Really good and polished game. It's really impressive that you drew everything by yourself alone while designing the levels and programming on top of it. Bravo.
But I do have a couple suggestions.
Your character plays the same through the entire game, with only the enemies being different. I wish there were some kind of progression, like having a dash ability, or some kind of alternative shot type you would be able to switch to (an example off the top of my head, pressing a button would switch your gun to shooting 3 spread out bullets that each deal less damage than your regular one, which would help more against swarms of enemies). Just to spice things up a bit.
Also, some indicator for new enemies that would spawn in would make this just a slightly less frustrating experience since it is really likely for an enemy to spawn in where you are at. A warning sign similar to the bombs or the like.
Lastly, maybe have fully planting the flowers, heal your character? This way it would help incentivize planting during combat rather than just doing it after killing all the enemies.
Overall great game. One of the best (and most complete) games I've played in the jam.