Plants may show preflowers before flowering starts, if that doesn't happen, you have to wait until there is 12 hours or more of dark per day for your area.

NOAA Improved Sunrise/Sunset Calculation
Fencewalker Reviewed by Fencewalker on . when will my plants show there sex? Ok so I planted my seeds about 3 weeks ago. How will I know when to check back for male/female signs. Is there a certain date or time period? And if not, how long do plants usually go before determining sex outdoors? Rating: 5