well the canna pk 13/14 that ive got is for any growing type, it says on the back of bottle something like add 15 ml for every 10 litres in resovour, apply for a week to 10 days, when theres 3 weeks left of flower! so it should be fine for what your doing, i normally start at around 4 weeks of flower and do it for 2 weeks to make sure i catch the right time, obviously once you've fed them for the 10 days you will have to change your res, having said that there's lots of people feed the pk from week 5 till end of harvest,
heres some info i found
What does Canna PK13-14 do?
Canna PK13/14 is a high percentage phosphorous and potassium bloom stimulator. Both elements play a big part during the flowering stage of fast growing plants and in turn enable much higher than normal yields. Due to their expertise in nutrient manufacture, Canna has succeeded in combining pure phosphorous (P), and potassium (K) in very high concentrations making Pk13/14 quickly and directly available to the plant.
Canna PK13/14 can be used in conjunction with all growing media (i.e. Canna Coco, Canna Professional soil, Hydroton clay pebbles etc.), all growing methods (i.e. NFT, Ebb and Flow, Pot culture etc.) and all nutrient regimens. PK13/14 is also suitable for outdoor cultivation.
How do I use PK13-14?
Canna Pk13/14 is an additive not a plant food so should be used as part of Canna's comprehensive nutrient and additive range. PK13/14 should be added to the nutrient solution during the flowering phase for one week, three weeks before harvest. This is when the plants have their greatest need for high phosphorous and potassium uptake.
PK13-14 Dose:
PK13/14 is added to your nutrient solution at a rate of 15ml/10litres, (1.5ml/litre). It should be noted that Canna PK13/14 will add to your overall EC/CF level, so should be added to your nutrient mix before you add your primary flowering food. In effect for the week you PK boost you should add a little less primary flowering food.