if you cover the sides of the building with black/white poly, and then put a layer of mylar on the inside of that, you might do ok; i'd give them as much time as possible, try not to put the covering up higher than you have to (more sidelight the better)


but, if your greenhouse is in plain view, i think covering up the sides would be suspicious in its own right; you know your situation though; i would avoid artificial lighting if you can