How do I start growing in the backyard? (iOS)

Wait until you see the task “Get the Gardener to pull out the first Bush” on your chalkboard.

Head to the backyard, and click on a bush. This will summon Charlie, the gardener; he’ll help you clear a bush for a price.

Make sure you wait to see the task on your chalkboard before you have him remove a bush, or else you won't get the XP for the task, and the task will become stuck on your chalkboard.