Effect of Season and Number of Glyphosate Applications on Control of Invasive Mexican Petunia (Ruellia simplex)

Effective control measures for invasive species are particularly needed in forested wetland-urban interface locales. Floodplain forest communities occupy a unique niche as a transition between upland and aquatic ecosystems and support botanically rich vegetation communities compared to adjacent habitats (Nilsson and Svedmark 2002). However, urban-bordered floodplain forests are particularly vulnerable to invasion by aggressive non-native species due to alterations in flooding regime, which may then favor invading non-native species (Predick and Turner 2008). Propagule pressure from invasive plant species is also especially high in the wetland-urban interface due to runoff from urban landscape sources (Loewenstein and Loewenstein 2005).

Mexican petunia, (Ruellia simplex, Syn. R. brittoniana) is a commonly cultivated ornamental herbaceous perennial. Environmental tolerance and abundant seed production have contributed to its spread from urban landscapes into natural areas. It has been documented in parts of Texas, Louisiana, Georgia, Alabama, Mississippi, and South Carolina (USDA-NRCS 2012a), and vouchered in 29 Florida counties (Wunderlin and Hansen 2011). The Florida Exotic Pest Plant Council lists Mexican petunia as a Category 1 invasive species defined as: “altering native plant communities by displacing native species, changing community structures or ecological functions, or hybridizing with natives” (Florida Exotic Pest Plant Council 2011).

Preliminary research on Mexican petunia in natural areas suggests herbicides may provide control of Mexican petunia (Hupp et al. 2009). When herbicide was applied twice, 120 days apart, all five herbicides evaluated resulted in low percent cover (< 0.5%) 183 days after initial treatment (Wiese et al. 2013). No work has been done, however, to address the efficacy of treatment as a function of application season, or to determine if additional glyphosate treatments are necessary to achieve optimal control of Mexican petunia. Preliminary research found that locations consisting primarily of Mexican petunia cover (75% or greater) pre-treatment shifted to ≥ 50% non-Mexican petunia composition within 6 months of treatment (Hupp et al. 2009) suggesting the potential for native species recovery following control. Establishment of native species likely suppresses further Mexican petunia invasion; Hupp (2007) found that survival of young Mexican petunia seedlings was reduced when native vegetation was present, compared to bare soil.

Our objectives were to evaluate the effects of glyphosate application season and number of applications on 1) control of Mexican petunia, and 2) species composition and quality of resulting post-treatment plant cover.

The study site was located at Paynes Prairie State Preserve, Alachua County, Florida (29°37'21.7" N, 82°19'20.8" W). The site was a bald-cypress (Taxodium distichum) dominated floodplain forest habitat with dense stands of Mexican petunia in the herbaceous vegetation layer. The soil was predominantly from the mulat sand (loamy, siliceous, subactive, thermic Arenic Endoaquults) series (USDA-NRCS 2012b). Six 3 × 3 m plots were randomly located on both sides of a branch tributary bisecting the bald-cypress preserve area (12 plots total). Each plot was divided into four 1.5 × 1.5 m subplots designated by permanent markers.

Herbicide application treatments were applied 0, 1, 2 or 3 times to each subplot in one of two application initiation seasons (fall or spring-initiated application). Percent cover of each species present was measured using a modified Mueller-Dombois scale (0 = 0 %, 1 = <1 %, 2 = 1–4 %, 3 = 5–24 %, 4 = 25–49 %, 5 = 50–74 %, 6 = 75–94 %, 7 = 95–100 %; Mueller-Dombois and Ellenberg 1974). Percent cover was measured before the first glyphosate application and every three months thereafter, with the final data collection occurring three months after the third spring glyphosate application (Figure 1). Pre-treatment percent plant cover data were collected in October 2010 and the first fall season herbicide application was applied immediately after (Figure 1). Spring pre-treatment plant cover data were collected in April 2011. As in the fall, pre-treatment data collection was immediately followed by the first spring season herbicide application (Figure 1). A foliar application of a 2% solution of glyphosate (Roundup WeatherMAX®, 48.8% a.i., Monsanto, St. Louis, MO) was applied with a...