A good thick (2-3") mulch of compost is all they need, especially their first season in the ground. If you want to encourage bluer flowers, work some cottonseed meal into the mulch. Or use worm castings or coffee grounds, if ya got 'em :-) Fertilizing doesn't have much of an effect on the length of bloom, unless one errs too far in a single direction, but that typically results in a shortened or nonexistant bloom, not longer. Compost will result in overall good health and is 100% natural. And will help retain moisture in the soil.