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Phragmites Management in a Tidal Marsh Utilizing Multiple Control Methods

William P. Bowman, PhD and Kelly Risotto
Land Use Ecological Services, Inc.

Abstract

A Phragmites management program was implemented by the Village of Mill Neck in 2011 to eradicate Phragmites australis in a 1.5 acre stand located along the shoreline of Mill Neck Creek. Two Management Areas were identified, one with a dense monoculture of high-vigor Phragmites and the second a low-vigor stand of Phragmites mixed with native high marsh vegetation. Control of Phragmites from 2012-2015 was via repeated cutting, which was effective at reducing Phragmites size and density. However, due to the tall height of native plants that recruited to the site by 2015 (i.e. saltmarsh bulrush Bulboschoenus robustus), Phragmites could not be cut to a height below. 2.0-3.0 feet, reducing the effectiveness of Phragmites control through repeated cutting.

In 2016 use of herbicides was introduced to control remaining Phragmites. Data collected in 2018 shows significant native plant recruitment and an overall reduction of Phragmites. Native tidal wetland vegetation is present throughout the Phragmites management area, and has increased in both abundance and diversity compared to monitoring performed from 2011-2015 Native wetland plants now established in the former Phragmites stands include saltmarsh bulrush (Bolboschoenus robustus), marsh elder (Iva frutscens), marsh mallow (Hibiscus moscheutos), chairmaker’s bulrush (Schoenoplectus americanus), marsh fleabane (Pluchea odorata), salt hay (Spartina patens), spike grass (Distichlis spicata), perennial saltmarsh aster (Symphyotrichum tenuifolium var. tenuifolium), and smooth cordgrass (Spartina alterniflora). Wetter portions of the site have small stands of broad-leaved cattail (Typha latifolia), narrow leaved cattail (Typha angustifolia), and soft-stemmed bulrush (Schoenoplectus tabernaemontani). Low vigor Phragmites is present at low density throughout the site, but comprises less than 10% of overall coverage. No high-vigor Phragmites (greater than 6 ft tall) is present at the project site. This project demonstrates that multiple control methods are often required for both successful management of Phragmites and recruitment of a diverse native plant community at a site.

Introduction

Invasive Phragmites australis has colonized large areas of high marsh and coastal fresh marsh habitats on Long Island. Phragmites has historically been found in slightly brackish, tidal fresh marshes, and the borders of salt and brackish marshes; but has increasingly been colonizing salt and brackish marshes (Orson, 1999; Tiner 2009).

In the wetlands associated with Long Island Sound embayments, Phragmites australis area increased by 106.6 acres between 1974 and 2005 (NEIWPCC, 2015). Phragmites expansion has contributed to the drastic loss of native high marsh communities and led to the near eradication of coastal fresh marsh communities.

Phragmites management was undertaken by the Village of Mill Neck on a 4.4 acre property located on the northern side of the DeGraff Causeway/Cleft Road separating Beaver Lake (North Shore Wildlife Sanctuary) and Mill Neck Creek (Oyster Bay NWR)(Figure 1). Phragmites has been present at this site since at least the early 1970s, as a well developed Phragmites stand can be observed on the 1974 NYS Tidal Wetland Inventory Maps (Figure 2). Before management, the site consisted of the an approximately 1.1 acre Phragmites monoculture with shoot heights exceeding 12 feet, an 0.5 acre stand of low-vigor Phragmites (i.e. 6-8 feet in height) mixed with marsh elder (Iva frutescens) and other native high marsh vegetation, a 0.15 acre area of herbaceous high marsh, and surrounding areas of intertidal marsh (Figure 3).


Figure 1. Location Map depicting the Phragmites management site at Mill Neck Creek.


Figure 2. Excerpt from NYSDEC 1974 Tidal Wetland Map #620 526.

Pre Project Photos

Figure 3. Pre project photos showing the extent of Phragmites at the site. Left (2) taken November 2010. Right (2) taken Summer 2011.

Phragmites Management Methods

The approved site plan for the implemented Phragmites management is shown in Figure 4 General details and specifications for the repeated cutting, herbicide application, and monitoring are provided below:

Repeated Cutting (2012-2015)

  1. In the first year, Phragmites shoots were cut to ground level (approximately 3" in height) in the early spring to remove standing dead biomass and accumulated thatch. Cutting was performed using commercial hedge trimmers. All cut material and thatch was hand-raked and removed.
  2. Phragmites shoots were typically cut three times per year. The first cut of each season typically occurred in late May to mid June when Phragmites shoots reached 4-5 feet in height. The second cut occurred in late August, after Phragmites flowering and when sufficient separation in height allowed Phragmites shoots to be cut above recruiting native plants. The third cut occurred in late October-early November to remove standing dead biomass.
  3. Phragmites shoots were only cut above the height of native vegetation that had recruited into the management areas. In areas where there was minimal growth or recruitment of native plants, Phragmites shoots were again be cut to ~3" above soil level.
  4. All cutting was monitored by a qualified ecologist to ensure that no native herbaceous plants or woody shrubs were cut or removed. Native plant stands or individuals were flagged or marked in the field prior to cutting to avoid incidental damage during cutting.
  5. All harvested plant material shall be disposed of in an approved upland landfill.


