You and I are in approximately the same climate, so you might circle March 1 as the target date -- after all threat of freeze is over. The best time being early spring right after they bloom, just about the time you see the plant beginning to push the growth buds -- the nitrogen will give them a boost. Continue to lightly fertilize once every month or two until August 1 -- then stop.

Forestal,
Thanks for your helpful information.
Felix

Autumn is the best time to fertilize permanent plantings (plants hardy in the local climate). And don't pour on a buncha fertilizer without first sampling your soil and having it analyzed. Application of unneeded nutrients wastes money, can damage plants and contribute to water pollution. Each nutrient has an optimal amount, below that level plants may be stunted, or look poor (discolored), above it they can also be hurt.