The first Wildflower Day was at the PPIE April 24,1915

ORIGIN OF WILDFLOWER DAY
[From Builders of Our Valley, A City of Small Farms, Bertha Marguerite Rice, 1957, p.70-72]

By ROLAND RICE

"So great was the wealth of marvelous beauty in the infinite variety of California's native flora, that a "Wild Flower Day" was founded on April 24, 1915, at San Francisco; with an exhibit at the Panama-Pacific International Exposition. This State Wild Flower Exhibit was for 10 years an annual event, presented by its founder, Mrs. Bertha M. Rice; and so profound was the interest awakened by these displays that thousands of San Francisco school children and flower lovers from far and near availed themselves of the privilege of studying the marvelous array of the State's flora. Men like David Starr Jordan, Luther Burbank and scientists and educators, whose names ranked with the world's great citizens, participated in the event. Indeed, Luther Burbank, in his address on a memorable occasion during the celebration of 1921, said: "I am not sure but that this is the greatest work that is being done in the world today. Its influence is so far-reaching."

"The origin of Wild Flower Day was due to the impression the colorful wild flower fields made upon Mrs. Rice, while journeying from San Francisco to San Diego on a visit to Madame Schumann-Heink, who, at her request had given at Festival Hall, on the exposition grounds, a free concert to children. Returning to an Francisco, Mrs. Rice presented her plan to the Exposition officials who promptly put the affair in her hand and offered their full cooperation. With the assistance of State officials, "Wild Flower Day" was celebrated in that year with an exhibit representative of California's native flora, and many beautiful and interesting features, including pageantry boys' band contests, musical and literary programs. 

The loveliness of the State's wild bloom made a deep impression upon the thousands of visitors who thronged the exhibit room. The judges of the county displays were headed by Edwin Markham America's greatest living poet who made a complete report of the exhibit. 

The Second Annual State Exhibit was held in April, 1916, in Norman Hall at the Fairmont Hotel, and in April of the following year, 1917, in the same beautiful quarter, was staged the most extensive and beautiful display of wild flowers, ferns , grasses, shrubs and trees ever gathered together in the history of the world. 

Mark Daniels, former Superintendent and Landscape Engineer of National Parks, was secured to arrange the exhibition and elaborate effects were successfully carried out by bringing characteristic trees and shrubs from the high Sierras. Miniature lakes, flower fields and forests were faithfully reproduced among typical scenes of the State shown on this occasion. 

For many years these annual affairs were held in the beautiful halls of either the Fairmont or St. Francis Hotel and were confined exclusively to wild flowers. Their beauty originality and educational value attracted world-wide attention. Every effort was made to utilize these exhibits as educational mediums in the interest of the conservation of wild life, and they became so popular that similar exhibitions on a smaller scale have become numerous. The Conservation League finally decided to abandon extensive exhibits and confine its activities to other line of educational work, and with the movement for National Wild Flower Protection Day headed by Albert E. Stillmann and Bertha M. Rice, backed up by the leading naturalists, educators and organizations of America, this work entered upon a new era of useful endeavor with much already accomplished. 

It was in 1917, also, that Dr. P. B. Kennedy president of th California State Botanical Society, assisted by other members, prepared an Annotated List of The Wildflowers of California which was published in book form of 165 pages, well-bound, and dedicated to our work. We were presented with 1000 copies of these books which were eagerly taken up by the libraries and flower lovers of the State."

Other Interesting things

• Builders of Our Valley, A City of Small Farms, Bertha Marguerite Rice, 1957 (only on Link+ in hard-copy, not online that I've found).
  Another section, "The California Wilflower Conservation League", talks about the formation of the league in 1923. No one alive remembers this now, but in the early 1900's there were huge stretches of Toyons over California. During the holiday season the berried branches were gathered for commercial purposes and these pickers ravaged the countryside. David Starr Jordon, Luther Burbank, Dr. William Bade (Sierra Club), and William Kent (who donated Muir Woods to Marin County) spoke out against this practice.
• Annotated List of the Wildflowers of California, 1917, 3rd Annual, held at Fairmont, by P.B. Kennedy, University of California, President of the California Botanical Society  (established in 1913)
• Popular Studies of California Wildflowers, Bertha M. Rice and Roland Rice, 1920. Photograph of the blazing star is from this book.
• The book is now digitized!
• San Jose Evening News, April 5, 1938 "Crowds visit Wildflower Display"
• More on Edwin Markham (poets.org)
• Roland Rice displayed hand-colored photographs of wildflowers at the Horticultural Hall at Treasure Island for the 1939 World's Fair. At one time, the San Jose library had these photos.
• "Naturalist to Give Wild Flower Talk Here Friday Night", Stanford Daily, June 1932
• I haven't found a PPIE calendar yet that mentions Wildflower Day.
• Overland Monthly, March 1922, 
• Saratoga Historical Foundation
• California Spring Blossom and Wild Flower Association 1925

From Calisphere, San Jose Public Library