (excluding common reed; 2–3 species/m²), non-reed density (7–23 stems/m²) and reed density (33–40 stems/m²). Methods: In 1995, two 0.4-ha plots were treated in a reed-dominated, tidal, freshwater marsh. In August, each plot was sprayed with herbicide (Rodeo® 1%). In autumn, one plot was cut by hand and one was mown mechanically; cuttings were left in place. A third adjacent plot was neither sprayed with herbicide nor cut/mown. The study does not distinguish between the effects of applying herbicide and cutting/mowing. In late summer before (1995) and after (1996–1998) intervention, plant stems were identified and counted in fifty 1-m² quadrats/plot.
Study and other actions tested
Referenced paper
Farnsworth E.J. & Meyerson L.A. (1999) Species composition and inter-annual dynamics of a freshwater tidal plant community following removal of the invasive grass, Phragmites australis. Biological Invasions, 1, 115-127.
A replicated, randomized, paired, controlled, before-and-after study in 1988–1991 in two wet meadows that had been cleared of vegetation in New York State, USA (Morrison 2002) found that controlling regrowth of invasive purple loosestrife Lythrum salicaria (by applying herbicide to large shoots and pulling up seedlings) had no significant effect on plant species richness, diversity or vegetation cover. After three years, plots with and without control of loosestrife regrowth had statistically similar plant species richness (control: 7; no control: 8 species/m²), plant diversity (data reported as a diversity index), total vegetation cover (control: 67–82%; no control: 79%), grass-like plant cover (control: 60–75%; no control: 70–73%) and forb cover (control: 5–20%; no control: 8–10%). Purple loosestrife cover was 0% in plots where regrowth had been controlled, but still only 2% in plots where regrowth had not been controlled. For data on the cover of other individual plant species, see original paper. Before intervention and within each meadow, plots destined for each treatment had statistically similar plant species richness (8–9 species/m²), plant diversity, total vegetation cover (103–143%), grass-like plant cover (16–58%), forb cover (25–56%) and purple loosestrife cover (23–63%). Methods: In 1988, six pairs of 1-m² plots were established across two loosestrife-invaded wet meadows. In September, all vegetation was dug up and removed from the plots (see Action: Physically remove problematic plants). In six of the plots (one random plot/pair), loosestrife regrowth was controlled twice/year thereafter (painting large shoots with glyphosate and pulling up seedlings; the study does not distinguish between the effects of these interventions). In the other plots, loosestrife regrowth was not controlled. Plant species and their cover were surveyed before initial removal (August 1988) and three years after (September 1991).
Study and other actions tested
Referenced paper
Morrison J.A. (2002) Wetland vegetation before and after experimental purple loosestrife removal. Wetlands, 22, 159-169.
A study in 1998–2003 in a degraded floodplain marsh in the Northern Territory, Australia (Paynter 2004) reported that following herbicide application, physical damage and prescribed burning to control invasive mimosa Mimosa pigra, some herbaceous plants recolonized the site along with mimosa. After one year, cover of all vegetation other than mimosa was approximately 31–80%. This included 12–45% total cover of grasses/sedges. Mimosa cover was approximately 0–17%, depending on the area within the marsh. The number of new mimosa seedlings each year declined over time, from 1 seedling/m² in the first year after intervention was complete, to <0.5 seedlings/m² in the second and third years, then 0 seedlings/m² in the fourth year. Methods: Three interventions were applied to a 100-ha patch of mimosa-dominated floodplain. In April 1998, the site was sprayed with herbicide (metsulfuron methyl). In October 1999, the dead vegetation was crushed using a chain tied between two bulldozers, then the site was burned (fire lasting several days). The study does not distinguish between the effects of these interventions. Vegetation was surveyed in the dry season (July–October), in up to three areas of the marsh (where no vegetation had been introduced) and for up to four years after intervention was complete. This study was in the same area as (7), but used a different experimental set-up.
Study and other actions tested
Referenced paper
Paynter Q. (2004) Revegetation of a wetland following control of the invasive woody weed, Mimosa pigra, in the Northern Territory, Australia. Ecological Management & Restoration, 5, 191-198.
