I’ve been growing pot outside for 10 years. I have ignored the advice of others regarding giant fan leaves. I start trimming them in late June, which is about 2 to 3 weeks before the flowering process begins and I continue to trim them right through until harvest. By removing these leaves from the plant tops, I am effectively increasing the sunlight to the many buds growing on the middle and lower parts of my plants. By removing the large leaves from the middle and lower parts of the plants, I am reducing the stress on the branches which is really evident in heavy rains. Two important notes. Make clean cuts when trimming leaves to avoid damaging the plants and never trim leaves on a wet plant or when rain is imminent. These tips will help minimize the chance of bud rot.
You prune for 2 reasons, 1 to thin out plants to improve airflow to prevent mold, 2 to get your branches into the light, yes you can prune autoflowers some strains dont mind other strains may slow up for a few, prune before plant starts to flower.