Poison ivy has three leaves, but the harmless Virginia creeper has five leaves. The rhyme “Fingers five, let them thrive!” has been cited in print since at least 1902. “Leaflets five, let it thrive; Leaflets three, turn and flee” has been cited in print since at least 1938. “Leaves of five, let them thrive” has been cited in print since at least 1951.
 
Similar poison ivy rhymes include “Leaves of three, let it be,” “Berries white, a poisonous sight,” “Hairy vine, no friend of mine” and “Raggy rope, don’t be a dope.”
 
 
Google Books
Stories of Woods and Fields
By Elizabeth Virginia Brown
Yonkers-on-Hudson, NY: World Book Company
1902
Pg. 80:
“All you have to do is look at the leaves in the summer and the berries in the fall,” said Fred. “If the leaves have five fingers, you may safely put your hand on them, for they belong to the friendly creeper; but if they have but three fingers, you must not touch them, for they are poison ivy. In the fall, the berries of the Virginia creeper are red, while those of the poison ivy are white.
 
“You can always tell the ivy from the creeper if you remember,—
 
“‘Fingers three,
Turn and flee!
Fingers five,
Let them thrive!
 
Berries white,
Poisonous sight!
Berries red,
Have no dread!’”
 
7 April 1948, San Antonio (TX) Light, “10 and 20 Years Ago, From Files of S. A. Light,” pg. 18, col. 5:
April 7, 1938
“Leaflets five, let it thrive; Leaflets three, turn and flee.”
With the above adage, persons answering the call of spring and planning expeditions in the country were warned to be on the lookout for poison ivy by Mrs. Ellen Quillin of the Witte museum.
 
3 June 1951, Morning Advocate (Baton Rouge, LA), magazine sec., pg. 13, col. 1:
If you’ll remember the little grade school jingle, it will be easier to identify:
 
Leaves of three
Turn and flee.
Leaves of five
Let them thrive.
 
24 July 1960, Morning Advocate (Baton Rouge, LA), “Communing With Nature Is Fun, But Beware of That Poison Ivy,” pg. 3-E, col. 3:
Her resulting discomfort could have been prevented had she but remembered the old child’s jingle: Leaves of three, turn and flee—Leaves of five, let them thrive.
 
14 July 1985, Cumberland Sunday Times (Cumberland, MD), “Nature Notes” by Ken Hodgdon, pg. A12, col. 5:
An old poem: Leaves of three let it be (poison ivy)! Leaves of five let it thrive (Virginia creeper)!
 
Google Books
The Gardener’s Weed Book:
Earth-Safe Controls
By Barbara Pleasant
Pownal, VT: Storey Communications
1996
Pg. 188:
Memorize this rhyme: “Leaves of three, let it be; leaves of five, let it thrive.”
(...)
The “leaves of five” part of the phrase refers to Virginia creeper, a harmless native vine that often grows in the same places as poison ivy.
 
Google Books
The Poisoned Weed:
Plants Toxic to Skin
By Donald G. Crosby
New York, NY: Oxford University Press
2004
Pg. 32:
Most people are familiar with the saying about poison oak and ivy, “leaflets three, let it be; leaflets five, let it thrive,” but although the 3-leaflet (palmately trifoliate) form is by far the most common, a few plants do have 5 or even 7 leaflets.
 
Google Books
The Poison Oak and Poison Ivy Survival Guide
By Sandra J. Baker
Medford, OR: Coleman Creek Press
2011
Pg. 13:
HISTORICAL RHYMES
(leaves changed to leaflets for botanical accuracy)
Leaflets three, let it be; leaflets five, let it thrive.
Berries red, have no dread; berries white, a poisonous sight.
Hairy vine, no friend of mine. (poison ivy vines)
Raggy rope, don’t be a dope. (poison ivy vines)
Red leaves in the spring, it’s a dangerous thing.