different between ragweed vs duckweed

English

rag +? weed

ragweed (plural ragweeds)

A plant of the genus Ambrosia. These weeds are particularly noted for producing pollen which people with hay fever are allergic to.

duck +? weed. Supposedly so-named for often being consumed by ducks and other waterfowl.

duckweed (countable and uncountable, plural duckweeds or duckweed)

Any of several reduced floating aquatic plants in the subfamily Lemnoideae of the family Araceae.
- 1909, H. G. Wells, The Beautiful Suit
  But his face was a face of such happiness that, had you seen it, you would have understood indeed how that he had died happy, never knowing that cool and streaming silver for the duckweed in the pond.

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