RCG has spread throughout North America, taking over natural wet prairies, streambanks, and wetlands. It constitutes a major threat to native riparian and wetland plant communities and is classified as a pest in nine states in the US (Lavergne & Molofsky, 2006). RCG is one of the most damaging invasive wetland grasses in Pacific Northwest and is able to rapidly spread and take over plant communities replacing diverse plant assemblages with monospecific stands, drastically reducing species diversity (Lavergne & Molofsky, 2006; Roche et al., 2019; King County, 2015). Studies continue to document RCG’s ability to decrease native species diversity through competitive exclusion. However, it is unlikely that allelopathy is the mechanism responsible for invasion success of RCG (Lavergne & Molofsky, 2004).
RCG’s prolific root system, high nutrient uptake capacity, relatively high water use efficiency, high stem elongation and leaf production, early emergence and persistent productivity into the fall, provides a competitive advantage allowing it to dominate both above and below ground resources (Lavergne & Molofsky, 2004).
RCG can clog waterways, increase sedimentation, alter water circulation, and reduce micro topography heterogeneity; which ultimately leads to the loss of valuable habitat for fish and wildlife. (Lavergne & Molofsky, 2006). By growing vigorously on streambanks, forming monospecific stands in floodplains, and clogging waterways RCG slows water flow which induces sedimentation potentially degrading salmonid spawning habitat (Miller, Martin, & MacConnell, 2008). The dense growth can form physical barriers to salmonid migration and flooded RCG fields have been known to confuse and strand migrating salmon; one RCG infestation caused conditions that resulted in the stranding and death of 158 prespawn male and female Coho in a western Washington stream (Miller, Martin, & MacConnell, 2008; King County, 2015).
RCG can outcompete native plant species, resulting in monospecific stands with subsequent loss of plant and insect diversity and ultimately to alteration in ecosystem function (Lavergne & Molofsky, 2006). RCG provides little food for native wildlife species and causes indigestion in livestock (King County, 2015). Few animals will eat the grass after flowering because of its rank growth, and most waterfowl are unable to use infested habitat for nesting, food, or cover (Miller, Martin, & MacConnell, 2008). RCG invasion could potentially impact streamside aquatic and terrestrial insects which are an important food source for rearing juvenile salmon. Very low soil insect diversity was observed in marshes dominated by monocultures of RCG, when compared to adjacent marshes dominated by native species (Lavergne & Molofsky, 2004).
RCG dominated communities that were historically forested are especially resistant to reestablishment of the original vegetation, presumably because of a lack of establishment sites. Grasses in particular can limit the establishment and growth of woody species (Hovick & Reinartz, 2007). The thick sod layer, and aggressive growth eliminate safe sites for establishment of native woody shrub and tree species. Over time canopy cover is reduced, which decreases stream shading and increases water temperatures (Hovick & Reinartz, 2007). Overtime, large woody debris recruitment into the stream channel is also reduced, which provides critical salmon habitat for returning adults and rearing juvenile salmon.
When in flower, RCG produces abundant pollen and chaff, which can aggravate hay fever and allergies; that poses a human health hazard (WA State NWCB).
RCG can reach up to heights of 9 feet tall but is usually between 3 and 6 feet tall (King County, 2015; WA NWCB). It produces dense crowns and prominent networks of vigorous underground rhizomes (Lavergne & Molofsky, 2004).
Plants bloom May to mid-June or July. Inflorescence turns from green to purple in full bloom, and change to a beige straw color when seeds form (WA NWCB; King County, 2015). Flowers are borne in densely clustered panicles on culms high above the leaves. Panicles 5-40 x 1-4 cm, whitish or tinged with pink or purple, usually lobbed near the base, branches appressed, spreading at anthesis; spikelets borne singly (Roche et al., 2019). Each spikelet has 3 florets, one floret is fertile while the other two are sterile (WA NWCB).
Stems are hairless, hollow, up to 0.5 inches in diameter, often reddish near the top. Dead stems remain erect and persist throughout the winter, making identification in winter possible (King County, 2015).
Leaves stick out at a 45 degree angle from the stem. Leaf blades are flat, rough textured, hairless, 0.25 to 0.75 inches wide, and up to 18 inches long (King County, 2015).
