A wonderfully innovative take on the tower defense genre! I love the idea of having to balance between leveling up individual plants and spawning new ones. An interesting quirk of the mechanic is that there's never any incentive to conserve resources - because a plant's level is determined by how long it's been on the map it's pretty much always a good idea to place plants immediately. I don't think this is necessarily bad, especially because it gets replaced by the equally (if not more) interesting decision of when to harvest plants, just something to keep in mind should you choose to continue with the idea.
I saw in another comment that the bugs going off-path in the third level was on purpose? If so, that's a wonderful mechanic that isn't communicated enough to the player. I thought that it was a bug (pun only a little bit intended) until I read your comment above stating that it was the intended behavior. I think just adding similar path assets to the other paths they can take would be plenty to tell the player that it's a feature and not a bug.
One last thing: the sensitivity on the mouse controls for moving the camera is pretty high. This wouldn't be a big deal since you can use WASD to move the camera as well, but because every other control is handled through the mouse I kept defaulting to using the mouse to control the camera. Even after discovering the WASD controls I used the mouse unconsciously because everything else in the game was handled through the mouse.
Overall, this is a genuinely fun and well-made game!