In Sonic Adventure 2 Battle how can you plant a seed

You actually can't plant the seeds yourself, you have to make your Chao do it for you. If one of your Chao wins the Crab Pool race, they get a small shovel as a reward; likewise, the reward for the Stump Valley race is a little watering can. Make sure you have a Chao that has won both of these items. Then hand the seed you want to plant to that Chao, and the Chao will plant the seed for you.