P. crispus is a productive, submersed macrophyte that is non-native and invasive in temperate areas of North America, New Zealand, and southern South America (Kaplan and Fehrer, 2004). The species is listed as a noxious or prohibited weed in several areas of the USA (USDA-NRCS, 2008). The species is a cold weather strategist, which allows it to establish early in the growing season (Nichols and Shaw, 1986). Unlike most macrophytes, P. crispus plants typically die back by early summer and lie dormant until temperatures decrease again in autumn (Bolduan et al., 1994). It is also a productive species that tends to form monocultures, thereby decreasing the amount of light available to other species (Engelhardt, 2006). However, its impact on native species is disputed in the literature. Some authors state that early emergence helps P. crispus out-compete natives, while others aver that its characteristic summer die-back removes it from such competition (Bolduan et al., 1994). However, this documented summer senescence is problematic, as the inevitable decay of organic material and resultant nutrient release can stimulate algal blooms and lead to dissolved oxygen crashes. Additionally, this species is particularly hard to control due to its prolific production of turions (Yeo, 1966).
Potamogeton crispus, of order Najadales, family Potamogetonaceae, is a monocotyledonous forb first described by Linnaeus in 1753. The genus Potamogeton contains approximately 100 species (Flora of North America Editorial Committee, 1993) and is cosmopolitan. Historically, the genus has been segregated into several sections and subsections (Hagström, 1916), although more recent examination of collected specimens has led taxonomists to regard these divisions as unnecessary (Flora of North America Editorial Committee, 1993). Within the genus, species have been classified by leaf shape, whether they are heterophyllous, seed morphology, and chromosome number (Iida et al., 2004). Although there is a high degree of morphological similarity among members of the Potamogeton genus, P. crispus is generally regarded as one of its more distinctive members. Hybridization among members of this genus is common. Hybrids have been reported worldwide, and include P. x cooperi (Kaplan and Fehrer, 2004), P. x bennetti (Strasse, 1997), P. polygonifolius and P. lintonii (Neveceral and Krahulec, 1994).
P. crispus is an herbaceous, submersed aquatic species that typically grows with stem up to 1m long. Its sessile, linear leaves are light to dark-green. They are typically from 1.2-9 cm long, 4-10 mm wide and are spirally arranged on flattened cauline stems. Leaves are homophyllous, often undulate, with obtuse apices and 3-5 veins. Margins are finely serrate. Lacunae are conspicuous and occur in rows of 2-5 along the midrib of the leaf. Stipules are not fused to the leaf and persistent, though inconspicuous. Leaves and stem are lax; the plant is either entirely submersed or nearly entirely submersed with some leaves floating at the surface. Nodal glands in this species are entirely absent. Inflorescences are unbranched and emersed, generally terminal (Flora of North America Editorial Committee, 1993). Flowers are tiny, with four petal-like lobes on spikes 1-3 cm long on stalks up to 7 cm long (Washington State Department of Ecology, 2008). Sessile reddish-brown single-seeded fruits are unkeeled and measure 6 x 2.5 mm. Fruits have a small recurved beak that measures 2-3 mm. Embryo has full spiral. Short, bur-like hardened turions, in which internode length is extremely shortened, measure 1.3-3 by ~2 cm, are common and can be either apical or axillary (Flora of North America Editorial Committee, 1993; USACE, 2002).
The Potamogeton genus is one of the most important, well-represented and cosmopolitan genera of submersed macrophytes; it is extremely important to the structure and function of aquatic ecosystems across the globe (Haynes, 1975). P. crispus is widespread throughout much of its native range, which is commonly reported to include Europe, Asia, African and Australia (Bolduan et al., 1994), whereas it is non-native and invasive in temperate areas of North America, New Zealand, and southern South America (Kaplan and Fehrer, 2004).
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.
