hard frost

I don't know the exact definition of a hard frost, but my understanding is that below-freezing temperatures that last for more a few hours qualify. At 5 a.m. my husband told me it was 23 degrees and when I checked at 7:30, the temp was still 23 degrees, so I'm calling this a hard frost. Plus a visit to the garden beds revealed rock hard ground - rock hard when rapped with the knuckles, but still diggable (the technical term) with a shovel.
(Fortunately for me no whistles sounded alerting me to the freezing temperatures and routing me from my bed so I could take the only blankets in the house outside to cover my precious, tender tomato plants on whose survival hinges my entire livelihood. See Eudora Welty's "The Whistle" for a stark portrayal of a night in the life of tomato farmers in Mississippi during the Depression).
I have been anticipating a hard frost and hoping I'd have straw on the premises in time to mulch my lettuce bed (see Bud Markhart's advice in Norther Gardener, sept/oct 2010 issue).
And lo and behold I got straw yesterday. But did I put it on the lettuce bed before temps dropped below freezing? Hmmm. As my kids like to say when they are on the defensive, "why would I?" And really it's a great defensive stance because usually, as in this case, the reasons why one would are so numerous there isn't time enough in the day to go into it.
So... the lettuce froze and I put straw on it anyway and we shall see what we see come spring.

This truly dynamic photo enlightens the viewer as to what straw looks like and how dead my marigolds are (upper right).

I dug out my scarlet nantes carrots, too, by the bye. They are now stored in a plastic bag in my hydrator. Potatoes - last plant - are still in the ground.