HYBRID VARIABLE WATER-MILFOIL
Myriophyllum heterophyllum X Myriophyllum laxum
NOT NATIVE TO MAINE - INVASIVE
NOTE: All leafy milfoils display a wide range of vegetative variability. Any milfoil found in Maine waters should be considered “suspicious” until a positive identification has been confirmed by someone with the appropriate expertise.
The invasive variable water-milfoil hybrid is not explicitly prohibited by Maine law. However, the hybrid is half variable water-milfoil
and, as such, is treated as a prohibited invasive plant by the Maine Department of Environmental Protection.
variable water-milfoil hybrid stems
Habitat: The invasive water-milfoil hybrid is an extremely well adapted plant, able to thrive in a wide variety of environmental conditions. It grows well in still and flowing waters, and can survive under ice. The hybrid grows rooted in water depths from 1 to 5 meters on various substrates including organic muck, silt, sand and gravel.
Description: This hybrid milfoil is a cross between variable water-milfoil (M. heterophyllum) and loose water-milfoil (M. laxum), a milfoil native to the southeastern United States. The hybrid is a submersed, aquatic plant with branching stems emerging from dense, spreading roots. Feather-divided leaves are arranged in densely packed whorls. (Some of the leaves in the whorl may be slightly offset.) There are generally 4 to 6 leaves per whorl and 5 to 14 pairs of thread-like leaflets on each leaf. The dense leaf arrangement give this plant a bottle brush appearance. Stems are
typically reddish in color (even bright red). Leaves may also be red. Flowers occur on emergent spikes.
Two features (both present on the emergent flower spike) distinguish the hybrid from its invasive parent, variable water-milfoil (M. heterophyllum): 1) The bracts and flowers of the hybrid are arranged both alternately and whorled, as opposed to the strictly whorled arrangement found on the flower spikes of M. heterophyllum. 2) The bracts of the hybrid range from pinnately lobed, to elongate and entire. (Bracts on M. heterophyllum are mostly serrated and blade-shaped.)
The hybrid does not produce winter buds; M. heterophyllum does.
Origin and U.S. Range: Currently unknown, however it is known to be in Maine. Annual Cycle: The invasive milfoil hybrid is an extremely hardy aquatic perennial that propagates primarily through root division and fragmentation. Flowering spikes typically emerge from the water in mid to late summer, but not all colonies produce flowers. Toward the end of the growing season some plants break apart and die back to their rootstalks; others overwinter intact. New growth sprouts from roots and overwintering plants and plant fragments as the water begins to warm in the spring, growing rapidly toward the surface. Certain milfoils are able to hybridize with other, closely related, milfoil species. (See above.) Look Alikes: May be confused with bladderworts, hornworts, mermaid weeds, water crowfoots, and other leafy water-milfoils. Print out a copy for the field 
Flowers and bracts are arranged both
alternately and whorled along the
emergent flower spike; bracts range from
pinnately lobed, to elongate and entire;
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 Water Quality Monitoring |  Aquatic Invasive Monitoring |
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