A replicated, randomized, paired, controlled, before-and-after study in 1997–1999 in a floodplain wetland invaded by mimosa Mimosa pigra in the Northern Territory, Australia (Paynter & Flanagan 2004) found that spraying the vegetation with herbicide did not reduce the cover of non-mimosa vegetation 1–2 years later. In three of six comparisons, non-mimosa cover was higher in sprayed plots (55–74%) than unsprayed plots (15–38%). In the other three comparisons, there was no significant difference between treatments (sprayed: 32–47%; unsprayed: 15–38%). Sprayed plots consistently had lower mimosa coverage (six of six comparisons) and density (six of six comparisons). Results for mimosa biomass were mixed, but never significantly higher in sprayed than unsprayed plots (see original paper for data). Before intervention, the abundance of both mimosa and other vegetation were statistically similar in plots destined for each treatment (data not reported). Methods: In April 1998, thirty-two 100 x 200 m plots were established, in four sets of eight, on a mimosa-invaded floodplain. Twenty-four plots (six random plots/set) were sprayed with herbicide in April 1998 and/or January 1999. In half of the plots, vegetation was also crushed with bulldozers in late 1998. Vegetation was surveyed one year before spraying (late 1997/early 1998) and approximately 1–2 years after the latest spray (late 1999), in four 1–5 m² quadrats/plot and by aerial photography (mimosa coverage). This study was in the same area as (6), but used a different experimental set-up.
Study and other actions tested
Referenced paper
Paynter Q. & Flanagan G.J. (2004) Integrating herbicide and mechanical control treatments with fire and biological control to manage an invasive wetland shrub, Mimosa pigra. Journal of Applied Ecology, 41, 615-629.
A replicated, randomized, paired, controlled study in a freshwater marsh invaded by antelope grass Echinochloa pyramidalis in eastern Mexico (López Rosas et al. 2006) found that spraying the vegetation with herbicide had no significant effect on overall plant density, richness or diversity, the relative abundance of common plant species, or the absolute abundance of common native plant species. After 4–8 months, sprayed and unsprayed plots contained a statistically similar overall plant density (six of six comparisons; sprayed: 57–81; unsprayed: 54–93 plants/0.49 m²), species richness (six of six comparisons; sprayed: 5–7; unsprayed: 3–5 species/0.49 m²) and diversity (two of two comparisons; data reported as a diversity index). Accordingly, all six monitored plant species had a similar relative abundance in sprayed and unsprayed plots (five native species, plus antelope grass; see original paper for data). The five native plant species had statistically similar cover in sprayed and unsprayed plots in 13 of 14 comparisons (both treatments: 0–19% cover/species). In contrast, antelope grass had lower cover in sprayed plots in four of six comparisons (for which sprayed: 22–78%; unsprayed: 99–100%). Methods: In January (year not reported), twenty-one 0.49-m² plots were established (in seven sets of three) in a degraded marsh, invaded by antelope grass. Fourteen plots (two random plots/set) were sprayed with glyphosate herbicide (Roundup®). In seven of these, the most abundant native plant species was shielded with plastic tubes. The final seven plots (one random plot/set) were not sprayed. All 21 plots were enclosed, underground, by a plastic barrier. Vegetation was surveyed between May and September later that year (relative biomass in September only).
Study and other actions tested
Referenced paper
López Rosas H., Moreno-Casasola P. & Mendelssohn I.A. (2006) Effects of experimental disturbances on a tropical freshwater marsh invaded by the African grass Echinochloa pyramidalis. Wetlands, 26, 593-604.
A replicated, randomized, paired, controlled study in 2000–2004 in two wet meadows invaded by reed canarygrass Phalaris arundinacea in Minnesota, USA (Reinhardt Adams & Galatowitsch 2006) found that plots sprayed with herbicide contained less overall plant biomass than unsprayed plots after 2–3 growing seasons, but more non-canarygrass plant biomass. Two to three growing seasons after the last herbicide application, sprayed plots contained less total above-ground plant biomass (320–720 g/m²) than unsprayed plots (520–900 g/m²). Sprayed plots contained less reed canarygrass biomass (10–480 g/m²) than unsprayed plots (420–880 g/m²). However, they contained more biomass of other plants. This was true for total biomass of sown species (sprayed: 0–70 g/m²; unsprayed: 0 g/m²) and species that had not been sown (sprayed: 170–550 g/m²; unsprayed: 30–100 g/m²). Methods: In the early 2000s, one hundred and sixty 25-m² plots were established, in 40 sets of four, across two canarygrass-invaded wet meadows. One hundred and twenty plots (three random plots/set) were sprayed with herbicide (Roundup® Ultra): in late May, August or September and in one or two years. The remaining 40 plots were not sprayed. Half of the plots under each herbicide treatment were also burned in mid-May. All plots were sown with a mixture of grass and forb seeds in the spring after the final herbicide application. Dry biomass samples were taken in August in the two years after herbicide application.
