Most perennials will not be greatly affected by late light frosts. And if they do get 'burned' a bit, well, established roots will get them going again quickly. Some plants don't get going until the soil warms up, so don't be too quick to give up on any particular plant. Do you know the average date of the last frost in your area?

I fertilize when the plants showed a couple inches of growth. I normally use a water soluable fertilizer and use it around the root zone. Weakly, weekly.

I toss around Milorganite and "scritch" it in around the plants and water it a bit when the plants are up a couple of inches and I leave it at that for fertilizing. Perhaps if I put in more I would get bigger plants or something but they look ok to me. Usually here in Kalamazoo we get enough rain so I don't have to water for ages! I water when it gets really dry as I want the plants to put their roots down deeply and not stick around on the surface. The deeper the roots the less water they need.

Good luck!
Kiwi in Kalamazoo

"Because the average perennial bed contains so many different types of plants, it is difficult to make a general recommendation for fertilizing. The best approach is to take a soil test and follow the recommendations. Otherwise, a single application in the spring as new growth develops may be adequate for most perennials." This is taken from the Michigan Master Gardeners manual. I have always read that perennials do not need the regular feeding that is required for annuals. The best thing to do is to treat each plant individually.

...and now you see the problem. Three gardeners. Three different opinions. That's the way it is with most questions about gardening!
Personally, I look at it this way...it's Michigan. Summer is short. I want to get as many blooms as possible. Therefore, I feed. I'm OK with doing more than what's adequate. In fact, I'm shooting for "exceptional."
P.S., before you use Milorganite, smell it and consider whether you can stand the smell. It's made from sewage. I put it on my garden one year and it chased my family indoors from the patio until I covered it with fresh mulch. Wheewwww! Even then, we could smell it after a rain. Or sometimes I'd dig up a plant and get a whiff of it. Gag! I admit I'm sensitive to smells.
I'm betting MagicKiwi uses it FAR away from the house! K.

If you give a flowering plant more fertilizer than it needs, you can end up with lots of nice green foilage and very few flowers.

Suppose to get down to 30 Wednesday and Thursday night. Should I wait to fertilize?

Lynn s: Plants can't use the fertilizer until they are in active growth, so it depends on what you're fertilizing and what you are using as a fertilizer. Generally speaking, though, with organics it doesn't matter because they'll sit there until they're needed so you could go ahead and put those down if you wish. Chemical fertilizers, either granule or liquid will wash down through the soil with the rain (or melting late snow!) and not be there for the roots when they need them - so wait on using those until it's more stable weather and the plants are up and really growing.