You could cover the days of creation and when plants were created, and their importance to our very existence. How God gave Adam the job of caring for the Garden, and how all their food was from plants at the beginning, even all the animals ate only plants, because before sin all of creation was in perfect harmony. You could have a special sunday school plot to then design and plant your own garden.

You could talk about the parable of the sower and the soils, and how important the right kind of soil is, and the cultivation of the "soil" of our hearts.

You could even talk about composting. How garbage breaks down to make compost which is good for plants. But garbage that enters our hearts is not good. God calls us to protect our hearts and think on those things that are lovely, pure, etc.

You could have a worm farm in your compost pile, or build ladybug houses and buy some ladybugs.

You could use the book, THE SECRET GARDEN WALK, and talk about how God transforms our lives.  You could create a 'secret spot' in the church garden with a 'secret entrance' and inside have a bench, and stepping stones with Bible verses on them to contemplate.

You could create an indoor or outdoor FOUNTAIN... water garden using a baby pool and submersible pump.   Pool stores sell small fountain kits and inexpensive plastic fountain heads which can be attached to hose, or you can make your own  

-----Pools of water for the deer. Jesus is the living water. etc   You could PAINT LETTERS on FLAT ROCKS which can be arranged into short verses or keywords from a Bible verse.