Freezing bulbs: Duration vs. low temperature

Jane McGary
Tue, 15 Jan 2013 11:23:55 PST
Until recently I was growing my bulbs in cold frames at 1600 feet 
(500 m) elevation in the Oregon Cascades foothills, so I had a lot of 
experience dealing with frost. When the temperature was predicted to 
drop below 24 F, I covered the plants inside the frames. First I used 
newspaper, but it absorbed water and was messy. Then I got a big roll 
(industrial size) of microfoam blanket as used by nurseries and cut 
it to size to fit inside the frames. I was able to use these sheets 
of microfoam for a number of years, discarding them when I moved to 
my new home and new bulb house. The microfoam blanket has thin 
plastic on one side and a puffy foam on the other, and I put the foam 
side next to the plants. It worked very well. This product may be 
available only in large rolls from commercial suppliers, which is not 
a problem here in the county with the most nurseries of all US 
counties. I agree that plastic film alone should not be used, and 
especially clear plastic.

The nonwoven fabric mentioned by one or two correspondents is sold in 
the USA under the name Reemay, and it comes in several different 
thicknesses. I would get the thickest one for frost protection. I use 
the thinner kind to protect amaryllids from bulb fly in the spring.

A number of frost protection products are available online from 
Territorial Seeds: in Oregon. One that I've 
just ordered (too late, probably, for the Canarina) is the Wall 
o'Water, usually used around tomato plants; it is a cylinder of 
plastic tubes that stands more or less upright when you fill the 
tubes with water, and I found it very effective around tender 
perennials as well as tomatoes.

We are experiencing a minor cold snap here in Oregon too. I now live 
in a "banana belt," so night temperatures have been in the upper 20s 
F and I haven't added any covers. Keeping the foliage fairly dry in 
my solid-roofed but wire mesh-sided bulb house is an aid to 
preventing cold damage, as the unheated cold frames were. Early in 
the morning some foliage (especially Arum spp/) appears wilted, but 
it recovers to normal later in the day. Many kinds of plants respond 
to freezing by wilting, so the cells have less water in them and will 
not burst when frozen. There are other kinds of responses, too, as 
when rhododendrons lower their leaves to allow snow to slide off 
rather than burdening and breaking the twigs.

I've noticed a pattern over the years: "marginal" bulbous plants that 
survive cold snaps are most likely to come from the Northern 
Hemisphere, especially the Mediterranean, west and central Asia, and 
the west coast of North America. Plants from South America and 
southern Africa are less likely to recover, though there are of 
course exceptions, such as Amaryllis belladonna (mine have perked up 
today after being flattened by frost, and they're not under cover), 
or Zantedeschia aethiopica (calla lily; some leaves mushy here, but 
it will recover well). Over millennia plants can relocate from one 
climatic regime to another, and their genomes can still harbor 
adaptations to cold even if their present homes rarely experience it. 
And remember that "coastal California" has a lot of topographic 
relief with frost pockets and steep hills where temperatures in the 
mid-20s F occur every winter.

Jane McGary
Portland, Oregon, USA

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