New Hampshire designated the purple lilac as the official state flower in 1919. The apple blossom, purple aster, wood lily, mayflower, goldenrod, wild pasture rose, evening primrose and buttercup were all considered, but the purple lilac was chosen because it symbolizes the hardy character of the men and women of the Granite State.

A lilac bush can live for hundreds of years. There are over 1,000 varieties of lilacs in several colors including white and pink. Originally from Europe and Asia, lilacs date back to the 1750's in America - they were planted in our first botanical gardens and both George Washington and Thomas Jefferson grew lilacs in their gardens. New York recongizes lilac as the official state bush.

Source:

State Flower and Wildflower: New Hampshire.gov

Links:

State Flowers - 50 States List
National Flower