gb, brussel sprouts actually taste better after cooler weather has set in, especially after a light frost, so i'm guessing you should be right on target for septembah haavesting up there in mass. they grow a very shallow root ball (relative to the size of the plant), so be careful to keep them well watered but not drowning. also, they require a lot of nitrogen (it's a green plant. all things green need lots of N), and from what i've read, boron. try using a fish emulsion fertilizer.
while it's growing, watch out for problems with aphids, cabbage worms, and powdery mildew.
there's lots of nasty chemicals out there like permethrin or sevin to get rid of the bugs, but i prefer to use my own organic cocktail of hot peppers and tobacco (the little bit left from a spent cigarette). it has to be applied often to be effective. also, a good blast of cold water from the hose will wash many of them away each day.
for the powdery mildew, sprinkle the plants with sulfur. it will retard the growth of leaf molds and fungi.