Figure 4. Phragmites Management Site Plan prepared by Land Use Ecological Services, Inc.

Herbicide Application (2016)

Due to the tall height of many of the native herbaceous plants such (as the bulrushes Bulboschoenus robustus and Schoenoplectus americanus), it was typically not possible to cut the Phragmites shoots to a height of 1.5-2.0 feet frequently throughout the growing season. Instead, Phragmites shoots were cut above the height of the bulrushes. This resulted in increased bulrush recruitment at the site, but also reduced the effectiveness of Phragmites control through cutting.

Targeted herbicide application directly to Phragmites shoots and leaves was conducted in 2016 to minimize potential disturbance to regenerating native vegetation, per the following methodology:

  1. A NYS certified herbicide applicator (Solitude Lake Management) applied a glyphosate based herbicide, AquaNeat with LI 700 surfactant, within the Phragmites management area.
  2. Herbicides were applied between August 15 and October 15.
  3. Herbicides were applied at low tide, so target Phragmites shoots were not in standing water.
  4. Herbicides were not be applied directly or indirectly to native vegetation.
  5. Herbicide was applied by back-pack sprayer and/or wiping to prevent damage to desirable planted vegetation or native vegetation Herbicide application to Phragmites shoots was not to the point of runoff of herbicide from the plant.
  6. Herbicides were mixed and applied in accordance with manufacturer’s specifications.
  7. Backspraying was done at times when wind did not exceed a velocity of 8 miles per hour, air temperature was greater than 50F, and no rain was expected within the next 6 hours.
  8. Herbicides were applied in accordance with the manufacturer's instructions, NYSDEC regulations and permits issued for the project, and pesticide labeling in reference to any restrictions to being utilized near shorelines, wetlands, and standing water.

Monitoring

Four transects were established at fixed points through the Phragmites stands and extending to the seaward edge of the existing marsh (Figure 4). The transects ranged in length from 55 to 110 feet (16.8 to 33.5 meters) Plant community composition and abundance were sampled at 1m2 sampling plots located every 15 feet (4.6 meters) along each marsh transect. Monitoring was conducted annually, between August 1 and October 1. Percent cover attributable to each observed plant species was qualitatively assessed to the nearest 5% and recorded. Durable 1.5 m Carsonite posts were installed at the landward and seaward edge of the monitoring transects to allow repeat monitoring of marsh positions.

Results and Findings

After three full seasons of Phragmites management through repeated cutting, a decrease in Phragmites cover was observed with % Phragmites cover declining from nearly 100% to 41.3 ± 30.2% within the site's high density Phragmites monoculture and from 57.0 ± 46.4% to 25.2 ± 27.5% across all monitoring plots (Figure 6). The management efforts clearly were successful in reducing the size and density of the Phragmites stand.

However, the repeated cutting did not result in complete control of Phragmites during the first three years of management. Phragmites control was limited, in part, to the increased the abundance of bulrushes (Bulboschoenus robustus and Schoenoplectus americanus). The bulrush recruitment was certainly a positive development; however, due to the 3.5-4.0 foot height of the bulrushes, it was not possible to cut Phragmites with sufficient frequency to attain control in portions of the site where Phragmites and bulrushes were mixed. In some locations within the project site, mixed stands of Phragmites and marsh elder presented a similar constraint.

After a targeted herbicide application in the early fall of 2016, Phragmites cover decreased further to 10.5 ± 15.6% within the site's Phragmites monoculture and to 6.3 ± 12.0% across all monitoring plots (Figure 6).

Analysis of the native wetland plant recruitment within the former high density Phragmites monoculture stand indicates that Phragmites was typically replaced by spike grass (Distichlis spicata) and saltmarsh bulrush (Bulboschoenus robustus) (Figure 5). Spike grass tended to replace Phragmites in locations where a salt panne developed after initial reduction in Phragmites cover. Saltmarsh bulrush tended to replace Phragmites in locations where freshwater seeping from Beaver Lake through the DeGraff Causeway berm resulted in more brackish marsh conditions

Plant species diversity has increased within the Phragmites management areas due to reduction of Phragmites cover and repeated removal of accumulated thatch and standing dead biomass. Only six plant species were observed during pre-project monitoring data in 2011 including Phragmites australis, Spartina patens, Distichlis spicata, Bulboschoenus robustus, Iva frutescens, and Spartina alterniflora. An additional twelve plant species were observed in subsequent monitoring events. A complete list of the native plant species that recruited into the Phragmites management area is provided in Table 1.