Found in a large variety of open wet habitats including shallow wetlands, marshes, swales, meadows, pastures, riparian floodplains and forests, streambanks, shorelines, river dikes, rights-of-way, and ditches. RCG is shade intolerant, it typically occurs in soils that are saturated or nearly saturated for most of the growing season and can tolerated extended periods of inundation. Although less common, RCG can be found on upland sites, where it can survive temporary droughts. Landscape sinks that receive debris, sediments, water, nutrients and species propagules; where water circulation encourages plant dispersal are more susceptible to RCG invasion. In addition, areas that are subjected to disturbance are usually more vulnerable to invasion; however landscape context, especially human activities and public use of waterways can influence the site sensitivity for establishment and spread. (Roche et al., 2019; King County, 2015; WA State NWCB; Lavergne & Molofsky, 2006; Lavergne & Molofsky, 2004)
RCG reproduces by seed and vegetative reproduction. RCG annually produces a large number of outcrossed seeds (due to self-sterility), each inflorescence can produce up to 600 seeds. Seeds exhibit dormancy and can remain viable up to 4 years. Seed germination is sensitive to overhead canopy structure and requires light with highest germination rates in water saturated soils. Seeds spread via animals, flowing water, humans (boots, clothing, tools) and machinery.
The more common form of spread is through creeping rhizomes that can extend over 10 feet per year and form a thick mat. It can also reproduce from vegetative fragments producing roots and shoots where they make contact with moist bare soil (WA State NWCB; King County, 2015; Lavergne & Molofsky, 2006; Miller, Martin, & MacConnell, 2008)
RCG overwinters in the rhizome stage and produces tillers in early spring, although in the Pacific Northwest it may begin to grow in late winter. RCG grows vertically throughout the spring and early summer followed by lateral expansion via rhizomes. In the Pacific Northwest RCG’s growth will peak in mid-June and decline by mid-August. Generally, RCG flowers from May to July, with seeds maturing in late July and early August (Waggy, 2010; King County, 2015; Lavergne & Molofsky, 2004).
Integrated Pest Management
Management of invasive species must switch from isolated efforts of stand eradication to landscape level approaches, emphasizing infestation prevention and accounting for surrounding human activities and the socio-economic context. No one method is sufficient, and most successful strategies will require a combination of prevention, physical control, cultural control, chemical control, monitoring and follow up maintenance (Lavergne & Molofsky, 2006). Management will require dedication over a number of years, and should allow for flexibility in method as appropriate. In most cases successful RCG control will involve at least two different control methods (King County, 2015). Generally, work first on isolated patches and in least infested areas, moving towards more heavily infested areas (King County, 2015). Deplete the seed bank by cutting or mowing before plants go to seed. Allow existing seed bank to germinate and grow and then remove the plants several times over at least two seasons for best results (King County, 2015).
Since RCG can cause major and long-term changes to ecosystem processes the reversibility of these changes after removal of the invader must be questioned. Restoration actions must aim to restore habitat characteristics and functions (Lavergne & Molofsky, 2006). Shade can be thought of as the “Achilles heel” of RCG and has been consistently found to limit its spread, germination and growth (Hovick & Reinartz, 2007). Replanting or seeding after the first initial control work is complete will improve long term maintenance needs and prevent re-infestation from the seed bank or from adjacent RCG populations.
Due to its high genetic variability, and diversity of adaptations using a mix of different strategies will be most successful in controlling RCG (Lavergne & Molofsky, 2006). As always, follow up monitoring and maintenance of any control site is imperative until desirable vegetation becomes well established. Any site left unmaintained will revert back to RCG within a few years (King County, 2015).
Prevention is the most cost effective method for managing RCG. Once RCG has invaded a natural area and displaced native species; removal becomes more expensive and problematic, requiring more resources and knowledge for control (Roche et al., 2019; Lavergne & Molofsky, 2006).
Prevent plants from spreading away from existing populations by washing vehicles, equipment, boots and animals that have been in infested areas. If you have a wetland dominated by native plants, monitor the edges and interior for islands establishing; immediately remove any RCG you find. Monitor for infestations in any disturbed wetland, including wetland restoration sites. RCG is identifiable year round, in winter when other grasses are dormant the straw-colored stalks are an easy feature to ID (King County, 2015).
Proper disposal of plant parts is another key component for preventing the spread of RCG. Many plant parts will form roots if left in contact with moist soil. If composting on site, dry thoroughly on a tarp or black plastic before composting. Rhizomes, plants in seed, and sod should be disposed of in a landfill. Plant parts can be burned where conditions allow. All plant parts, including seed, roots and sod, can be buried a minimum of two feet deep in weed-free soil. Buried RCG must remain undisturbed for at least four years (King County, 2015).
