What temps constitute a frost?

Question

I live in Zone 5 (Chicago). For the next few nights, our lows are being forecast around 32 degrees, however, I don't hear anyone talking about frost damage to the plants that have begun to come up in the last few weeks, such as lillies, salvia, astilbe, and sedum. I remember reading once that 32 degrees does not necessarily constitute a frost, but I can't find any reference to that on the Internet. Is that true and if so, what exactly does constitute a frost? Thanks!

Answer 1

WIKI  explains what is need for frost in there, Moisture is another factor

Answer 2

A frost is not a specific temperature.   A frost is the freezing of plants caused by temperatures below 32 F.  The lower the temp., the more powerful or "killing" the frost will be.

Answer 3

From the National Weather Service glossary:

"Frost: The formation of ice crystals on the ground or other surfaces in the form of scales, needles, feathers, or fans. Frost develops under conditions similar to dew, except the temperatures of the Earth's surface and earthbound objects fall below 32oF. As with the term "freeze", this condition is primarily significant during the growing season. If a frost period is sufficiently severe to end the growing season or delay its beginning, it is commonly referred to as a "killing frost". Because frost is primarily an event that occurs as the result of radiational cooling, it frequently occurs with a temperature reading in the mid 30s."

"Frost/Freeze Advisory:  This product is issued by the National Weather Service when freezing temperatures or conditions conducive to the formation of frost occur during the growing season.

Frost Point: Dew point below freezing."

Answer 4

1. Some plants are more frost tolerant than others.

2. If the average last frost date in your area is yet to come, there is no reason to issue a frost warning

Ground frost can occur even when forecast lows are a few degrees above freezing (not always, but sometimes). It's worse when the ground is dry. Moist soil retains heat better.