To clarify, special missions where you aren't allowed to plant any plants are possible. You just need to level up the plants, increasing their range, and then put them into attack mode, further increasing their range.
The one problem with this game at the moment is that there is only one gamemode and it soon gets boring. Controls are easily reachable and every ship has different weapons and play styles. This makes for a very interesting game when every ship is good for different things.
Because it's so short, or did you get an unsavory ending? Either way, thank you very much for playing. :)