Study and other actions tested
Referenced paper
Reinhardt Adams C. & Galatowitsch S.M. (2006) Increasing the effectiveness of reed canary grass (Phalaris arundinacea L.) control in wet meadow restorations. Restoration Ecology, 14, 441-451.
A replicated, randomized, paired, controlled, before-and-after study in 2004–2005 in two freshwater marshes invaded by alligatorweed Alternanthera philoxeroides in Alabama and Georgia, USA (Allen et al. 2007) found that spraying the vegetation with herbicide had no significant effect on native plant biomass after 1–2 growing seasons. Native plant biomass varied a lot depending on herbicide type, dose and application date, but was statistically similar in sprayed and unsprayed plots in 24 of 24 paired comparisons (sprayed: <1–210 g/0.25 m²; unsprayed: 76–129 g/0.25 m²). Alligatorweed biomass did not significantly differ between treatments in 14 of 24 comparisons (for which sprayed: 18–92 g/0.25 m²; unsprayed: 54–78 g/0.25 m²) but was lower in sprayed plots in the other 10 comparisons (for which sprayed: <1–22 g/0.25 m²; unsprayed: 54–78 g/0.25 m²). The study also provided short-term data on alligatorweed cover. This was depressed after 2–4 weeks for all herbicide types, doses and application dates (sprayed: 0–32%; unsprayed: 25–69%; before spraying: 17–62%). Methods: Sixty-four 5 x 5 m plots (in four sets of 16) were established across two alligatorweed-invaded freshwater marshes, managed for waterfowl. Herbicide was applied to 48 of the plots (12 random plots/set), in all possible combinations of herbicide type (triclopyr amine or imazapyr), dose (low, medium or high) and application date (April or July 2004). Alligatorweed cover was surveyed one week before spraying and for 12 weeks after. Vegetation was cut from plots, then dried and weighed, in October 2004 and 2005.
Study and other actions tested
Referenced paper
Allen S.L., Hepp G.R. & Miller J.H. (2007) Use of herbicides to control alligatorweed and restore native plants in managed marshes. Wetlands, 27, 739-748.
A study in 2005–2007 of a dune slack in England, UK (Smith & Kimpton 2008) reported that after cutting and applying herbicide to grey willow Salix cinerea scrub, ground vegetation recolonized. In 2006, approximately one year after removing willows, 80% of the site was covered with vegetation (mostly herbaceous). There were 108 vascular plant taxa, including 98 natives. Approximately 54 taxa were characteristic of dune slacks. In 2007, approximately two years after removing willows, 95% of the site was covered with vegetation (still mostly herbaceous). There were 111 vascular plant taxa, including 107 natives. Approximately 65 taxa were characteristic of dune slacks. Twenty-eight taxa recorded in 2006 were not present in 2007, but 31 new taxa had colonized the site. Methods: In November/December 2005, dense grey willow scrub in a dune slack (low-lying area amongst dunes) was controlled. Grey willows were cut at ground level, then herbicide (Roundup® Biactive Plus) was applied to the largest stumps. The study does not distinguish between the effects of these interventions. Cut material was burned on site. Vascular plant taxa and their overall coverage were surveyed in August/September 2006 and 2007.
Study and other actions tested
Referenced paper
Smith P.H. & Kimpton A. (2008) Effects of grey willow Salix cinerea removal on the floristic diversity of a wet dune-slack at Cabin Hill National Nature Reserve on the Sefton Coast, Merseyside, England. Conservation Evidence, 5, 6-11.
A replicated, randomized, paired, controlled study in 2006–2008 in a wet meadow being invaded by hybrid cattail Typha x glauca in Wisconsin, USA (Hall & Zedler 2010) found that spraying cattail with herbicide had no significant effect on cover of sedges Carex spp. after two growing seasons. Plots where cattails had been sprayed with herbicide had statistically similar sedge cover (14–29%) to unsprayed plots (11–38%). No sedge seedlings were found in any plot. Methods: Sixteen 4 x 8 m plots were established (in two sets of eight) on the boundary between native wet meadow vegetation and a patch of hybrid cattail. In May 2006, cattail plants in eight plots (four random plots/set) were sprayed with herbicide (Rodeo® 0.75%). The other eight plots were not sprayed. Sedge cover was surveyed in October 2007, in four 1-m² quadrats/plot.
