Technically I would say it's only too late to plant outside if you are into the later half of August/September. Cannabis does not flower until there is a change in the photo period, AKA, the transition between Summer and Fall. As the hours of daylight diminish during the seasonal change, cannabis enters the flowering phase in an attempt to procreate before the onset of winter and eventually death. This phase of growth can take anywhere between 50 - 75 days (with exceptions to both min and max).

You should be just fine if you get them outdoors by July. If you would like to keep them a little more inconspicuous, you could wait to plant them until early August as well. You will see first hand that the sun is a far more efficient light source than any man made bulb.