January 5, 2013

Cold Sink

A few days ago when I woke up on a very still winter morning, the official outside temperature on my iPhone app was 5 degrees F (-15 C). But on the thermometer here at the house it was 0 (-18 C). Zero.

On such mornings I am reminded that I live and garden in a cold sink.

My yard sits at the bottom of a ridge, and on a still morning the temperature in my garden can be four or five degrees cooler than the general conditions.

This shows you the bare contours of the rise behind our house, when we first moved in. My gardens have been built in the flat area below this little hill:












Cold air is heavier than warmer air because the molecules slow down and stay closer together, making it denser. Heating air makes the molecules expand and move apart, and the whole air mass becomes lighter, and it moves upward.

Cold air shrinks and sinks.

On cool spring mornings the new leaves on my trees in the yard at the bottom of this ridge get frost nipped, when there is no general frost around. It's that much colder. The heavier air sits hard on my garden.

Years after we moved in, the bare hillside is now covered in lupines and daisies in spring:

In fall the new trees I have planted hide the contours of the hill:

But this 15 foot drop drives the conditions in my gardens and sometimes, when conditions flirt with frost, disastrously so.

Orchardists around here used the principles of sinking air when they planted apple trees. Have you noticed that old apple orchards spill down slopes?



Partly for the drainage that apple trees want, but also because earlier blooming apple varieties were planted at the top where it would be five degrees warmer on a spring morning,

Cold air would sink down the slope on a still morning in early May, and the trees at the bottom would bear the brunt of a frost. If those trees at the bottom were later blooming types, with their buds still tight until late May, the apples were not as vulnerable. The early blooming varieties planted upslope, with their delicate flowers open, would be spared.

These heirloom apple trees at Tower Hill Botanical Garden in western Massachusetts march down a hillside that drops very steeply off to the right of this picture. Late blooming kinds like Northern Spies are growing below, earlier blooming Pearmains further up:

Almost every suburban town in New England has a road called Apple Hill, a reminder that developments were built in old orchards, and old orchards were built on slopes with the hardiest trees at the bottom.

Most of the time it makes no difference in my garden. When the breezes blow it equalizes the temperatures, and when there is no danger of frost it doesn't matter.

But on a still morning like the other day when it was 5 degrees in the neighborhood, but 0 degrees at my house, I am aware that my half acre of land sits in a cold trap.

28 comments:

  1. Laurrie, I perfectly understand you, I hate so low temp, we had -18C a week ago. I put the thickest clothes on.Is the snow covers plants it's easily for them to resist the frost.

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    1. Nadezda, get ready for more cold --- we are still only at the beginning of winter! The plants do like to be covered up in their protective snow blankets.

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  2. Brrr. It has been this cold here too. We still have snow cover which I am glad to have over my new tender plants. I am wondering how much they can take. My garden sits lower too. Not as exaggerated as your garden but I have often thought about how my garden sits in a bowl. I enjoy seeing your before and after photos. Your garden has developed into a beautiful place.

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    1. Lisa, thanks. It does't take much to affect the conditions in a garden -- just a few feet of elevation or a bowl shaped area will change things.

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  3. Your fine explanation of apple orchards' sloping reminds me of the North Georgia mountains, where we had a cabin. Growers of course used the same techniques. Nice to know that horticultural principles connect us no matter where we live.

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    1. Lee, yes, it really is nice to know we are connected to other geographies and to other times in history, just by tending our gardens as others have. A continuum!

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  4. Interesting that you even had temperatures as cold as 5 F. We don't live that far apart. At my house in Wethersfield, I've yet to see single digits. I'm always amazed when I see weather maps for CT. At times the variation in temperatures and conditions can be so dramatic for such a small state.

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    1. Sue, it does amaze me how variable our little state's weather is. We back up to the Litchfield hills, and you'd think it was a major mountain chain controlling the conditions in my garden nestled in their foothills!

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  5. Very interesting post! I hope your cold sink at least provides you with cooler weather in the heat of summer.

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    1. Jason, Ha! A hot still day at the top of the ridge is 96 degrees, and 92 below in my garden, but the summer humidity is drenching all over!

