What is a hard, killing frost?

Mel B. states in his book: A hard frost is indicated by a crunchy feel to the ground and a thin film of ice on the birdbath. Could someone be more specific? My fall garden, with red lettuce, cabbage, beets, spinach, Swiss chard, and turnips, has experienced several frosts with no ill effects. At some point, though, it will become so cold that even the toughest plants will be frozen solid and spoiled. Are we talking about temperature lows of 5,10,15, 20, 25, or 30 degrees. At what point should I call it quits and harvest what I can before it is too late? I am not really interested in extending the season into winter.

My garden is up on a hill (cold air flows down and hot air rises) and gets a great deal of sunshine all day long.

Experience will enlighten soon enough.

My understanding is a light frost is temperatures 29°F-32°F, these temps will kill off very tender, tropical and sub-tropical plants such as tomato, pepper, eggplant, basil and the like. Hard frost is 28°F or below for several hours. I have had weeks of hard frost during the overnight hours. I currently still have Brussels sprouts, kale, carrots, parsnips, beets and Swiss chard left in the garden. I harvested the last of my broccoli 2 days ago. All of these have tolerated these low overnight temps for weeks. Cell division will occur at 36°F and above, so these plants are still growing when the days are in the 40s. I will probably pull everything soon, with the exception of the brussels sprouts that I plan on leaving in the garden and harvesting as needed up until Christmas.
Within the next week (or 3) the temperature will rapidly drop and everything will freeze and stay that way until late March. If it gets cold quickly, we can expect to be playing pond hockey on Christmas Eve.

Will it be a killing frost? Unless you have advance warning of a sudden, dramatic temp drop, you might not know until it's already happened. Crunchy ground, having to scrape frost of your windshield, etc, in days before.... you have to think it's coming.

Correct me if I am wrong, please, but I wonder if what you're really asking is, when do I pull my crops? I don't have a good answer for that. My dad might have, but I was too young when we had on a working farm. Lacking that... I think the real questions are: A) Are your crops mature enough to harvest now, and B) Will leaving them in the ground a few more days, at risk of frost, gain you anything? If you're doing eat-as-you go, then you will only be harvesting what you need anyway, and hopefully make the best of what's left. If you are canning and/or freezing, and your plants are sufficiently mature as-is, you might want to pull them now and get the job done.

Just my $0.02, and I really want to hear what more experienced gardeners have to say on this!

I've always heard the difference that really matters is between a frost and a freeze. Well, for 52+ years anyway! I looked this up once...

Frost is caused by radiational cooling. It happens only on clear or nearly clear nights. The ground looses heat and freezes on the surface when the temperature falls below freezing. Most hardy fall plants can take this. I'm sure there are different levels of frosts too, but covering your plants will more than likely be enough to keep them around longer.

A freeze is caused by advective cooling. We call it the North Wind or Arctic air! A mass of cold air comes in from the North and stays. It can be clear or cloudy, night or day -- covering plants won't help, because it's not just the surface that will freeze -- everything freezes!

So far we've only had frosts here in the mountains of Pendleton County, WV. One night last week it got down to 22 degrees, but I covered my lettuces, greens and broccolini and they still live...and I'm still eating it!

pattipan