Use herbicide to control problematic plants: freshwater swamps
Overall effectiveness category Trade-off between benefit and harms
Number of studies: 4
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.
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.
Supporting evidence from individual studies
A replicated, randomized, paired, controlled study in 2002–2005 aiming to restore swamps in three reed canarygrass Phalaris arundinacea stands in Wisconsin, USA (Hovick & Reinartz 2007) found that spraying the vegetation with herbicide typically increased plant species richness and diversity, and increased tree seedling density. After 1–2 growing seasons, overall plant richness was greater in sprayed than unsprayed plots in two of three comparisons (for which sprayed: 3.2–6.6; unsprayed; 1.9–2.3 species/m2; other comparison no significant difference). The same was true for overall plant diversity (data reported as a diversity index). However, native plant richness did not significantly differ between treatments in two of three comparisons (for which sprayed: 1.7–2.6; unsprayed; 1.3–2.2 species/m2; other comparison higher in sprayed plots). The density of non-planted tree seedlings was greater in sprayed plots in three of three comparisons (sprayed: 3–25; unsprayed: <1–4 seedlings/m2). For one of the three swamps, the study also reported data on the abundance of individual plant species (see original paper). Methods: Sixteen plots of varying size were established across three canarygrass-invaded wetlands. Ten plots (1–8 random plots/site) were sprayed with herbicide (Roundup®) in autumn 2002 or 2003. Six plots (1–4 random plots/site) were left unsprayed. All plots were planted with tree/shrub seedlings (roughly 1 seedling/m2) in spring 2003 or 2004. In August 2004, plant species and their cover were surveyed in ten 1-m2 quadrats/treatment/swamp, ignoring planted trees/shrubs.Study and other actions tested
A replicated, controlled study in 2006–2009 in a floodplain swamp clearing invaded by reed canarygrass Phalaris arundinacea in Wisconsin, USA (Thomsen et al. 2012) found that cutting, disking and applying herbicide to invaded plots increased tree seedling abundance after 1–3 years, and increased cover of herbs other than canarygrass after three years. In three of three years following intervention, treated plots contained more tree seedlings (4–44 seedlings/m2) than untreated plots (0–5 seedlings/m2). At the same time, treated plots had lower reed canarygrass cover (7–31%) than untreated plots (83–92%). Cover of herbs other than reed canarygrass did not significantly differ between treated and untreated plots in the first two years after intervention (treated: 15–47%; untreated: 16–22%), but was higher in treated than untreated plots in the third year (treated: 35–58%; untreated: 12%). Methods: In November 2006, twenty plots (roughly 810 m2) were established in a storm-created clearing within a floodplain swamp. Sixteen canarygrass-dominated plots were treated by cutting the vegetation (with a mechanical mulcher), disking the soil, and applying herbicide (four combinations of herbicide type and dose; repeated applications in summer and autumn until November 2008). The other four plots received none of these interventions. The study does not distinguish between the effects of cutting, disking and applying herbicide. Some tree species were planted and/or sown across the whole clearing. Vegetation (excluding planted trees) was surveyed in August 2007–2009, in four 2.25-m2 quadrats/plot.Study and other actions tested
A replicated, paired, controlled, before-and-after study in 2006–2008 in six freshwater swamps invaded by Old World climbing fern Lygodium microphyllum in Florida, USA (Hutchinson & Langeland 2015) reported that spraying the fern with herbicide had no clear effect on native plant richness or ground cover after two years. Unless specified, statistical significance was not assessed. Before intervention, plots destined to be sprayed contained 7–10 native plant species and had 46–72% native vegetation cover (mostly ferns). After two years, they contained 8–11 native plant species and had 33–67% native vegetation cover (mostly weedy species). Meanwhile, unsprayed plots contained 9 native plant species (sprayed plots statistically similar in ≥10 of 12 comparisons both before and after) and had 93–107% native vegetation cover (sprayed plots significantly lower in ≥11 of 12 comparisons both before and after). Herbicide treatments did reduce cover of the climbing fern (e.g. sprayed plots before: 59–72%; two years after: <1–4%). Methods: In September/October 2006, thirteen 20-m2 plots were established in each of six fern-invaded swamps. Seventy-two plots were sprayed with herbicide (1 plot/swamp for each of 12 different herbicides). Initial treatment was followed up with spot-treatments every 6 months. The final six plots (1 plot/swamp) were left unsprayed. Ground-level vegetation was surveyed on a 10-m-long transect in each plot, immediately before initial spraying (September/October 2006) and every 6 months after (until September/October 2008).Study and other actions tested
A replicated, randomized, controlled study in 2013–2014 in a floodplain swamp invaded by Mexican petunia Ruellia simplex in Florida, USA (Smith et al. 2016) found that spraying the vegetation with herbicide had no significant effect on overall plant species richness. Averaged over 3–15 months after intervention, overall plant species richness was statistically similar in sprayed plots (2.8 species/2.25 m2) and unsprayed plots (1.8 species/2.25 m2). Sprayed plots also had statistically similar Mexican petunia cover to unsprayed plots (sprayed: 55%; unsprayed: 71%), but contained fewer Mexican petunia stems (sprayed: 5–35 stems/0.56 m2; unsprayed: 23–76 stems/0.56 m2) and, after 15 months, contained less Mexican petunia above-ground biomass (sprayed: 8 g/m2; unsprayed: 15 g/m2). Methods: Fourteen 1.5 x 1.5 m plots were established in a petunia-invaded floodplain swamp. In August 2013, seven random plots were sprayed with glyphosate herbicide (AquaPro®). The other seven plots were not sprayed. Vegetation was surveyed between November 2013 and November 2014: plant species and their cover every three months (whole plot), petunia stem density every month (two 75 x 75 cm quadrats/plot), and petunia biomass in November 2014 only (vegetation cut from one 15 x 15 cm quadrat/plot, then dried and weighed).Study and other actions tested