Frost Warning? I'm Just Sayin'...

You remember last year how, not only was there frost but SNOW on December 4th?

Cabbages fight it off!

I don't think it'll snow this week, but we've already had a frost at my house, and it looks likely for Tuesday night.  Frost can damage plants even if the air temperature doesn't actually get below freezing.  This always seems like a hard fact to accept here in semi-tropical zone 9A.  But this is the time of year for that sort of thing.  Plants aren't hardened off.  The weather changes rapidly from mild, sunny days to bleak, chilly days.  What to watch out for?  Calm, clear, dry nights.

No ill effects.

The heat absorbed by the earth is released or radiated into the atmosphere at night.  Clouds help keep that layer of heat closer to the surface of the earth, and slow the radiation.  Wind helps mix cold air and warm air, maintaining a bit warmer air than might otherwise occur on still nights.  And humidity slows temperature change.  We notice this most in the summer, when our hot humid nights barely cool down to 80° before it's time to heat up again in the morning.

Amaryllis soft tissue ruined, but plants survived.

Radiation frosts can occur even if the air temperatures remain above freezing.  What's happened?  The surface temperature of the plant has dropped below freezing, even though the air has not, and if sufficient moisture is in the air, ice crystals may form. 

Night photo of frost-bitten tomatoes

The frost last week got the top of the tomato forest, but overall the plants look okay.  I think it's best to leave frost or freeze-damaged foliage on the plant, to provide a bit of protection throughout the winter.  If I pruned back to live, healthy tissue, more of the plant would suffer damage in the next frost.  Yes, it's ugly. After I harvest the tomatoes, I'll pull them out.  But landscape plants are best left alone, unpruned, until March.  You don't want to encourage new growth midwinter, either.

We like to get all excited about winter here, because we get so little of it!  I imagine those zone 4 and 5 gardeners are laughing at us right now, worrying about our little frosts.  That's okay -- I chuckle to myself when they complain about humidity!