Manual and Mechanical Control
Early in the colonization process, the physical removal of the invasive species can be an easy and efficient control method. Mowing or cutting stems can be a valuable method to control RCG, since they remove stems, leaf canopy and seed heads before maturation and expose the ground to light, which could promote the growth of native species (Lavergne & Molofsky, 2006).
Ideally cut before plants are in seed. Cut small patches with hand clippers or loppers as close to the ground as possible to prevent seeding. Cutting alone will not kill the plants (King County, 2015).
Mow using brush cutters, line trimmers, walk behind field and brush mowers, and tractor-drawn mowers. Mow at least 5 times per year for several years to prevent seeding and exhaust root reserves. Always clean tools after use to minimize risk of spreading plant fragments (King County, 2015). Lavergne & Molofsky, 2006; reported twice-yearly mowing (early June and early October) was shown to slightly increase native species diversity to the detriment of RCG in comparison to control plots. Removal of RCG above ground growth and mulch offers the advantage of substantial natural tree regeneration, establishing from natural seed rain after RCG cover has been reduced by mowing or other forms of control (Hovick & Reinartz, 2007). Like most invasive perennial plants with vigorous rhizomes, mechanical methods alone are not a sufficient control strategy for RCG because it can vigorously regrow from rhizome fragments and the seed bank (Lavergne & Molofsky, 2006).
Cultural control includes burning, manipulation of nutrient availability, covering, and revegetation.
In regions where fire was originally an important disturbance, managing fire may be effective because it could remove growing material in spring, weaken plants, prevent seed production and eliminate seed bank, possibly kill rhizomes, and allow more fire tolerant native species to compete successfully; however, fire can also harm native spring-blooming species (Lavergne & Molofsky, 2006).
The choice of fire date appears critical to the success of burning as a control strategy. Burns should be conducted in the late spring, and early to mid-summer (WA State NWCB; Lavergne & Molofsky, 2006). A two to three-year burn rotation or repeated late spring fires over five to six years can control RCG (Lavergne & Molofsky, 2006).
The use of fire can be easily included in an integrated management plan. Burning as a pre-treatment with other methods such as tilling, covering, or herbicide application can be very effective since it will remove above-ground dead litter. However, burning can stimulate growth if done at the wrong time, fall is the wrong time of year to burn RCG in Western Washington. Burning alone fewer than 5 times per growing season has been shown to increase reed canarygrass density. Carefully monitor the recovery of native wetland plant communities following burning stands of RCG (King County, 2015; Lavergne & Molofsky, 2006).
Manipulation of Nutrient Availability
Competitive control of RCG is possible if nitrogen availability is reduced by carbon enrichment, this could possibly favor native species with a higher nitrogen uptake capacity under nitrogen poor conditions. Offering a competitive advantage to native species previously outcompeted by RCG (Lavergne & Molofsky, 2006).
Covering with shade cloth or landscape fabric is another option. Cover with commercially available shade cloth, secure tightly with stakes, rebar, large garden staples or other appropriate devices. Overlap sections of cloth by at least one foot, and extend the coverage at least two feet beyond the edge of the infestation. Shade cloth should be left in place for at least two growing seasons (King County, 2015).
Similar to covering with shade cloth sheet mulching involves using several layers of thick, clean cardboard (no tape or staples). Overlap pieces by at least one foot and extend coverage at least two feet beyond edge of infestation. Cover cardboard with at least four inches of mulch (King County, 2015).
Combining covering with a pre-treatment of herbicide, burning or cutting/mowing beforehand can increase efficacy. Dense planting of willow livestakes (2-3 feet apart) has also been successful.
Covering RCG could limit regrowth of RCG, but this method may not be fully effective against RCG since rhizomes can still persist after two years under plastic and some shoots can grow through it. Seasonal flooding can also displace covering materials (Lavergne & Molofsky, 2006).
Shading is the best long-term control strategy. Where possible, establish a multi-layered dense canopy, preferably with conifers in the overstory (King County, 2015). Empirical evidence has shown that RCG is sensitive to competition for light at germination and earlier development stages (Lavergne & Molofsky, 2006). Revegetate with shade producing or highly competitive native species. Choose plants that will thrive in your location. Dense plantings of willows, cottonwood, red alder, and Sitka sprucehave shown to reduce RCG once a canopy is formed. Competitive ground covers such as slough sedge and Lyngby’s sedge can also compete with RCG if combined with other forms of control. 6ft tall willow or cottonwood livestakes installed at a density of 2-3 feet on center in areas of shallow inundation or high soil moisture content, can diminish RCG within two growing seasons (King County, 2015).
