When to start with OK 13/14?

PK 13/14 is not good for beginners, it only helps if you know what you are doing. Your grow will be great without this stuff. If you use it incorrectly, it can stop growth or even cause bud rot. first learn to grow properly. Find out what the plant does with PK. It is not necessary under any circumstances and is more for advanced users.

Depends on how much P/K you already give... more is not always better. this is easier to give numbers for soilless/hydro contexts. Feeding over 190ppm K is very unlikely to have any positive effect on yield or potency. With P i've seen studies peg this at 30-40ppm, but 50-60 is more than safe and limts waste. those numbers won't translate well to soil, but gives an idea that it is about a happy zone, not gassing the shit out of it. if you already give lots of p and k, you will only cause a p or k toxicity or needless waste of p/k in your runoff by adding more.

As soon as there are small "budlets" forming........which is now in this picture.