How about making the contest a little more focused, but have several contest throughout the years. You could have contest like.
Gardens - categories Flowers Garden Your Yard, Flower Garden Public Gardens, Vegetable Gardens etc.
Wildflowers - categories Individual Flowers, Multi-Plant Photos, Landscapes.
Ornamental Plants - categories Perennial, Annuals, Tropicals, Close-ups, Garden Setting, Non Flowering.
You could have a contest centered around popular plants. Like Roses and Daylilies. Pick the top 5 or 10 most popular plants and make each one a category.
