It came from Mars...

Our house was once owned by an eminent Victorian Extra-Terrestrial Horiculturist. He brought back this innocuous-looking plant from one of his trips to the Dark Side of Mars. For many years, it quietly grew, biding its time, waiting, waiting...
But, last night, at the first full moon after the Martian equinox, it started to bud.
It is half way up the house already. I don't know how much longer we can hold out.
Any help will arrive too late for us. Your only chance is to launch a missile strike on our house. Now! Before it is too late... And just pray that it hasn't launched its spoors already...