Board Thread:General Discussion/@comment-1785597-20140623012837/@comment-15575766-20140623020147

"As of September 18th 2013, Regen Farming is no longer possible on BTD5. It can still be achieved on BTD5 Deluxe and BTD5 iOS."

- Regen Farming page

Ninja kiwi prevented us from using regen farming in an update. But you can still do it in Deluxe and mobile versions. I don't know much more about it.