If you use a commercial potting compost it should already have enough food for six to eight weeks, and after that you just use a liquid feed. You might find Growmore has more nitrogen than you want - I would go for more potassium eg tomato fertiiser every now and again.

Or add some slow release fertiliser. Growmore tends to give out the food in one big blast.

I would go for the best of both; add some slow release granules when planting as any fertilizers already in the compost will quickly become washed away with constant watering. As the plants begin to produce flower buds some potassium enriched feed will make a difference.

Thank you