Most of the trees in Nebraska were planted by its citizens, and >he first “Arbor Day” was founded in Nebraska in 1872. “Tree Planters State” was suggested as a Nebraska state nickname in October 1894, and the “Tree Planters’ State” became official in 1895.
   
Nebraska has also been called the “Antelope State,” the “Bug Eaters State” and the “Cornhusker State.”
 
Nebraska’s official nickname was changed from “Tree Planters’ State” to Cornhusker State” in 1945.
 
     
Wikipedia: Arbor Day
Arbor Day (or Arbour; from the Latin arbor, meaning tree) is a holiday in which individuals and groups are encouraged to plant and care for trees. Today, many countries observe such a holiday. Though usually observed in the spring, the date varies, depending on climate and suitable planting season.
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United States
Arbor Day was founded in 1872 by Julius Sterling Morton in Nebraska City, Nebraska. By the 1920s, each state in the United States had passed public laws that stipulated a certain day to be Arbor Day or Arbor and Bird Day observance.
 
National Arbor Day is celebrated every year on the last Friday in April; in Nebraska, it is a civic holiday. Each state celebrates its own state holiday. The customary observance is to plant a tree. On the first Arbor Day, April 10, 1872, an estimated one million trees were planted.
 
Nebraska Newspapers
31 October 1894, Columbus (NE) Journal, pg. 3, col. 2:
“Tree Planters State” is the name recently adopted for Nebraska by the State Historical society. Excepting along the rivers, nearly all the trees in the state have been set out by the hand of man.
 
Nebraska Newspapers
29 March 1895, McCook (NE)

, pg. 1, col. 5:’
We presume that Nebraska deserves the name of the Tree Planter’s state because it has so many tree planters and so few trees.
 
Nebraska Newsapapers
3 April 1895, Columbus (NE) Journal, pg. 2, col. 2:
SENATOR SLOAN made quite a speech in favor of naming Nebraska “The Tree Planters’ State.” The Nebraska Pioneer association, the State horticultural society, the State board of agriculture, the State dairymen’s association, the State press association, and also the State live stock and breeders’ association had all endorsed the name and the senator closed by saying: “The name of a sister state was by legislative enactment pronounced ‘Arkan-saw.’ Other states have in a like manner adopted symbols suggestive of their ambitions and apparent destinies. And why may not Nebraska by legislative authority, doff the undeserved stigmas of ‘Coyote’ and ‘Bug Eater’ and don the suggestive appellation, ‘Tree Planters,’ in line with the ambition of Nebraska husbandmen to replace the old arid plains with alternate fields and groves, that the comfort of the men may be assured by the cooling breezes of summer and tempering the rigor of winter’s blast; that the rains of heaven may be reasonably controlled and the seasons of earth favorably modified. I trust that upon the roll call there will be no dissenting voice and thus baptize anew Nebraska with a new name worthy of her highest industrial hope and moving on to her manifest destiny shal recognize as her best citizen the man who shall cause a tree to grow where only a blade of buffalo grass has thriven.”
   
Google Books
Laws Passed by the Legislature of the State of Nebraska
Omaha, NE: Omaha Printing Company, State Printers
1895
Pg. 441:
CHAPTER 119.
(Senate File No. 270.)
A joint resolution to designate Nebraska in a popular sense, “The Tree Planters State.”
WHEREAS, The state of Nebraska has heretofore, in a popular sense been designated by names not in harmony with its history, industry, or ambition; and Whereas, The state of Nebraska is preeminently a tree-planting state; and
WHEREAS, numerous worthy. and honorable state organizations have by resolution designated Nebraska as the “Tree Planters state,” Therefore,
Be it Resolved by the Legislature of the State of Nebraska:
That Nebraska shall hereafter in a popular sense be known and referred to as the “Tree Planters State.”
 
7 July 1945, Evening World-Herald (Omaha, NE), pg. 1, col. 3:
It is Official Now: We’re Cornhuskers
By James Keogh
   
Google Books
Encyclopedia of Nebraska
By Nancy Capace
St. Clair SHores, MI: Somerset Publishers, Inc.
1999
Pg. 2:
The nickname, the Tree Planters State, was officially adopted by an act of the Nebraska Legislature, approved on April 4, 1895, because it is “preeminently a tree planting state.”
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The Nebraska legislature changed the State’s official nickname to the Cornhusker State in 1945.
 
Google Books
Flags of the Fifty States:
Their Colorful Histories and Significance
By Randy Howe
Guilford, CT: The Globe-Pequot Press
2010
Pg. 147:
Nebraska was first known as the “Tree Planters’ State,” from 1895 to 1945, because Arbor Day has its roots in Nebraska.