After control, sowing seeds of natural competitors alongside replanting or as a standalone method may also suppress RCG infestation. Seeding native forbes and herbaceous species may benefit the survival of installed willow stakes and conifers (Lavergne & Molofsky, 2006). A diverse and native plant community has consistently been found to be an important factor in plant survival because of the variety of safe sites the community has available for establishment (Hovick & Reinartz, 2007).
One of the most crucial requirements for a successful restoration is the creation of suitable safe sites where trees and shrubs can establish and that they remain present for the initial growth and establishment of native species (Hovick & Reinartz, 2007). Pre and post-planting herbicide treatments will enhance survival by creating safe zones where installed natives are free from the aggressive resource allocation of RCG. Monitor and spot control RCG for several season until trees and shrubs are large enough to compete with RCG and provide adequate shade to limit regrowth. (King County, 2015). (Miller, Martin, & MacConnell, 2008).
Biological control is any strategy that uses an herbivore or pathogen species to contain the spread of an invasive species. Bio-control assumes that the invasive species benefits from relaxed herbivory pressure in its introduced range so that the introduction of a specialist exotic enemy will control the invasive plant (Lavergne & Molofsky, 2006). Currently, there is no evidence that RCG’s invasiveness in North America is due to herbivore release and no biological control methods are currently used against RCG (Lavergne & Molofsky, 2006).
Never apply non-aquatic herbicide formulations directly or indirectly to water, since many include ingredients toxic to aquatic organisms (King County, 2015). In aquatic environments, herbicide application can increase levels of water toxicity in the short term and modify water chemical properties, pH, nutrient status, and bacterial populations. Post-emergence chemical treatments may have less adverse effects on the entire ecosystem and are more commonly used for RCG control (Lavergne & Molofsky, 2006).
The best time to apply herbicide is in late spring to actively growing plants before flowering or in the fall before the first hard frost. Flowering plants that are sprayed, may still be able to after ripen and produce viable seed. Always follow the herbicide label as specific herbicides have specific timing and application requirements.
Small patches of RCG can be spot sprayed or wick wiped, being careful to avoid damaging surrounding vegetation. Small patches less than 2 feet in dimeter can also be tied in a bunch just before flowering, then cut above the tie and treated with a 33% glyphosate solution (King County, 2015).
Larger patches will require multiple treatments, treat for at least three years to exhaust the seed bank (King County, 2015; Lavergne & Molofsky, 2006). To reduce herbicide used, start by spraying the outer boundary of the patch and working your way into the center in layers over multiple years. Another, option is to mow 3ft diameter circles 2-4 ft on center in August, followed by an herbicide application in the fall once regrowth is 2ft tall, and installation of an aggressive site specific shade producing tree or shrub in the winter.
The following herbicides provide good control of RCG, however monitor treatment areas as RCG may recolonize from adjacent stands or from seed bank recruitment. Only glyphosate and imazapyr are licensed for use in aquatic systems in WA (WA State NWCB).
- 1.5 to 5% solution with a nonionic surfactant approved for aquatic areas (King County, 2015).
- Both early and later herbicide applications with glyphosate (late May and late August) the two treatments alone or in combination, significantly reduce RCG stands (Lavergne & Molofsky, 2006).
- Apply to actively growing plants at early heading or in fall from mid-September to after first light frost (PNW Weed Management Handbook).
- 1.5% solution with a nonionic surfactant approved for aquatic areas (King County, 2015).
- Imazapyr remains active in the soil and should not be applied to sites that will be replanted within the following year.
- Apply in boot stage (seed head starting to come out of the leaf sheath before flowering) through fall, when plant is actively growing (PNW Weed Management Handbook).
- Selective herbicide for grasses and monocots and appropriate for late fall treatments (Lavergne & Molofsky, 2006).
- Not approved for aquatic use in WA State.
Combinations of Control Methods
Depending on feasibility, site conditions and resources, use mowing, covering, planting, burning or herbicide alone or in combination to remove or kill established plants (King County, 2015).
Mow in late spring and again in August, spray in October to November, repeat for at least three growing seasons (King County, 2015). Replant or follow with heavy seeding of native species to limit RCG establishment or secondary exotics (Lavergne & Molofsky, 2006).
Burn in late spring, mow in late summer, and spray in October to November. Replant in winter and follow up with herbicide spot treatments for at least three years. Additionally an herbicide application in the fall prior to the spring burn can increase control efficacy (Hovick & Reinartz, 2007).