Wassup GR420. I often get asked what PK I use, when, for how long. So I figured id do a post with what I fo and why. This is based around coco growing as that's my chosen medium.
What is a PK product:
PK which stands for Phosphorus and Potassium, yep, neither start with a K...

When to use:
Depending on the product, the instructions will be different. But in true stoner style, we make it up as we go along, and rightly so

One of the common ways I've seen growers use these products is to add once they see their plants in true flower. On average this will be 14-21 days from changing the lighting cycle to one suited to induce flower. It makes sense, the plants need more Phosphorus and Potassium for flower, they are now it true flower, hit them with it. Over the years of doing this it started to occur to me that when we see the plants going into true flower, they have already gone through huge hormone changes to start this process. What we are seeing is the end result of these changes.
Based on the above I started to use PK from day 1 of the flowering light cycle, in small amounts, but straight away. One thing I noticed straight away was plants getting into true flowers much quicker. Instead of the 2 - 3 weeks, I was now seeing true flower in 1 -2 weeks, I had one grow recently go as fast as day 5 of the flowering light cycle. After a few years of growing like this I'm of the firm belief that having a small amount of PK available in your plant feed from day 1 flower will mean they have the extra Phosphorus and Potassium the second they need it, not when we see they need it. An extra week or 2 in flower over a 10 week flowering cycle on average is a long time.
The amount I feed:
This will vary from grower to grower, system to system, product to product. I get asked a lot so Ill list what I use based on the above thinking.
I use Canna PK 13/14 primarily, it's a cheap, tried, tested, and proven product. Before I started adding PK from day 1 of the flowering light cycle, Id add 0.5ml of Canna PK 13/14 once I see true flower, run for a few weeks, then change to another product. I now stick to a set of numbers that work well no matter the strain I'm growing.
Day 1 - 5 of 12/12: 0.25ml per litre of water.
day 6 - 10 of 12/12: 0.4ml per litre of water.
Days 11 - 21 of 12/12: 0.5ml per litre of water.
I adjust the base nutrients around this as needed.
After day 21 of the flowering cycle ill change the product to a lower ratio PK product. Many of these are not labeled as "PK" products, flower enhancers, or boosts, are names they often fall under. Higher ratios of PK for long periods through the flowering cycle can cause issues, mainly aging of plants early. You obviously don't want this. You can just reduce the amounts per litre of your chosen product, I prefer switching to a different product. A really good product that's a great addition
to the flowering cycle, has been around for years, isn't too high in its ratios of PK, cheap as chips, is Vitalink Buddy. It's got some other goodies in there too.
Since using PK the way I describe above I've definitely noticed a faster transition to true flower.