PK covers quite a lot of products and each have been formulated to be used at different times. 13/14 is quite an aggressive ratio and is normally given later on in flower when the plants are really going for it. 9/18 is a little less aggressive and would normally be used a little earlier and for a longer period. Vitalink Buddy is a 3/5 but includes sulphur. The manufacturer recomendations is to start early at half strength and increase later into flower. Then you have the all-in-one PK boosters like green sensation. Those can be used from the early flowering stage all the way through. You also have products like shooting powder and bud blood that are the same thing but comes with two recommendations. Advanced Nutrients suggest you add bud blood for the first two weeks of flower to stop stretch and increase bud sites but House & Garden suggests that you use their product in the very final stages to give them a boot up the arse before harvest. I don't think there is a correct answer TBH
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If your plants are healthy and growing well then they will benefit from a little extra P/K mid to late flower. You have also got to consider other factors like strain etc. For example a long flowering sativa may not appreciate a dose of P/K too early. One thing I will say is these additives are super available to the plant so less is more because you can really burn the shit out of your plants and by the time you realise that you've overdone it you're about two weeks past the point of no return.
Those who can make you believe absurdities can make you commit atrocities.