There seems to be a bit of misinformation here. Roundup will work in the winter, it just takes longer. On a summers day it will start to kill weeds within 2-3 hours, with visible signs in within 4-5 days it takes 6 or more hours in the winter. If it rains it will be much less effective. Roundup fumes are not toxic, it's a herbicide not a pesticide so try not to drench yourself in it or drink it. If you use a mix of 1 litre of roundup for every 40 litres of water it will be devastating.