Freezing bulbs: Duration vs. low temperature

Michael Mace
Mon, 14 Jan 2013 17:57:52 PST
In California we're currently going through one of the nastier winter
freezes we've had in a few years.  The overnight low last night in my part
of San Jose was probably under 20 F (-7C) at dawn, and when I got up in the
morning there was more than half an inch of ice on the puddle of rainwater
in my wheelbarrow.  That probably sounds like a nice spring morning to those
of you who live in places that have real winters, but for coastal
California, it's serious cold.


Yesterday I put a clear plastic tarp over my most exposed bulb pots, along
with a string of Christmas lights to add a little warmth.  It looked festive
when I went to bed last night, but this morning despite the tarp and the
lights the soil on top of the bulb pots was frozen rock hard.  There was a
fuzz of hoarfrost on many of the bulb leaves, a sight that would normally be
beautiful but that made me feel a little bit sick in this context.  I
worried that I had done serious damage to my collection.  However.


As usual for cold snaps in California, the weather was completely clear
today.  The sun melted the frost on everything it touched, and by afternoon
the pots were totally defrosted.  And the bulbs look just fine.  One pot of
Tritonia crocata shows moderate damage, and a Brunsvigia marginata growing
in the ground has some tip burn on its leaves, but those are literally the
only problems I can find.  Lachenalias, Oxalis, Amaryllids.even the
first-year seedlings look fine.


Other things in my garden are not as happy.  There's serious freeze damage
on a coral tree, a Chorisia, and some sages.  My Bougainvillea is probably
dead (again).  But the bulbs look happy.


This matches a pattern that I think I've seen in other years.  Freezes that
last only overnight don't seem to do serious damage to most of my bulbs.
I'm sure there is some level of overnight cold that would kill them, but I
haven't hit it.  What seems to do serious damage is sub-freezing
temperatures that last more than a day.  Based on previous cold snaps, a
full 24-hour period of 27 F seems to do a lot more harm than an overnight
low of 20 F.


I'm curious to know what other folks have seen.  Do you think a very cold
night is less damaging than a prolonged period just below freezing?  And
what low temperature do you start to see freeze damage at?



San Jose, CA

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