Thank you :) I have drawn the title screen art myself, it was a lot of fun :)
As for now, this is indeed the only way to get rid of the weed: you basically shoot at the planet and because the weed moves so fast, it will get under your fire. One thing that is not intentional is how easy the "impossible mode" is. I changed a parameter last minutes to make the weed more visible but it made the "impossible mode" too easy.
In the original plan, the head of the weed would sometimes spawn some sort of buds on the planets in addition to the spikes. You can see these buds in the art of the title screen, actually; they look like glowing strawberries. The head would have had more health, and move slowly at first. Then the more the head spawns buds, the quicker it moves, the harder it gets to kill and the faster it conquers planets. Also some buds would fire stuffs at you. So in the original plan, you would have needed to keep the number of buds low, so that you can slow down the spread of the plant and attack it. Unfortunately, I had to scrap this part of the plan.
Hopefully I will find some time after the votes to implement the original idea :)