You may find the Virginia Tech Grass Weed key useful, as well as the
Purdue University Turfgrass Identification Tool. The University of
California also posts the UC Guide to Healthy Lawns identification key
to weeds in turfgrass. BASF also provides a Weed ID Guide for Grassy
Weeds. You can find links to these and other online plant ID databases
on Ergonica World of Weeds EWIRM.

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EWIRM: One good weed deserves another.