You'll receive this promo via a pop-up like the one at right, giving you a link to head directly into CastleVille. Even if you were one of the game's earliest adopters, and have long passed Level 5 in the game, this window will still appear, and it looks like you'll need to click on the game to trigger the rewards into your Gift Box. For my game, as an example, my Gift Box only received the five free Instant Grow after I went into CastleVille and then reloaded my farm in FarmVille.
These Instant Grows look to only apply to growing crops, rather than trees or animals, but they'll still come in mighty handy if you're trying to catch up on goals - specifically those Bonus Challenges in Lighthouse Cove that may require two or three days of growing the same crop due to being limited on the amount of plots you can have in your game. Regardless of how you use these freebies, remember that once you get to Level 5 in CastleVille, you'll unlikely unlock rewards that we've yet to see across the rest of Zynga's catalog of games. Play once and get rewarded multiple times? Yes please.
Have you already played CastleVille to Level 5 for your free Instant Grows, or did you reach that milestone for the 20 Zoning Permits in CityVille instead? Let us know in the comments.