Actually when I played it I noticed that if you wait from time to time it turns to infinite positive money, so it's a glitch you can use to buy more stuff without restriction. The problem is when it's negative, you can't buy anything and have to wait until it alternates to positive.
I played it again recently since I really liked this game, and what I think though is once you see you are getting a rate of about 146 or higher and the tree looks really huge in a specific way (I put some screenshot somewhere further down), just don't fertilize the tree anymore past that point and you can avoid getting this issue for now.