The first verified report of P.crispus in North America came from Philadelphia, PA in 1841-42, and after initial introduction, the plant began to establish quickly (Bolduan et al., 1994). By 1860, its growth in Delaware and Pennsylvania was described as abundant, and it spread rapidly in areas of Massachusetts and New York by the early 1880s (Bolduan et al., 1994). It is likely that spread inward from this eastern population accounts for the first Canadian record in 1891 (Stuckey, 1979). Occurrences were subsequently reported just west of the Mississippi River by 1903, and by 1896, the population had spread to the western seaboard (Bolduan et al., 1994). Stuckey (1979) hypothesized accidental introduction during stocking activities as the primary vector for spread in New England, and also noted that the plant was at first intentionally planted due to its suitability as habitat and food source for wildlife. P. crispus currently remains widespread throughout temperate North America, where local populations continue to expand. It has been estimated that it currently occupies anywhere from 30-90% of its potential range (Tomaino, 2004). The first record in New Zealand occurred in 1940, although the first unofficial collection occurred earlier. At least some of the New Zealand introductions can be attributed to accidental or intended plantings (Healy and Edgar, 1980). Scant information is available on the population reported in South America.
Fragments of P. crispus can spread long distances, especially via the dormant apices that are so prolifically produced during summer. Thus, unintentional introduction via fragments transported on boats and equipment is a significant risk (ISSG, 2006). Maki and Galatowitsch (2004) also found that 10% of aquatic plant mailings in the horticultural trade contained regulated noxious species, including P. crispus, indicating a significant risk of introduction through horticultural activities. This species is easily acquired for intentional planting, even in states in which its sale, transportation, and release are regulated (Maki and Galatowitsch, 2004). Additionally, P. crispus is spread naturally over long distances via waterfowl, especially in areas along migratory routes (Boylen et al., 2006). Fragments can locally expand populations by passive spread in flowing water or during flood events.
P. crispus occurs worldwide in rivers, streams, freshwater and brackish lakes, marshes and ponds (Iida et al., 2004), and generally prefers alkaline, calcareous eutrophic waters (Bolduan et al., 1994). It is disturbance-tolerant and is commonly associated with impacted, disturbed, sometimes highly polluted, sites (O’Hare et al., 2006). It is also able to survive in a wide range of sediments, from gravel or fine sand with low organic content to loamy mud and clay (Bolduan et al., 1994). This is in part due to the ability shared by many aquatic species to acquire nutrients from the surrounding water as well as through roots. It is also important to note that this cold-tolerant species is evergreen and will grow through winter, often under thick ice cover (Stuckey et al., 1978).
Given this species’ tendency to grow in monocultures with high productivity, it has been reported to cause decreases in biodiversity by out-competing native plants (Tomaino, 2004). However, it should be noted that the impact of this species on the native community is disputed, with some authors concluding that because the plant acts like a winter annual it does not negatively impact native species (Bolduan et al., 1994). It can be productive, but is not generally reported as a nuisance in its native range.
The genus Potamogeton is highly morphologically and ecologically diverse. Species within the genus have been classified by leaf, seed and pollen morphology as well as chromosome number. Recent molecular analyses have allowed for a detailed exploration of the phylogenic relationships within the genus, indicating that this monophyletic group can be split into two major groups (Iida et al., 2004). P. crispus is unique in carrying a long deletion in the trnT-trnL sequence; results of allozyme analysis suggests that P. crispus is remotely related to and diverse from other taxa in the genus (Iida et al., 2004). Rutland (1941) reports P. crispus as having a chromosome number of 2n=52. Missouri Botanical Garden (2009) notes reports of other chromosome numbers of 26 and 82-85. The species itself has been described as being only moderately genetically variable, with little isozyme genetic diversity within New Zealand populations (Hofstra et al., 1995). Many hybrids have been reported in the literature, including P. x cooperi (with P. perfoliatus) (Kaplan and Fehrer, 2004), P. x bennetti (Strasse, 1997), P. polygonifolius and P. x lintonii (with P. friesii) (Neveceral and Krahulec, 1994).
P. crispus reproduces mainly vegetatively via rhizomatic spread as well as with vegetative propagules called turions. Turions are formed from buds along the stem at or near peak biomass depending on day length, water temperature, and light intensity (Bolduan et al., 1994). Production is quite prolific: a single turion planted in a 5.9 square metre container yielded 23,250 turions in a single growing season, and densities from 236 – 1648 turions per square metre have been reported in the field (Nichols and Shaw, 1986). High rates of germination have been reported in the lab (100%) and in the field (> 60%) (Bolduan et al., 1994). Turion germination is controlled by light and temperature, and requires a cold (5ºC) or hot (30-35ºC) period to break dormancy (Bolduan et al., 1994). In a South African lake, turion germination was initiated when water temperature fell below 25ºC (Rogers and Breen, 1980) and was inhibited by darkness (Jian et al., 2003). The species does produce seeds, sometimes at very high densities, but field germination rates are extremely low (e.g. 0.001%) (Rogers and Breen, 1980).
