When you have a baby, various stores encourage you to sign up for monthly emails. Because the store knows the date of birth, they can send accurate developmental updates. e.g. "Your baby is X months old. You may notice she is starting to pay more attention to the world and is able to hold up her own head..." (etc).
This is fascinating for the parent due to the timeliness of the message. It is also a great sales opportunity, as lower down the message it suggests things the user might want to buy.
You could do the exact same thing. Don't just send a message saying "plant your seeds now" but send (opt in) periodical follow up emails that explain how the garden should be doing, common gotchas, and so on.
If you ever add any social aspects to the site, it might make sense to expose the 'age' of the user's garden. People at a similar stage of growth have something in common and may want to interact with each other.
Good luck with it!