Study and other actions tested
Referenced paper
Hall S.J. & Zedler J.B. (2010) Constraints on sedge meadow self-restoration in urban wetlands. Restoration Ecology, 18, 671-680.
A replicated, randomized, controlled study in 2007–2008 in a freshwater marsh invaded by common reed Phragmites australis in Ohio, USA (Back et al. 2012) reported that plots sprayed with herbicide were more likely to contain free-growing filamentous algae than unsprayed plots, but found that all plots contained a similar abundance, diversity and community of biofilm algae. After approximately one year, free-growing algae occurred in 13 of 30 samples in sprayed plots (vs 1 of 15 samples in unsprayed plots; statistical significance not assessed). Meanwhile, biofilm algae reached a statistically similar abundance in sprayed and unsprayed plots. This was true for both density (sprayed: 1,700–2,800 cells/cm²; unsprayed: 1,100–1,700 cells/100 cm²) and biomass (sprayed: 5–41 μg chlorophyll/cm²; unsprayed: 5–41 μg chlorophyll/cm²). Sprayed and unsprayed plots also supported a similar diversity of biofilm algae (data reported as a diversity index) and a similar community composition of the most abundant group: diatoms (data reported as a graphical analysis; statistical significance of difference not assessed). Common reed was less abundant in sprayed than unsprayed plots, in terms of both density (sprayed: 2–3 live stems/m²; unsprayed: 36 live stems/m²) and cover (sprayed: 1–3%; unsprayed: 49%). Methods: In June 2007, fifteen contiguous 20 x 20 m plots were established in a reed-invaded, lakeshore marsh. Ten random plots were sprayed once with herbicide (five with glyphosate-based AquaNeat®; five with imazapyr-based Habitat®). The other five plots were not sprayed. Vegetation was surveyed in June–August 2008. Free-growing algae were surveyed in 10 x 10 cm quadrats. Biofilms were surveyed on fallen, submerged reed stems. Reeds were surveyed, in 0.5-m² quadrats, along a central transect in each plot.
Study and other actions tested
Referenced paper
Back C.L., Holomuzki J.R., Klarer D.M. & Whyte R.S. (2012) Herbiciding invasive reed: indirect effects on habitat conditions and snail-algal assemblages one year post-application. Wetlands, 20, 419-431.
A replicated, randomized, paired, controlled, before-and-after study in 2005–2008 in five wet meadows in South Dakota, USA (Bahm et al. 2014) found that controlling problematic plants by mowing, applying herbicide and planting native upland plants increased plant species richness and cover of unplanted native species. All plots were initially dominated by reed canarygrass Phalaris arundinacea (>80% cover). After 1–3 growing seasons, plant species richness was higher in treated than untreated plots in 19 of 21 comparisons (for which treated: 2–5 species/0.25 m²; untreated: 2 species/0.25 m²). Treated plots also had greater cover of unplanted native species in 17 of 21 comparisons (for which treated: 8–57%; untreated: 3–21%) and lower cover of reed canarygrass in 21 of 21 comparisons (treated: 1–66%; untreated: 91–93%). After 2–3 growing seasons, no treatment outperformed those involving imazapyr. Plots treated with imazapyr never had lower plant species richness and unplanted native cover than plots treated with other herbicides, and never had higher cover of reed canarygrass (see original paper for data). Methods: Forty 3 x 40 m plots were established across five canarygrass-invaded wet meadows (eight plots/meadow). Between autumn 2005 and spring 2006, thirty-five plots (seven random plots/set) were mown, sprayed with herbicide (seven chemical x timing combinations), and planted with 14 native upland species. Subsequent targeted mowing of “noxious weeds” was also carried out. The study does not distinguish between the effects of these interventions. Vegetation was surveyed at the end of each growing season 2006–2008, in nine 0.25-m² quadrats/plot.
Study and other actions tested
Referenced paper
Bahm M.A., Barnes T.G. & Jensen K.C. (2014) Evaluation of herbicides for control of reed canarygrass (Phalaris arundinacea). Natural Areas Journal, 34, 459-464.
