Terp1answered grow question 3y ago From the beginning! It's best to use water-soluble powder and administer it while watering. So far, I've transplanted my Automatics twice and added mycorrhiza during the process. That worked really well too. You just have to make sure to keep the transplanting as stress-free as possible. So, I started them in a 1L pot, then after about a week, I moved them to a 6L pot, and finally (around the 3rd week) into the final 15L pot.