On their web page, The National Gardening Association explains that it "works with sponsoring companies and organizations to provide in-kind grants to projects that actively engage kids in the garden and improve the quality of life for their communities.
To be eligible for these awards your school or organization must plan to garden with at least 15 kids between the ages of 3 and 18."
On the right sidebar there is a list of the upcoming deadlines with links that will help you apply for any and all of these awards for your group. The first deadline is September 18th for the Hooked on Hydroponics award that helps teach children and teens how to grow plants hydroponically (without soil).
There are six other grants listed in order of their deadlines and you can apply for as many as you wish. Winning one does not disqualify your group winning another. In fact, multiple applications are encouraged.