I really like this game, but there are so many balancing issues. The largest one I've found so far is that enemies' health scales too fast each wave. I used to be able to get to wave 30+ in normal mode but now I can barely get past 15.
When your plant gets the coloured spots in it click on them.
RED: Don't click on it.
YELLOW: Gives you money.
GREEN: Raises odds of something
BLUE: Gives you a magic crystal.