a couple critiques
1. the largest one, by far - there's no reason to go fast! if, persay, with time the garden gnomes got faster, stronger, etc. there would be a simple incentive to upgrade quickly! but at the moment, there is none, leading to a game where the optimal strategy is to go incredibly slowly, which is boring and highlights the other problems with the game.
2. god the rose is not good enough as a final flower. its incredibly easy to cheese and then just slowly rack up money by planting 5 in a diagonal and then killing them all by sitting on that diagonal, leading again, to boring and stale gameplay. perhaps again as an incentive - by mixing flower or planting more you could gain a monetary multiplier on their drops? (goes really well with the above idea by incentivizing fun and difficult gameplay)
3. to go along with the rose being easy to cheese, the other flowers dont have that good of patterns - the sunflower should have the blue flower's basic attack, the blue flower maybe gets a spread of three or perhaps a bullet that explodes, and the lilac maybe a shotgun spread? (the lilac is incredibly easy to cheese, just stand far away and shoot from a distance.)
other than these pretty major problems, the art is great - the sprite work having a unique art style that really gives it personality and flair - and the core idea is pretty strong!
(also why is this tagged as idle)