Myriophyllum heterophyllum Michx.
Broadleaf Water Milfoil
Basis for Listing
As of 2017, the Minnesota Biological Survey has completed floristic surveys of over 2,000 lakes. One notable result is that Myriophyllum heterophyllum (broadleaf water milfoil) was found only five times (always in the Laurentian Mixed Forest Province). There is little doubt this species is very rare in Minnesota, though other doubts do persist. For example, empirical evidence seems to support the conclusion that M. heterophyllum is native to Minnesota; however, some sources (NatureServe 2015) believe it is likely native only in southern states, and that some populations in northern states (including Minnesota) could be adventive (non-native; not fully naturalized). It is also uncertain why it has been found in so few lakes, and why it had not been found anywhere in Minnesota until 1992.
Based solely on the documented rarity of M. heterophyllum in Minnesota, it might be deserving of a status of endangered or threatened. However, it was given the lesser status of special concern in 2013 pending more information and the results of additional survey work.
Myriophyllum heterophyllum is a submerged aquatic species. The stem can grow 1-3 m (3-10 ft.) long and up to 3 mm (0.12 in.) thick. The stems are derived from a rhizome, which is rooted at the bottom of the lake, and in some cases they can grow so long that the upper portions form a tangled mat at the surface. The leaves are whorled, 2-4 cm (0.8-1.6 in.) long, and seldom over 1.5 cm (0.6 in.) apart on the stem. Each leaf is divided into 4-10 pairs of threadlike segments. The flowers are on a stout spike that rises 5-15 cm (2-6 in.) above the surface of the water. The bracts that subtend the male flowers are sharply serrulate and much exceed the flowers in length. This last character is diagnostic and separates it from the common M. sibiricum (shortspike watermilfoil).
In Minnesota, M. heterophyllum has been found in five lakes, ranging in size from 104 to 145 ha (256-355 ac.). Plants were found in both clear water and dark-stained water, at depths ranging from 1-2 m (3.3-6.6 ft.). Substrates included sand, gravel, and silt.
Biology / Life History
There is very little published information concerning the biology and life history of M. heterophyllum. Like most members of its genus (Myriophyllum; watermilfoil), the stems and leaves grow underwater, and the flowering spikes rise above the water. The flowers are unisexual and attract flying insects, which are the vector for pollination. Although seed dispersal has not been documented, it is likely facilitated by water currents and possibly aquatic animals, specifically birds and mammals. It is also believed the species can spread by vegetative methods. This would occur when a stem fragment (with leaves) is carried by water currents or an aquatic animal to a previously unoccupied habitat. Such fragments may, under certain conditions, have the capacity to produce roots or new stems.
It has been reported (fide Aitken 1981) that at a site where M. heterophyllum occurred, receding water stranded a colony of M. heterophyllum. When this happened, the plants survived in a terrestrial form that was characterized by tiny shoots, rarely over 10 cm (4 in.) high, and cutinized leaves, which are resistant to drying. Although this terrestrial form presumably does not produce flowers or fruits, it probably allows the survival of individuals, and perhaps whole populations, in years when the water level is unusually low.
It has also been reported that M. heterophyllum forms winter buds, which are commonly called "turions" (fide Aitken 1981). Such buds typically survive when the stems and leaves die-back at the end of the year and act as vegetative propagules.
Conservation / Management
The populations of M. heterophyllum should be closely monitored, annually if possible. This would require that the spatial extent and density of the populations be determined as accurately as possible. If the populations are small, then a complete mapping of the population boundaries ought to be undertaken. If the populations are large or dispersed, then it may be necessary to use sampling techniques such as transects.
Assuming M. heterophyllum is native to Minnesota (which is the prevailing opinion), and if the populations begin to decline, and if that decline is determined to be the result of human action, rather than a natural fluctuation, then the situation needs to be mitigated.
Best Time to Search
Mature inflorescences are useful for identification of M. heterophyllum. Unfortunately, phenological information is rather scanty. However, it is estimated that the best time to search is from the beginning of July to the middle of September.
Welby Smith (MNDNR), 2018
(Note: all content ©MNDNR)
References and Additional Information
Aiken, S. G. A conspectus of Myriophyllum (Haloragaceae) in North America. Brittonia 33(1):57-69.
Crow, G. E., and C. B. Hellquist. 2000. Aquatic and wetland plants of northeastern North America. Volume 1. Pteridophytes, Gymnopserms, and Angiosperms: Dicotyledons. University of Wisconsin Press, Madison, Wisconsin. 448 pp.
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.
NatureServe. 2015. NatureServe Explorer: an online encyclopedia of life [web application]. Veresion 7.1. NatureServe, Arlington, Virginia. Accessed 22 August 2016.
Robert W. Freckmann Herbarium. 2009. Flora of Wisconsin: Consortium of Wisconsin Herbaria [web application]. University of Wisconsin, Stevens Point. <http://wisflora.herbarium.wisc.edu/>. Accessed 8 July 2009.