Morphological identification
Although some species can be readily recognized using morphology alone, for many species the differences are subtle or restricted to flowers, fruits or seeds which are rarely present. Furthermore intraspecific variation can be large and several species are known to hybridize, e.g., L. minor and L. gibba, and L. minor and L. turionifera, impeding a reliable identification.
A recent determination key can be found here.
Molecular identification
Molecular identification of duckweed using plastid or nuclear barcodes (Borisjuk et al., 2015; Bog et al. 2015) or PCR based methods such as amplified fragment length polymorphism (AFLP) fingerprinting (Bog et al. 2015), tubulin-based polymorphism (TBP) fingerprinting (Braglia et al. 2021), is well established. Nevertheless there is no method that can reliably identify all 35 species and hybrids.
Integrated approach
Morphological and molecular identification both have their advantages and drawbacks. As such the most reliable identification combines both. Additionally we recommend to use flow cytometry to verify the ploidy level.
Registration
It is recommended that novel identified duckweed clones get registered via the Rutgers Duckweed Stock Cooperative. By doing this, other researchers can order this clone and repeat experiments or perform additional experiments using the same clone. This will facilitate future comparison of data reported in the literature related to duckweed and will minimize the proliferation of many different coding systems and nomenclature used by researchers in the field. Registration is done by assigning a 4 digit code number to the clone (see http://www.ruduckweed.org for more information).