You've done your homework and you know your child's got a real green thumb of a teacher--or at least a definite green thumb wannabe. :) Here are some good no-clutter gardener gifts:
Burt's Bees Gardener's Kit
Dish Garden with Pinks
European Dish Garden
A Borders Gift Certificate for them to buy one of their gardening books on their wishlist.
A Yardiac Gift Certificate for them to pick out some fun new gardening toys from one of my personal favorite gardening catalogs. I love them because they will price match anyone. They have everything starting with organic fertilizers all the way up to decorative trellises.
Don't forget--A gift certificate right there in your neighborhood for your local green house or nursery.
What ideas do you have for the gardening teacher? Are you the gardening teacher? What would you like?