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  6. Very well described and illustrated. I'm always talking to customers about micro climates but this is a lot larger. Where I live is in a big bowl of sorts, the temperature here (Manassas) can be 5 degrees lower or more than in Washington DC. The wind comes howling down from the mountains too, causing desiccation on our broad leaved evergreens. For such a short distance between the two areas its amazing to see how different plants will grow.

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    1. Rob, the gardener needs to get to know the conditions in the garden -- a zone map or a weather station aren't enough when microclimates affect the conditions so specifically. As you point out, it is amazing to see how variable two nearby locations can be. And that can be especially true for summer rains -- a town away can get drenched when your own garden is dry.

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  7. Five degrees may not make much of a difference, but somehow when the thermometer reads 0, it makes me feel 10 times colder! Very interesting about the apple planting; I didn't know this information about orchard planting before, but then around here there aren't many hills to worry about. That's very smart to take into account the temperature differences in planting the different varieties.

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    1. Rose, apple orchards are everywhere here, and I grew up in a house built in an old orchard nearby. Boy did we have apples --- what a mess in a suburban lawn! But how pretty the neighborhood was in spring when they all bloomed.

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  8. It's not nearly as cold here, but living in a relatively low place means that we live in a cold sink too. Cold sink is a new term to me btw.

    The most difficult place I ever gardened in winter was in western PA. All of the freezing and thawing wrecked havoc on the plantings.

    Beautiful transformation in your yard. :)

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    1. Sweetbay, thanks so much. You can be in a cold sink in any climate, it's just the slight difference in elevation if there is frost around. What a troublesome climate western PA sounds like!

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  9. Thanks for the education about apple orchards Laurrie. This was news to me and really interesting seen as we have an old apple orchard here. Good thing you know about the difference in the temperature and why. At least knowing you are able to work with it instead of always wondering why your plants are getting frost bitten. On a positive side, I really like the rise around your property. It makes a lovely backdrop to your garden.

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    1. Marguerite, thanks. I agree that the hillside makes a bit of a backdrop that encloses us. I take it your old apple orchard is on flat land -- and probably all one variety? Whatever kind of apple you have, it must be suited to your frost schedules, to have survived all these years!

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  10. Cold indeed! I never new about cold sinks and apple planting. It makes sense to put the hardier trees below in an area like New England. Wow! To see the before shots of the ridge and the after is amazing! You have transformed that whole area!!!

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    1. Nicole, thanks so much. It really is a transformation from the barren hillside we first had, and it pleases me no end, even in my frosty garden!

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  11. This is really interesting. While it may be a pain in the winter, does it prove advantageous in summer? I love the before/after shots. What a gorgeous transformation!

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    1. Tammy, I guess it must help in summer, but not so's you'd notice, when the temps are in the 90s!

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  12. This post was a real education for me. I never that about apple orchards, although it makes perfect sense now that you have described it. But forget the freezing apple buds - do you have a problem with the pipes freezing? We used to have that problem when I was growing up in New York.

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    1. Sarah, before this house I lived in an older house and we did have pipes freeze in winter. We never did get the insulation fixed right in the area that froze. Now, in my newer home, everything is better insulated. It's still a long, cold winter here, though!

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  13. The slope and low area gives you a real appreciation of hardiness zones. Want to go to Tower Hill. I am thinking of a trip there this week. The winter garden should be glorious and the Orangerie and Lemon House are always a nice winter treat.

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    1. Layanee, I haven't been to Tower Hill in winter. The Orangerie will be just the spot --- it was lovely in summer when I was there, but it certainly is the best place to be on a winter's day!

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  14. Brrr! Our temp is at 55 right now. I haven't experienced 0 in a while! Very interesting post. You explained cold sink very well. Now I realize why some plants bloom later than others in my garden. My woodland garden is in a cold sink, which I never knew before!

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    1. Deborah, a cold sink is just a microclimate formed by a slope. You have so many Japanese maples, and they are sensitive to slight temperature fluctuations -- when your area flirts with frost it probably affects them!

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