Yeah, for sustainability it is the most logical way (I think you should do this and encourage people to play for leaderboard).

What about this:

1. A tree lives xxx days (real time)
2. Boosts, watering from friends can speed up time before your next watering (yield), that way if you time your waterings accordingly you get more returns compared to just passively farming once a day. This to me would encourage speed up items, drones and being active.