Saturday, November 27, 2010

First Freeze

I grew up in western Oregon, land of clouds, cold rain, and Douglas firs.  For over a decade, I've lived in central Texas, land of heat, sun, and drought-tolerant shrubs.  Every week I talk to my parents and we compare the current conditions in our respective states.  For several months of the year I complain about the unending heat, while, for the other several months of the year, they complain about the unending rains.  In my mind, we live in opposite lands.

Yet when I look at a USDA Hardiness Zone map, we are merely one zone apart.  Austin, Texas is located in zone 8B, while Salem, Oregon is located in zone 7B.  The problem with the hardiness zone map is that it is created based on only one measure of climate - the average annual minimum temperature in an area.  In central Texas, despite our overall warm climate, we receive arctic fronts that drop the temperatures down to an uncharacteristic 15˚ to 20˚ F once or twice a year.  The opposite is true in western Oregon, where, despite the long months of near-freezing temperatures, the proximity of the ocean keeps the lowest temperature of the year from dropping below 5˚ to 10˚ F.

Recognizing that plants are stressed by heat as well as cold, the American Horticultural Society came up with the AHS Heat Zone map to categorize climate based on the average number of hot days a year, which are defined as days above 86˚ F.  On the heat zone map, central Texas is in zone 9, which experiences 121 to 150 hot days a year, while western Oregon is in zone 4, experiencing only 15 to 30 hot days in a year.  This contrast, of heat zone 9 versus heat zone 4, seems to characterize the differences between our climates much better than the hardiness zone map.

But, while heat may define the climate in Austin, the fact that it does freeze here can't be ignored.  The difference between cold and freezing is the difference between life and death for many plants.  So, every year, around the first of December, I begin worrying about my plants, scanning the 7-day forecasts for potential freeze nights.  This fall, freeze warnings in the midst of an otherwise warm November confused me, causing me to question my mental freeze timeline, which went something like, Freezes begin in December, become threatening around Christmas, get less severe after Martin Luther King Jr. Day, and end sometime in March

For the most part, I was right on.  According to the data compiled by Dave's Garden, Austin is "at-risk" for frost between December 6 and February 17 each year, and is most likely to receive killing frosts between December 28 and January 23.  Our frost-free growing season extends from March 15 to November 14, giving us a full eight months, or 292 days a year, without frost.  (In contrast, Salem, Oregon, is "at-risk" for frost from October 24 to April 23, is likely to receive killing frosts from November 16 through March 23, and experiences only 184 safe-from-frost days a year.  Again, the hardiness zone map just doesn't say enough.)

I have noticed that my yard, which seems to be located in a milder microclimate than the Austin weather stations, only experiences a freeze when the official Austin temperature drops below 30˚ F, which only happens a few days a year.  In fact, my first four winters in this house were mild, with nothing but several mild freezing nights each year.  Nonetheless, I dutifully carried my motley assortment of potted plants up the stairs into the kitchen before each freeze, only to haul them back down the stairs a day or two later.  The winter before last I noticed that my curry plant was hosting termites in its heavy pot and declared that, next year, I wasn't going to haul the plants up the stairs.  My mom, who has an uncanny ability to catalog every wouldn't it be nice if I had ... statement that a relative makes, heard my declaration and sent me a pop-up greenhouse for my next birthday.

In any other year, the pop-up greenhouse would have been a perfect solution, but last year we actually experienced, even here in the warmth of town, temperatures as low as 20˚ F, which killed most of the plants in the picture above.  Luckily, in the spring a few volunteer curry plants resprouted from their roots to carry on the legacy of the momma curry plant.  And the greenhouse, I'm confident, will be quite useful in the gardening seasons to come.

Meanwhile, this year's frost season is approaching and I have the front-yard garden to consider.  For the most part, cool-season vegetables do fine with a light freeze.  Kale, spinach, and bok choy aren't harmed by a light freeze, while chard, beets, and lettuce may lose their outside leaves to the frost, but are able to grow back.  My garden is so packed with potential food at this point, though, that I hate the idea of losing any of the tasty leaves to an early frost.

I also count on the frost season to begin after leaf fall, which is currently in progress.  Once the leaves are on the ground, I can rake and pile them on the garden as mulch, which helps to keep the bases and roots of the plants from freezing, which in turn allows the plant to regrow if frost-bitten.

Last night's temperatures were predicted to be in the low- to mid-20's.  I found Plankets at the Home Depot in the 10' x 20' size, almost big enough to cover my whole garden.  The fabric was heavy and soft, and the large size made them easier to work with than narrow strips of row-cover fabric.  Using the wealth of bricks from this property and two Plankets, I covered the garden for its first freezing cold night.  In reality, I think it was only 32˚ F here last night, judging from the unprotected Cosmos plants that were just slightly frost damaged, but now I have a plan for the freezing nights to come, in the killing-frost month of late December through late January.

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