Elodea bifoliata    St. John

Two Leaf Waterweed 


MN Status:
endangered
Federal Status:
none
CITES:
none
USFS:
none

Group:
vascular plant
Class:
Monocotyledoneae
Order:
Hydrocharitales
Family:
Hydrocharitaceae
Life Form:
forb
Longevity:
perennial
Leaf Duration:
deciduous
Water Regime:
aquatic
Soils:
silt
Light:
full sun
Habitats:

(Mouse over a habitat for definition)


Best time to see:

 Foliage Flower Fruit 
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Minnesota range map
Map Interpretation
North American range map
Map Interpretation

  Basis for Listing

Elodea bifoliata (two leaf waterweed) is an aquatic plant species that reaches the eastern edge of its range in Minnesota. While it is less scarce in western North America, it is apparently not common anywhere. Elodea bifoliata has been documented only twice in the state, at two widely disjunct locations:  in the St. Louis River near Duluth in St. Louis County (Laurentian Mixed Forest Province) and Crystal Lake in Blue Earth County (Prairie Parkland Province). These collections are dated 1949 and 1958, respectively, though it was some years later before an expert correctly identified them. 

It had been thought that E. bifoliata might begin to turn up once Minnesota lakes were inventoried by skilled aquatic botanists, but that has not been the case. Between the years 1995 and 2015, a total of 2,025 lakes were surveyed, and this plant was not found even once. Clearly there are many more lakes to be searched, and extremely small populations could have been missed, but it seems almost certain this species is very rare in Minnesota.

Unfortunately, E. bifoliata has gone so long without being seen in Minnesota that Minnesota botanists have no first-hand experience finding it. Listing it as endangered in 2013 may seem premature; however, the hope is it will serve to alert aquatic botanists to the importance of this cryptic species.

  Description

Elodea bifoliata is a perennial plant of fresh water. The stems are elongate and root in the substrate. Rhizomes and stolons are absent. Nodes of the stem have leaves either in pairs or in threes, never in fours. The leaves are sessile, 4.7-24.8 mm (0.19-0.98 in.) long, 1.8-4.3 mm (0.07-.17 in.) wide, usually linear in shape, with mostly parallel sides; the apex is acute. The flowers have three white petals, which are very small (< 0.66 cm [0.25 in.]), and occur singly at the ends of long and slender tubes. The flowers are unisexual, with male and female flowers on different plants. Male flowers have pollen in monads and sometimes detach and float free. The pollen is usually shed directly on the surface of the water. Female flowers also float on the surface of the water but remain attached. The fruit is capsular and produces 1-5 seeds (Haynes 2000). 

There are three species of Elodea (waterweeds) in North America, and all three occur in Minnesota. Elodea bifoliata can be distinguished from other members of the genus by having leaves occurring mostly in pairs at the nodes, rather than in threes; and by having pollen in monads, rather than tetrads.

  Habitat

In 1958, E. bifoliata was found in Crystal Lake in Blue Earth County. Crystal Lake is a shallow, very fertile and turbid hard-water lake. This lake is described as hypereutrophic due to the presence of nutrients such as phosphorus in the water. The lake has a history of algal blooms dating to at least the 1940s. The cause of this problem dates from around 1900, when a ditch was dug that drains into Crystal Lake. The ditch contributes considerable amounts of water and silt during spring thaws and hard rains. It appears that water quality in the lake was very poor in 1958, when E. bifoliata was known to occur there, and it has not improved substantially since. No subsequent plant surveys have been done in Crystal Lake, and it is not known if E. bifoliata still survives there. If it does, it must be extremely tolerant of poor water quality.

The only other known location of E. bifoliata in Minnesota is the St. Louis River at New Duluth in St. Louis County.  Conditions in the river in 1949, when a specimen of E. bifoliata was collected there, were quite different than those in Crystal Lake, but there is too little information to draw useful conclusions.

  Biology / Life History

Studies of the biology and life history of Elodea have largely been restricted to the more common and widespread E. canadensis (Canadian waterweed). Many of the general features of E. canadensis are probably similar to those of E. bifoliata and might serve as a useful stand-in.

It has been reported (Bowmer et al. 1995) that E. canadensis is able to grow slowly under ice cover and can survive inside ice. Also, E. canadensis produces vegetative propagules at the onset of winter or in unfavorable conditions. These propagules are condensed shoots and are able to root readily at the nodes. Ordinarily, Elodea is rooted at the bottom of a lake or stream, but free-floating stem fragments can continue to grow.

Roots overwinter in the substrate, with swollen shoot apices that begin to grow leafy stems in the spring (Bowmer et al. 1995). Seeds of Elodea are rarely found, though it has been reported that E. bifoliata is highly fertile (Cook and Urmi-König 1985).

Dispersal by seed seems likely, though perhaps infrequent. However, stem fragments and vegetative propagules easily float on water currents and can be carried throughout a water body and between water bodies, if they are connected by a stream. It also seems likely that propagules could be carried from one isolated water body to another by water birds.

  Conservation / Management

At this time, there is too little occurrence data to make useful management recommendations. It appears that E. bifoliata may not be dependent on high water quality, but that will remain uncertain until at least one extant population of this species can be found in Minnesota.

  Best Time to Search

The species should be visible most of the summer, but positive identification will be aided by the presence of flowers, which would likely be present from late June through July.

  Authors/Revisions

Welby Smith (MNDNR), 2018

(Note: all content ©MNDNR)

  References and Additional Information

Bowmer, K. H., S. W. L. Jacobs, and G. R. Sainty. 1995. Identification, biology and management of Elodea canadensis, Hydrocharitaceae. Journal of Aquatic Plant Management 33:13-19.

Catling, P. M., and W. Wojtas. 1986. The waterweeds (Elodea and Egeria, Hydrocharitaceae) in Canada. Canadian Journal of Botany 64(8):1525-1541.

Cook, C. D. K., and K. Urmi-Knig. 1985. A revision of the genus Elodea (Hydrocharitaceae). Aquatic Botany 21(2):111-156.

Haynes, R. R. 2000. Elodea. Pages 32-34 in Flora of North America Editorial Committee, editors. Flora of North America north of Mexico. Volume 22. Oxford University Press, New York, New York.

Minnesota Department of Natural Resources. 2003. Field guide to the native plant communities of Minnesota: the Laurentian mixed forest province. Ecological Land Classification Program, Minnesota County Biological Survey, and Natural Heritage and Nongame Research Program. Minnesota Department of Natural Resources, St. Paul, Minnesota. 352 pp.

Minnesota Department of Natural Resources. 2005. Field guide to the native plant communities of Minnesota: the prairie parkland and tallgrass aspen parklands provinces. Ecological Land Classification Program, Minnesota County Biological Survey, and Natural Heritage and Nongame Research Program. Minnesota Department of Natural Resources, St. Paul, Minnesota. 362 pp.

NatureServe. 2009. NatureServe Explorer: An online encyclopedia of life [web application]. Version 7.1. NatureServe, Arlington, Virginia.