Sunday, August 21, 2011


            I regard my email inbox with the same jaundiced eye that used to be reserved exclusively for the telephone. As far as I’m concerned, these communication devices take up space in our house for my convenience, not anyone else’s. Over the years, I’ve perfected the selective deafness required to ignore phone summons (a trait inherited from my dad, the lone male in a six-person household, whose ability to tune out female voices was legendary). The insidious nature of internet technology is harder to circumvent. Email hooks me with written words. I’m the kind of person who reads everything: signs, labels, every word of anything I’m asked to sign. Ergo, it’s hard for me not to read something right in front of my face.

            Yes, there’s a point to subjecting you to yet another of my peculiarities. The emails I’m having a tough time deleting lately have to do with fruits for fall planting.

            Burpee’s started it. Cooks Garden and Gardens Alive! chimed in yesterday. It’s like they know my weaknesses and are out to make a few bucks off them. Opening these siren–song solicitations, however, led me down the unexpected path of discovering how many chilling hours the average Brunswick County winter is good for.

            What’s a chilling hour? you ask.

            A chilling hour is a 60-minute period wherein the temperature is 45°F or less. You add up all these hours between November and February, the result being a number that tells you how much cold weather you have.

            What does that mean for my garden? you persist.

USDA cold-hardiness zone map
for the Southeast
            Chilling hours are the basis for the USDA’s cold-hardiness zone map, a tool to help you decide what plants will most likely survive an average winter where you live. But many edibles and ornamentals also have chilling requirements that determine whether or not they will fruit or flower.

            In my neighborhood, for example, tulips, lilacs, rhododendrons, peonies and fritillaries cannot be counted on to produce the gorgeous blooms that are the reason we grow them in the first place. Why? Not enough chilling hours. We have the same problem with apples, raspberries and cherries. On the other hand, figs, pomegranates, okra and other native hibiscus (what? you didn’t know okra’s a hibiscus relative?) do well here because we don’t have too many chilling hours. Capisce

No peonies for me
            So anyway, I was drooling over the online offerings of berries and stone fruits when it occurred to me that I didn’t know how many chilling hours I can depend on from a regular Brunswick County winter. (It may surprise you to learn that I am not obsessive enough to track chilling hours on my own. Hourly temperature checks around the clock for 18 weeks are required: nobody likes me when I’m sleep-deprived.) Not having a suitable reference work to hand, I went net-surfing.

Minimum chilling hours
across the southeast
           What I found, after some considerable time wandering around in the ether, was this map, at AgroClimate: A Service of the Southeast Climate Consortium. It specifically refers to growing peaches with varying chill needs in South Carolina, but the broader information’s revealing. (What really struck me was that most of the United States has more than 1200 chilling hours per winter: the national map was mostly white. No wonder people are moving south in droves.)

            Searching for chilling-hour statistics segued into learning about heating degree days and cooling degree days. I’ve seen those abbreviations on the National Weather Service’s almanac pages for years, and vaguely knew what the letters stood for, but not what they meant.

            Which is…? you prompt.

Barrow, Alaska
            Heating degree days (HDDs) are a measure of demand for energy to heat buildings. You settle on a base outdoor temperature below which the house gets nippy so you turn on the heat. Most calculations use 65°F. So you average the actual high and low temperature of a day (adding the high and the low together and divide by two) and subtract the result from the base. For example, on January 13, 2011, Oak Island’s high was 41°; the low was 28°. (Brrrr.) The average of the two is 69, divided by two is 34.5; a base temp of 65 minus 34.5 = 30.5. That means Oak Island had 30.5 HDDs on January 13.

Miami Beach, Florida
            Cooling degree days (CDDs) are, as you probably already suspect, a measure of demand for energy to cool buildings. To figure these, you subtract the base from the average of the high and low temperatures of a day. To continue with the Oak Island example, on July 7, 2011, the high was 93° and the low, 79°. We add those numbers, getting 172, and divide by two for an average of 86. Subtracting our base from the average (86 – 65 = 21) gives us 21 CDDs for July 7.

            I hate math, you grumble. But I’m assuming there’s a payoff?

            Well, maybe. Mostly I just think this stuff is fun to know, and I do like math. I found a really cool (pun intended) website called Degree Days that’ll calculate the number of HDDs and CDDs of any location with a reporting weather station, which usually means a controlled airport. Then I compiled a little table, comparing the HDDs and CDDs of six representative cities.
Barrow, Alaska
Los Angeles, CA
Miami, FL
Montreal, Quebec
New York, NY
Wilmington, NC

            Isn’t this neat?

            Payoff? you remind me.

Pink Lady apples: yes
            Well, you can monitor the efficiency of your heating and cooling systems by dividing the total number of HDDs or CDDs for the period covered by your monthly power/oil/gas bill into the total kilowatt hours/gallons/therms used. Establish a baseline, then look for discrepancies. Tim and I replaced our 13-year-old HVAC in June, so I compared the August bills from 2010 and 2011. In 2010, with the old heat pump, we slurped up 2.05 kilowatt hours per CDD. This year, we only used 1.61 KwH per CDD. Now, that’s efficiency! (For a more detailed explanation, read Jack Williams’ “Answers: Heating Degree Days” article linked here.)

Lapins cheeries: no
             Oh, yes. To circle back to the beginning: based on all my new knowledge, I decided which fruits to order. Blackberries, yes. Raspberries, no. Southern highbush or rabbiteye blueberries, yes. Northern highbush, no. Pink Lady apples, a hot-climate cultivar from western Australia, yes. Lapins cherries, bred in Canada, no. Lilac, maybe. There’s a “new,” “heat-tolerant,” “reblooming" version out there called Bloomerang Purple Penda. I’ve tried all the rest of them—without success—over the years: might as well give this one a go. Will keep you posted.

Now all I have to do is find a place to plant them.

Thanks for dropping. Stay cool. Or whatever.


Lilacs: maybe