OMG. I really like your game. The graphics are very lovable and your idea of wilting plants is really cool. I really see potential. Well some things to improve first is obviously balance, I played 3 times and the third I just planted 2 sunflares with higher speeds of attacks and it just kills every bug. A description reminding you the strengths of a plant would be nice after the 1st phase.
And well maybe for making it more interesting I would remove the shovel and wilted plants would dissapear in day. That way is also game about managing space.