A general rule of thumb that I've learned around here is that if stores around you are selling tomato plants, it's okay to plant. As I said this is a general rule of thumb so it may vary from area to area.
Growing outdoors? after the frosts have gone, like Indianatoker says, if you can grow tomato plants then you can grow MJ. good luck
but remember dont think that you can grow them in 40 degree weather, these a tropical plants, and thought they can live with a pretty broad temperature spect. they cant be to cold, or hot, or you'll kill them...
While I agree with you on the 40 degree temp, I disagree as classifying them as tropical plants. Check out the Audobon Society Fieldguide of North American Wildflowers. The original range of wild marijuana is pretty much widespreada from BC to Maine. And the Afghan strains originated from Afghanistan, which does not appear to be tropical from the scenes on CNN.
alright, i was wrong with the tropical plant clasification, but weed grows best at high70's and higher80's, which i find to be a tropical climate... thats why i said that... i mean, find a place that after the last frost, grow weed in 60 degree weather... if it does actually grow, it wont be too successful...