The sign for garden is a compound sign, looking like several "plants" sprouting out of the ground. Your non-dominant arm will form the ground, the forearm lying flat with your fingers curled as if holding a small pot, and it will come up through the non-dominant hand, opening up your fingers as the "plant" passes the top of your non-dominant fist. Do this same sign for plant or grow, but 2-3 times more.
If you have a yard with a garden, use the garden sign when you go outside. Teaching your baby this sign will help her to tell you when she is feeling like playing outside.
We spend a great deal of time outdoors. Every year, we help our kids plant a vegetable garden, which helps the smallest ones contextualize the concept.