Iwould wait until they,re planted out in the ground before you start feeding them, you could put growmore on the beds 2 or 3 weeks before planting, or use a liquid feed once they are in.
where you are, i wouldn,t plant out before mid june.
hope this helps

There is enough fertiliser in bought compost to last a plant for six weeks, which should see you through almost to planting out time.

Cheers, Derekm and Steragram I will take your advice