Starts off slow but boy does it speed up by level 4 or so. I noticed that a good strategy is to save up seeds to stock up on some of the bugs in the bug store. Share scores on facebook, invite friends to play, try completing some of the achievements -- anything to be rewarded some seeds. Those definitely help knock out those plants. Also try setting your phone on a flat surface and tapping the plants with both fingers! Worked for me, got to level 21 so far.