Milfoil identifications


The identification of watermilfoils is difficult based on morphology because diagnostic morphological characters are related to flowers and fruits primarily.  However, because milfoils can reproduce both sexually and asexually, they are not always in flower.

Submerged watermilfoil in a swamp in South Carolina. Vegetative material for watermilfoils is very difficult to identify to species because most species essentially look the same!


The identification of watermilfoils is important because they are a heavily managed group of aquatic plants.  In particular, several species are considered as invasives in all or part of the US.  For example, Eurasian watermilfoil is a nuisance invasive throughout much of the US, especially the Great Lakes region.  Similarly, although variable leaf milfoil is native to much of the US, it is considered invasive in the northeastern and western US.  However, invasive milfoils commonly look very similar to native species.  For example, Eurasian watermilfoil is very similar in morphology to its native congener northern watermilfoil.  Furthermore, invasive milfoils can be hybrids, which can also be difficult to distinguish from natives.


Thus, we employ genetic tools to identify milfoils for various state agencies across the US.  The most common method of molecular identification is using DNA sequences from the internal transcribed spacer (ITS) regions of the nuclear ribosomal DNA.


In addition, we are exploring amplified fragment length polymorphisms (AFLPs) as an identification tool (see Matt Zuellig’s thesis project page).  While ITS may reliably identify species and hybrids, they may not be sufficient if 1) species share ITS alleles because of incomplete lineage sorting instead of hybridization, 2) hybrid genomes rapidly homogenize for ITS DNA sequences, or 3) the amount of genetic variation within and among species at ITS is too low to identify ecologically meaningful genotypic variation.


If you would like to send specimens for identifications, click here.

Page last modified April 15, 2010