I finished a whole first adventure of Plants vs Zombies, but NO ZEN GARDEN. I have Quickplay, some awards, a key to Crazy Dave's shop, but no Zen Garden. Is it because you have to have a certain device to get the Garden? I'm confused. Do you unlock it in the second adventure? That I would understand. I even defeated the Zombot on the very last level! Do you need the Yeti Zombie to get it?
The latest version from Google Play has all the features included in the desktop game. If you bought your game from Google Play, update it and Zen Garden would be available.
I searched the internet and found a possible reason: if you downloaded it with the Humble Bundle for Android, there's no Zen Garden, as the lasts updates aren't available in the Humble Bundle library. It seems that the Humble Bundle team didn't want to promote in-app purchases added in last updates.