The first signs of stigma, (white pistons, white hairs) is when your plant is classed as flowering. When your plants start flowering you have to make sure that you're providing higher doses of phosphorus and potassium, these are the main minerals your cannabis plants need to produce buds. Lower the N, nitrogen is more KEY in the growth phase the P-K part is more for flowering. If you need more detailed info please feel free to DM me. The info that comes with the plant arw guidelines and flowering can be delayes in different environments, lightning, nutes, temps and so on. Some autos if stalled might not flower until wk 8 like mine, or migjt start on wk 2, depending on stress and condi, it all plays a part. Happy growing Buddy.