Bay Grass Identification Key

​​The bay grass key was designed to allow you to identify most species of bay grasses found in Maryland. Although bay grasses are notoriously difficult to identify using standard taxonomic keys, the flexible format of the Internet allows us to combine detailed pictures, simple line drawings and text messages in a stepwise sequence that makes identifying bay grasses simple. You may find it useful to have a clear metric ruler with millimeters marked, a magnifying glass, and a Ziploc plastic bag to help you in the process of identifying your plant.

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Common Name:  
Scientific Name:

Common Name:Eurasian Watermilfoil

Scientific Name:Myriophyllum spicatum

Native or Invasive:Non-native to Chesapeake Bay; invasive

Illustration:Photo collage of Eurasian Watermilfoil

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Introduced from Europe and Asia, Eurasian watermilfoil is now found throughout the United States. Explosive growth of Eurasian watermilfoil during the late 1950's covered large areas of the Chesapeake Bay and its tidal tributaries. Eurasian watermilfoil choked waterways until the epidemic came to an end in the early 1960's, possibly due to spread of a virus-like organism in combination with pollution, grazing, and herbicide and harvesting programs. Eurasian watermilfoil is still present in the Chesapeake Bay and its tributaries today, inhabiting non-tidal fresh to moderately brackish tidal water and preferring soft mud to sandy mud substrates in slow moving streams or protected waters. Eurasian watermilfoil does not tolerate strong current or wave action. It is often the first species to appear in the spring in tidal tributaries with fairly degraded water quality and may be followed by other native species.

Up to 2.5 m (9 ft) tall, leaves in whorls of 4 or 5, finely divided (pinnate), 0.8 cm to 4.5 cm (1/3 in to 2 in) long with 9 to 13 hair-like segments per side. When removed from water these delicate leaves compress and lose their shape. Lower portions of the stems may be devoid of leaves.

Ecological Significance:
Eurasian watermilfoil was introduced to the Chesapeake Bay in the early 1900's. In the early 1960's, the population exploded in the Bay and could be found in almost all the tributaries. By 1970 the populations had died back and stabilized. This plant, while not considered a great food source for waterfowl, provides excellent cover for young fish, crabs and invertebrates. Fishermen recognize watermilfoil beds as excellent places to catch large mouth bass, which are often found lying in ambush near and amongst the stems of watermilfoil plants.

Similar Species:
Eurasian watermilfoil usually has whorls of 4 pinnate leaves whereas parrot feather (Myriophyllum brasiliense) usually has whorls of 5 pinnate leaves. Appearance is similar to coontail (Ceratophyllum demersum), however, coontail has whorls of 9 to 10 leaves at stem nodes, has stiffer leaves (especially when taken out of the water), and lacks a root system.

During late summer watermilfoil grows flower spikes on stem tips that protrude above the water surface. Self-pollination does not occur because the pistillate flowers on each individual reach maturity before its staminate flowers. Aerial pollination produces nut-like fruits that sink to the bottom where they can remain viable for years. Asexual reproduction occurs by fragmentation.

Print out a complete version of the key in PDF format (Adobe Acrobat file 18MB)

For permission to reproduce individual photos, please contact Mike Naylor

The text and photos used in this key were produced through a collaborative effort among the following partners.

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