Gladden (Oxford English Dictionary)

“They took a boat and went down to the Gladden Fields, where there were great beds of iris and flowering reeds.” (LotR - i. ii)

Gladden is Tolkien's updating of the Old English word glaedene 'iris'. This is recorded in an Old English document from the 9th (or possibly the 8th) century. In the names Gladden River and Gladden fields in The Lord of the Rings, Tolkien intended the word to refer to the 'yellow flag' (Irispseudacorus), which grows by streams and in marshes (Lett. 297). The Old English word in fact continued in later use in the slightly different form gladdon, though, as Tolkien went on to explain, the name is now usually applied to a different species, the purple-flowered Irisfoetidissima (or 'stinking iris'). The word is now rarely found outside regional dialects.

The OED entry (gladdon) quotes a line from the Middle English Romance The Wars of Alexander, evoking an image reminiscent of the Gladden Fields: a dryi meere was full of gladen & of gale & of grete redis (a dry lake was full of gladdon and bog-myrtle and great reeds).

The Ring of Words ~
Tolkien and the Oxford English Dictionary
Oxford University Press (2006)