On 3/28/2009 12:53 PM, Bill wrote:
In article ,
"Denis Mitchel" wrote:

Does the temperature have to be at or below freezing for frost to form on
plants?

Thanks

http://en.wikipedia.org/wiki/Frost

Notice the mention of dew point.

Bill who thinks the frost acts like a magnifying glass hence the plants
burn. Not sure if this is correct only know that late frost about here
has me out spraying water on tender new plants.


Liquid water from a hose is above freezing. In the process of turning
into ice, such water must first give up heat just to be chilled to 32F
(0C); then it must give up significantly more heat just to become ice
even if the temperature remains at 32F. That's why farmers in my area
spray water on citrus, avocados, strawberries, etc if there is a late
freeze. For "radiation frost" with air temperatures above freezing
(described in the Wikipedia article you cite), they do nothing since
those crops are generally hardy enough to withstand the chill.

--
David E. Ross
Climate: California Mediterranean
Sunset Zone: 21 -- interior Santa Monica Mountains with some ocean
influence (USDA 10a, very close to Sunset Zone 19)
Gardening diary at http://www.rossde.com/garden/diary