Post Project Photos

Figure 5. Post project photos showing the extent of Phragmites at the site following management activities. Photos taken in Summer 2017

Table 1: Native Plant Recruitment into Phragmites Management Area During Six Years of Management
Plant Species observed in 2011 Additional Plant Species observed in Subsequent Monitoring (2012 2018)
Common Reed (Phragmites australis) Glasswort (Salicornia sp.)
Salt Hay (Spartina patens) Seaside Lavender (Limonium carolinianum)
Spike Grass (Distichlis spicata) Marsh Fleabane (Pluchea odorata)
Saltmarsh Bulrush (Bulboschoenus robustus) Seabeach (Orach Atriplex mucronata)
Marsh Elder (Iva frutescens) Seaside Goldenrod (Solidago sempervirens)
Smooth Cordgrass (Spartina alterniflora) Chairmaker’s Bulrush (Schoenoplectus americanus)
Perennial Saltmarsh Aster (Symphyotrichum tenuifolium var. tenuifolium)
Soft stemmed Bulrush (Schoenoplectus tabernaemontani)
Narrow leaved Cattail (Typha angustifolia)
Broad leaved Cattail (Typha latifolia)
Groundsel Bush (Baccharis halimifolia)
Marsh Mallow (Hibiscus moscheutos)

Figure 6. Change in Phragmites % cover (left) and Native Plant Composition in Phragmites plots after management (right).

Adaptive Management

Ecosystem processes are complex and ecological restorations typically result in unforeseen conditions. The repeated collection of monitoring data, informal site inspections, and ecological supervision of management actions provided the necessary site data to modify restoration methods to achieve the project’s ecological goals. Several examples of adaptive management of this Phragmites control project area provided below:

  1. The beneficial recruitment of tall bulrushes into the Phragmites management area reduced the effectiveness of repeated cutting to control Phragmites. In order to allow portions of the site with freshwater seeps to continue to develop as a brackish marsh community, Phragmites management through herbicide application was initiated in 2016.
  2. Marsh wren (Cistothorus palustris) nests were observed in the mixed stands of low-density Phragmites bulrush, and marsh elder resulting from the initial cutting events (Figure 7a). Areas with abundance of nests were excluded from cutting or a minimum 10-ft no-cut buffer was established around nests.
  3. After initial Phragmites cutting events, the upland berm adjacent to the road surface was largely vegetated by upland weeds including green foxtail, poison ivy, and orchard grass. Switch grass (Panicum virgatum), Long Island-ecotype, seed was purchased and hand broadcast on the road shoulder/berm in late May 2012 after a Phragmites cutting and raking to remove cut biomass to the maximum extent practical (Figure 7b).
  4. Native plant recruitment was persistently poor in a narrow band of the upper marsh located at the toe of the road shoulder/berm. To facilitate native plant establishment, approximately 250 marsh elder tubelings were planted along the seaward toe of the road berm in late April 2018.
  5. The final cutting of Phragmites shoots and removal of biomass was intended to remove shoots close to ground level to provide open marsh substrate the following spring. Due to bulrush recruitment, final Phragmites biomass removal had potential to result in the collateral cutting and removal of senesced bulrush shoots and seeds within inflorescences (Figure 7c). Bulrush inflorescences were manually broken apart on standing shoots to release seeds to marsh substrate prior to final Phragmites cutting and biomass removal in late October-early November.

Figure 7. (a) Marsh wren nest, (b) switch grass seeded along roadway and berm, and (c) bulrush shoots and inflorescences.

Conclusion

Phragmites management greatly reduced % Phragmites cover and increased native wetland plant diversity at the site. The multiple control methods (i.e. repeated cutting and herbicide application) had complementary benefits that were necessary to achieve the Village of Mill Neck’s ecological objectives. Benefits of the repeated cutting phase of the project include:

  • Avoidance of creation of large, unvegetated marsh areas in high density Phragmites stands. More gradual control of Phragmites resulting from repeated cutting allowed native plants to concurrently recruit into the management area, resulting in mixed Phragmites-native plant stands. Maintenance of marsh vegetation resulted in maintenance of wildlife habitat availability, e.g. marsh wren habitat, throughout the project.
  • Several years of repeated cutting reduced Phragmites living and standing dead biomass, reduced shoot heights, and likely reduced shoot density. Accordingly, the quantity of applied herbicides were reduced and herbicides could be more effectively and selectively applied.

Benefits of the herbicide application phase of the project include:

  • Provided Phragmites control in areas of the site where recruitment of tall native wetland plants (i.e. saltmarsh bulrush) limited the effectiveness of repeated cutting.
  • Herbicide application is less costly and labor intensive. However, cutting and removal of biomass is still necessary for herbicide-based Phragmites control, but is limited to only cutting and removal event approximately 60 days after a late summer/early fall herbicide application.

Future Management

It is anticipated that maintenance herbicide treatments via backpack sprayer and/or wiping will be conducted as needed every 2-4 years. The next herbicide treatment at the project site is scheduled for the late summer/early fall of 2019.

Literature Cited

Orson RA 1999 A paleoecological assessment of Phragmites australis in New England tidal marshes: changes in plant community structure during the last few millennia Biological Invasions 1: 149-158.

New England Interstate Water Pollution Control Commission (NEIWPCC). 2015. Long Island tidal wetlands trends analysis. Prepared for NEIWPCC by Cameron Engineering and Land Use Ecological Services.

Tiner RW. 2009. Native Phragmites Located on Long Island. The Quarterly Newsletter of the Long Island Botanical Society 19(2): 9-12.