Marin Master Gardeners
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Marin Master Gardeners

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Protect your plants from winter’s freeze

December 5, 2014
Wendy Irving

Your favorite TV weatherman has just predicted clear, calm, cold weather with overnight temperatures dropping to, or below, freezing. At my house, this usually meant last- minute scurrying around in the dark, grabbing the hose, and armloads of old sheets to toss over my sensitive plants.

Last year's hard freeze (temperatures sinking to the upper 20s or less for more than a few hours), the loss of several favorite plants and the realization that climate change may mean more frequent freezes in the future, caused me to rethink my haphazard approach. I vowed to be more prepared.

I consulted a USDA Plant Hardiness Zone Map (Marin's zones are 9b, 10a or 10b) for information about my area, and the Sunset Western Garden Book (zones 15, 16 or 17) for more cold-hardy plants to replace some I had lost. I inspected my garden with the idea of understanding its unique microclimates and found that exposed areas, and low spots where cold air settles, are not the best places for plants like citrus, fuchsia, bougainvillea, and many succulents and subtropicals.

But I love these plants, so now my fuchsias are under an arbor and my bougainvillea on a warm south-facing wall. Citrus is planted on a rise against a southwest-facing fence, where I can easily cover the smaller trees if a hard freeze is eminent. Larger citrus are more durable, but I have wrapped the trunks of the trees that are too difficult to cover with insulation, and strung them with C7 or C9 Christmas lights to switch on if it gets really chilly.

At first frost, all of my container plants (many are succulents and subtopicals), will be indoors, or grouped together under shelter close to the house, and well watered if appropriate.

I plan to consult long-range forecasts, especially leading into December and January, when freezes are most likely. Irrigation, the most important thing you can do when expecting cold temperatures, should be done in advance (a correctly watered plant is healthier, and wet soil holds heat, protecting roots and warming the surrounding air). Keep succulents dry, though. Since damage comes from the formation of ice crystal in plant cells, their plump leaves are particularly vulnerable when water-laden.

Most tender plants tolerate some frost on their leaves and short exposure to freezing temperatures, but if temperatures are below freezing for more than a few hours, extra care is needed (many weather apps predict hour-by-hour overnight temperatures by zip code). Keep sheets, blankets, plastic, etc. handy, as well as stakes to support these covers and keep them from touching foliage. Erect supports ahead of time. Cover plants all the way to the ground, removing covers during the day. Low plantings can be covered with lightweight mulch removed as soon as the danger of freeze passes. An anti-transpirant spray will help protect leaves, but polymers like this aren't dependable in hard freeze conditions. Aforementioned old-fashioned Christmas lights (or strategically-placed 100-watt outdoor bulbs) can raise temperatures but should not touch any protective covering you may also be using.

Unfortunately, drought conditions have made our gardens vulnerable in many ways, and in our area, infrequent rain makes freeze conditions more likely. But being prepared will make your job easier when cold snaps threaten.

If your garden does sustain damage, don't despair. Plants can be amazingly resilient. Don't prune or dig up damaged plants immediately. Be patient, wait for spring, or even longer if damage has been severe, and you just may see new leaves sprouting on "dead" branches, or new growth at the base of a plant that you were sure was a goner!

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