Physiology and Phenology
P. crispus has a unique life cycle; it typically acts as a winter annual. After achieving peak biomass (in May in North America) the plant produces turions and dies back completely (Bolduan et al., 1994). The turions remain dormant through the summer months. As the water cools off near the end of summer, the turions germinate, producing the winter growth form. Thus the plant has two periods of peak biomass, once in the spring and once in the autumn. After autumn germination, the plant spends the winter actively growing; its low light requirement allows it to subsist even under ice (Nichols and Shaw, 1986; Bolduan et al., 1994). The species therefore appears to be a cold-weather strategist; this allows the plant to establish early and either avoid competition with or out-compete other macrophytes (Bolduan et al., 1994).
P. crispus is a cosmopolitan species, associations have been reported in the literature for more than 30 plant species, including many other Potamogeton species (Bolduan et al., 1994). The species hosts great numbers of invertebrates, chironomids being the most important group (Nichols and Shaw, 1986).
P. crispus is cold-weather and low-light adapted (Tobiessen and Snow, 1984), allowing it to exist in deeper or more turbid waters than many other species (Jian et al., 2003). It has been reported to typically grow in water from 1-3 m deep, although sometimes it can be found in water up to 7 m deep. Photosynthetic rate is highest at 30ºC, but vegetative growth has been reported to survive temperatures of 1-4ºC in the field (Bolduan et al., 1994). USDA-NRCS (2002) reports an absolute minimum temperature of -33ºC; active growth stops when temperatures drop below 5ºC. The species is typically associated with eutrophic alkaline sites, and is extremely tolerant of high nutrient systems (7.5 mg P L-1, 75 mg N L-1) (Mulligan et al., 1976). Its main phosphorus source is the sediment, whereas it acquires nitrogen and potassium from the surrounding water (Nichols and Shaw, 1986). A study in Wales shows that P. crispus lakes all had conductivity >150mS and Ca+Mg/Na+K hardness ratios >3 (Bolduan et al., 1994).
P. crispus spreads vegetatively via rhizomes and turions. Turions can be distributed passively via water flow (ISSG, 2006).
Turions are small and probably spread over long distances by wildlife such as waterfowl. Dispersal via waterfowl has been documented along major migratory routes in New York, USA (Boylen et al., 2006).
P. crispus can increase algal blooms (Nichols and Shaw, 1986), which can decrease the aesthetic value of a water body. Monotypic stands of this species can be quite a nuisance, presenting significant navigational difficulties to recreational users (Bolduan et al., 1994).These factors have a significant impact on the recreational and real estate value of a water body, and may also have an impact on the tourism industry. Impacts are greatest in the species’ introduced range, where it is considered a noxious weed (USDA-NRCS, 2008).
Massive stands of P. crispus substantially alter a water body’s internal loading; it can also reduce the fetch of a lake, sometimes inducing stratification in normally unstratified systems (Bolduan et al., 1994). It has been shown to produce the highest shoot biomass in a comparative study that evaluated four related macrophyte species (Engelhardt, 2006). It can grow in dense monotypic stands and affect habitat structure, which may have impacts on fish species, including those sought both commercially and recreationally (Crowder and Cooper, 1982). P. crispus has been reported to decrease the amount of light reaching the sediment surface (Engelhardt, 2006). However, the plant may have positive effects in extremely degraded systems. Feng et al. (2002) report that planting of P. crispus in enclosures improved water transparency, decreased electric conductivity and increased pH, and was shown to have an inhibitory effect on green algae.
Impact on Biodiversity
Several sources report that P. crispus has a negative effect on macrophyte biodiversity and often out-competes native plants (ISSG, 2006). The species is found at sites where a globally rare species exists, where it probably competes with it (Tomaino, 2004). However, the variety of fungus species reported growing on dead fragments was high in P. crispus relative to other species (Czeczuga et al., 2005).
