duckweed

But the farm had fallen on evil days at the hands of Paddy Magragh, and the reservoir was choked with cresses and duckweed.
Lives of the Fur Folk|M. D. Haviland
Waterlilies and duckweed are not the chief part of this floating mass.
The Last Journals of David Livingstone, in Central Africa, from 1865 to His Death, Volume I (of 2), 1866-1868|David Livingstone
There is a large pond in the midst of the garden, covered with duckweed, and full of beautiful gold and silver fish of many kinds.
A Voyage in the 'Sunbeam'|Annie Allnut Brassey
The river is rising fast, and bringing down large quantities of aquatic grass, duckweed, &c.
The Last Journals of David Livingstone, in Central Africa, from 1865 to His Death, Volume II (of 2), 1869-1873|David Livingstone

Out of this lagoon, and by this stream, the chief part of the duckweed of the Shiré flows.
A Popular Account of Dr. Livingstone's Expedition to the Zambesi and Its Tributaries|David Livingstone

British Dictionary definitions for duckweed

duckweed

/ (ˈdʌkˌwiːd) /

noun

any of various small stemless aquatic plants of the family Lemnaceae, esp any of the genus Lemna, that have rounded leaves and occur floating on still water in temperate regions

Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012