The fourth line is quoted as "Against the sweet earth's flowing breast;" (with "sweet" at a different position) in text archive Bartleby.com's copy of the poem. (This seems to be a typo rather than a variant.)
van der Vliet, Henk (composer). "Trees / tekst van J. Kilmer" from 5 Songs for medium voice and piano (1977) (Amsterdam: Donemus, 1978 - publisher's number 05099). Retrieved July 21, 2013.
elcore.net
poetry.elcore.net
Kilmer, Joyce. "Trees" in The Poems of Joyce Kilmer (1918), reproduced at ELCore.net
foreverdreaming.org
transcripts.foreverdreaming.org
Transcript for episode 08x24 - The Simpsons Spin-Off Showcase [1]
hathitrust.org
catalog.hathitrust.org
McMillan, Margaret. Labour and Childhood. (London: Swan Sonnenschein, 1907), 127.
inkyfool.com
blog.inkyfool.com
Although an unscientific poll, Kilmer's couplet is ranked 26th of 50 with 1,080,000 Google hits, see: Forsyth, M(ark). H. "The Most Quoted Lines of Poetry" in The Inky Fool: On Words, Phrases, Grammar, Rhetoric and Prose (blog). (February 8, 2010). Retrieved May 22, 2013.
jstor.org
Garlitz, Barbara. “Uprooting "Trees" in College English 23(4) (January 1962), 299-301.
leoraw.com
Kilmer Oak Tree, Highland Park (NJ) Environmental Commission (no further authorship information given). Retrieved December 26, 2006.
Tietjens, Eunice. Trees and Other Poems by Joyce Kilmer (book review) in Poetry: A Magazine of Verse (December 1914), 140–141. Retrieved July 23, 2013.
publicradio.org
weekendamerica.publicradio.org
Hadas' comments were on the public radio program segment exploring legacy of Kilmer's poem as "one of the most quoted poems in American history" and its many interpretations in "A poem as lovely as a tree" on American Public Media's Weekend America (December 3, 2005), segment producer: Sarah Elzas, editors: Amanda Aronczyk and Jim Gates. Retrieved July 21, 2013.