When is a "frost" a "frost"? Is it any time temps dip below freezing after spring?

I live in Zone 5 (Chicago). We've had some mild weather recently and I've got lots of stuff starting to pop up in my garden -- crocus, bleeding heart, clematis, sedum, hydrangea, a magnolia tree and many shrubs are starting to leaf out. Some of these things have only little buds on them and some (many the bulbs) have flowers. For the next few nights now, temps are supposed to go below freezing. Is this considered a "frost" or do temps have to be mild for a while before you can call it "frost"? Do I have to cover any of my stuff?
When is a "frost" a "frost"? Is it any time temps dip below freezing after spring?
Frost is a combination of humidity level and temps. The plants that already present themselves are hardy and can withstand another freeze or frost! Your region is subject to possible frost thru April and into early May.

Protect your plantings by using a 10-10-10 with mineral granular fert the weekend after Easter. Put directly on soil at recommended amount. Mulch lightly with a hardwood shredded material. Any visable dieback from frost or freeze remove with hand shears ( dip those shears in a Clorox and Water solution thereafter).

Plant any really non-hardy plants around Mother's Day. Fill in the bed with a two-three inch layer of mulch and treat with a Preen with Pre-emergent agent. You'll be fine that way.

The fact that the plants reappear shows you they can and will survive. Good Luck
Reply:A frost comes when the dew freezes. This is why you usually have frost in the fall and the spring, in canada anyways. Below freezing does not always mean frost. You can simply protect your plants by laying some blankets over them, and then weight them so they wont blow all over your yard if a wind should come up. This way it protects them from the direct frost. Hope this helps, have a good one!