A replicated, controlled, before-and-after study in 2013–2014 in eight artificial marshes invaded by hybrid cattail Typha x glauca in Michigan, USA (Lawrence et al. 2016) found that applying herbicide to the vegetation reduced native plant richness, density and biomass. After one year, there were no living plants in marshes treated with herbicide: richness, density and biomass, of both native plants and hybrid cattail, were zero. All metrics were significantly lower than in untreated marshes (where native richness: 4.5 species/2 m²; native density: 300 stems/m²; native biomass: 440 g/m²; cattail density: 44 stems/m²; cattail biomass: 745 g/m²). Results were generally similar after one month, although native plant richness had not yet declined in treated marshes (see original paper). Before treatment, vegetation was statistically similar in all marshes (native plant richness: 3.5–4.3 species/2 m²; native plant density: 190–230 stems/m²; native plant biomass: 350–430 g/m²; cattail density: 66–69 stems/m²; cattail biomass: 1,080–1,130 g/m²). Methods: In July 2013, glyphosate-based herbicide was spread onto all plant stems in four experimental marshes (1 x 2 m area, 1 m soil depth). The marshes had been created in 2002 and planted with cattail (i.e. deliberately invaded) in 2004. Four additional marshes were left untreated. Plant species, density and height were recorded in all marshes immediately before, one month after and one year after treatment. Dry above-ground biomass was calculated from height measurements.
Study and other actions tested
Referenced paper
Lawrence B.A., Lishawa S.C., Rodriguez Y. & Tuchman N.C. (2016) Herbicide management of invasive cattail (Typha × glauca) increases porewater nutrient concentrations. Wetlands Ecology and Management, 24, 457-467.
A before-and-after study in 2006–2010 of a floodplain wetland invaded by limpo grass Hemarthria altissima in Florida, USA (Toth 2016) reported that over the four years after applying herbicide, cover of wet-prairie indicator species and total plant species richness were typically higher than before intervention. Statistical significance was not assessed. In the spring before applying herbicide, the wetland had 96% limpo grass cover and <1% cover of native wet-prairie indicator species. There were 3 plant species/100 m². Between one and four years after applying herbicide, limpo grass cover ranged from 2% to 22%. Indicator species cover ranged from 1% to 13%. There were between 13 and 30 plant species/100 m². Methods: In May 2006, glyphosate herbicide was applied to 6 ha of a recently rewetted, limpo-grass-invaded floodplain. Plant species and their cover were surveyed in twelve 100-m² plots, before intervention (spring 2006) and for approximately four years after (spring 2007–summer 2010). This study was in the same area as (17), but used a different plot.
Study and other actions tested
Referenced paper
Toth L.A. (2016) Cover thresholds for impacts of an exotic grass on the structure and assembly of a wet prairie community. Wetlands Ecology and Management, 24, 61-72.
A replicated, before-and-after, site comparison study in 2007–2010 of floodplain wet prairies invaded by limpo grass Hemarthria altissima in Florida, USA (Toth 2016) reported that applying herbicide reduced limpo grass cover and maintained cover of other wetland-characteristic herbs in some plots 2–3 years later, but failed to do so in others. Statistical significance was not assessed. In three of seven treated plots, applying herbicide reduced limpo grass cover (before: 47%; 2–3 years after: 4%). Cover of other wetland-characteristic herbs was similar before (21%) and after (18%) applying herbicide. In the other four of seven treated plots, applying herbicide failed to prevent an increase in limpo grass cover 2–3 years later (before: 47%; 2–3 years after: 59%). Meanwhile, cover of other wetland-characteristic herbs declined (before: 21%; after: 10%). Nearby untreated plots were always dominated by wetland-characteristic herbs (before: 54%; after: 43%) with little limpo grass cover (before: 5%; after: 1%). Across all seven treated plots, three key metrics increased over time after herbicide application, reaching similar or higher levels three years after intervention than before: cover of native wet-prairie indicator species (before: 2–5%; after: 9%), total vegetation cover (before: >80%; after: >80%), and total plant species richness (before: 13–17; after: 23 species/100 m²). Methods: Eighteen 100-m² plots were established in wet prairies on a recently rewetted floodplain, with varying limpo grass cover. In autumn 2007, glyphosate herbicide was applied to seven plots with the greatest limpo grass cover. Plant species and their cover were surveyed in all 18 plots, before intervention (spring 2006–summer 2007) and for approximately three years after (spring 2008–summer 2010). This study was in the same area as (16), but used different plots.
Study and other actions tested
Referenced paper
Toth L.A. (2016) Cover thresholds for impacts of an exotic grass on the structure and assembly of a wet prairie community. Wetlands Ecology and Management, 24, 61-72.

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.

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This Action forms part of the Action Synopsis:

Marsh and Swamp Conservation

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