Really fun and addictive, and it looks amazing for a flash game.
You should really consider approaching one of the gaming console companies to see if you can put it on one of their online store or something.
With some better graphics, it wont be too bad.
Definatley got a great base for a fun game happening. Work on creating more interesting graphics, Maybe a background too. Try to make the score text not just look like it's text, say, put a border around it or an object of some sort so it's not just text. Have a more interesting and involving display, and you'll easily have this at 7-10 stars.
Try Expanding it a bit
The game is only really interesting for the first ...half a minute. You could definatley make this into a better game by using what you have here as "framework". Try adding some difficulty levels, and working on making the Special Effects (basically the animation that shows when you whack a plant) look better. Maybe add abilities that you can unlock with some sort of currency system, abilities such as slow down etc. that work toward's the player's advantage.
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