Frost

Dew this morning.

10 feet away from the dew there was frost

Every spring people come in looking for annuals before the frost date.  Knowing that frost kills most young plants, we don't bring them in to sell 'till the frost date is past. I guess we should because we would satisfy our customers AND we could sell them again to the same customers when the little plants are killed by frost like the one we had surprisingly last night. I guess that I am so disgusted at the efforts required to protect plants from frost here at the nursery we just decided to wait till the danger is past. That doesn't stop big box stores from selling the plants but we hope our customers appreciate our approach to protect them from unnecessary efforts and loss.

Conventional wisdom is to wait until the frost date (ok to plant time) on Memorial Day.

What is frost and when does it occur?

I have seen frost at 41f. How could that be if the freezing point of water is 32f?

Don't ask me.

Frost always:
1. seems to be preceded by cool windy afternoons
2. needs cloudless nights
3. 41f and lower
4. migrated directly through plastic if there is plant contact (need space between) but not so with a fabric like burlap
5. quiet still nights
6. can be postponed by the presence of water on the leaves (water freezing gives off enough heat to protect young leaves for a period of time)

Frost never:
1. occurs under any kind of overhang
2. occurs under any kind of woven blanket
3. happens on cloudy nights
4. happens on windy nights

The mechanics are extremely complicated. Heat radiates from everything out into space but the clouds, burlap, overhangs, etc, block that loss, radiating the heat back to the surface. The wind disturbs that loss somehow. 

When there is frost on let's say, lettuce, water inside the cells freezes and the ice crystals pop through and break the membrane of the cell. This breakage also occurs on stems of young plants, and can actually kill the entire plant if it is fleshy and youthful enough. 

​Hope this helps.