Good take on the theme, and I like the 3D art. I especially love the art at the beginning title screen.
I like the effect of the weeds growing a bubbling around the planet, and the spaceship is really cute with its puffs of smoke.
I don't really see how to kill the weed other than just holding fire on to a planet. I beat all difficulties by aiming towards the planet and holding space. The red thing moves too fast to really be able to aim well at it in my opinion, so its better to just hope it crosses your shooting path.