This plant can cause substantial nuisance to recreational users by impeding navigation and tangling fishing line. This species can also reduce swimming access, and stimulate unsightly, possibly toxic algal blooms. Its environmental effects can decrease the aesthetic value of a water body as well as affect property values and tourism.
P. crispus can be used in the treatment of industrial aqueous waste, obviating the need for chemical treatment (Hafez et al., 1998).
P. crispus has been proven to be a good resource for carotenoids, which are often used in medicine and cosmetics for their anti-oxidation, immunity-regulation and tumour proliferation-slowing properties. Carotenoids like the ones extracted from P. crispus plants are also used as colourants and antioxidants in food additives (Ren and Zhang, 2008). The species has been used as an ethnobotanical treatment of cancer (Duke, 2008).
P. crispus aids in the self-purification of water bodies over the winter by aiding in resettlement of suspended mud and sand (Cao and Wang, 2007). The plant has also been shown to improve water transparency, decrease electrical conductivity and increase pH in eutrophic systems (Feng, 2002). Given the species’ pollution tolerance, it is a viable candidate for the revegetation and restoration of extremely impacted sites. The plant also provides good fish and invertebrate habitat and is a valuable food source to aquatic herbivores.
P. crispus is easily distinguished from other pondweeds by leaf and fruit morphology, phenological characteristics, and turion production (Iida et al., 2004). P. crispus is unique in that its leaf margins are finely serrate. Difficulties in identification may occur very early or late in the growing season, when turions are germinating. At these times, the plant develops a winter growth form with very slender, limp, blue-green leaves (Bolduan et al., 1994). Serrations are present in the winter growth form, though are not as conspicuous. However, members of the genus Potamogeton hybridize readily, and produce individuals with intermediate morphological characteristics (Kaplan and Fehrer, 2004).
Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.
The vegetative propagules of this species are very easy to spread. Therefore, educational programs are usually necessary to decrease this form of human-mediated spread. Teaching users how to clean equipment in a way that decreases the chance of transmission is one way to lessen the impact of the human vector. Several of the USA states have legislated regulation of the purchase, transportation, and introduction of this species.
This species produces turions prolifically, and because the turions can stay dormant yet viable for at least 2 years (Tomaino, 2004), rapid response to decrease turion deposition is integral to successful management.
Numerous educational campaigns have been directed at informing the public about the danger of aquatic invasive species. Agencies in areas in which P. crispus is particularly problematic commonly distribute informational materials about its identity as well as how to report new invasions. Other educational campaigns have been directed towards informing the public about how to clean equipment in order to prevent the transportation and spread of invasive species.
No reports of eradication exist in the literature.
Cultural control and sanitary measures
Turions are easily transportable and can remain dormant for up to 2 years. Thus, it is extremely important to decrease the instances of accidental introduction by addressing humans as a vector. By establishing guidelines on how to properly clean equipment, dispose of water, and identify target plants, it is likely that instances of accidental transportation and release will be fewer.
Mechanical harvesting may be used to obtain some nuisance relief, but reviews of efficacy of control are mixed. In Michigan, USA the dominance of P. crispus was only reinforced by harvesting at the expense of natives (Bolduan et al., 1994), whereas it has been shown elsewhere that early season cutting at the sediment surface prevented turion production (ISSG, 2006). Some have used winter drawdowns as a means of control, but the literature reports no significant impact of overwinter drawdown on P. crispus (Nichols and Shaw, 1986). Shallow dredging also has mixed reviews, at times there seems to be little effect, and in some cases lasting control has been achieved (Tobiessen and Snow, 1984; Tomaino, 2004).Other mechanical methods including benthic barriers, hand removal, rotovation and shading have been reported as successful (USACE, 2002).
Screening has been used to stop the movement of turions. However, because plants can spread via fragments, much attention has been given to decreasing human-mediated dispersal. The plant is on a number of state noxious lists. Some states have put in place legislation to regulate the sale, transportation and introduction of P. crispus.
P. crispus is sensitive to 2,4-D, especially during early spring (Wolf and Madsen, 2003; Belgers et al., 2007). P. crispus is susceptible to endothall-based herbicides (Skogerboe and Getsinger, 2002). It is also suggested that treatments occur in early spring in order to lessen the impacts on the native plant community (ISSG, 2006). The herbicides fluridone and diquat have also been used and in general, chemical treatments provide relief for one growing season (ISSG, 2006).