This is an absolute must visit. Anytime in season. Ten minutes north of the falls a beautiful park setting is home also to the Niagara Parks School of Horticulture so they are constantly creating beautiful setting using flowers that bloom at different times of the year. Make sure that you give yourself enough time to walk about and experience it in its entirety.
The photos below were taken in August 2009
Niagara Falls Botannical Gardens

Bottanical Gardens in Niagara Falls

Yellow Lillies