At the moment, I'm using a far cheaper option - a packet of blood, fish and bone. I sprinkle a little around shrubs, Clematis and roses (I ususally do this in early spring, and again in July). Plus, of course, mulching with home made compost.

If you want to feed your plants as you've suggested, then later in the season would be best - it's formulated to help them to produce and maintain flowers, so use it on any bedding plants in particular, especially any in containers.

I use Miracle Grow and the packet directions instruct you to feed it fortnightly from March through to September.
I use it on all the perennials and the shrubs !

Watch for the ones that go dormant in late summer--many bulbs and some woodland perennials--and go easy around them once they fall asleep.