A NEIGHBOR with a new cold frame emailed me the other day, seeing colder weather finally in the forecast and wanting to know how to extend his season even longer inside the unit. Well, serendipitously, later that day I went to the post office and found a review copy of Niki Jabbour’s new book, “Growing Under Cover,” waiting for me, and had some answers for my neighbor.
Despite living in Nova Scotia, writer Niki Jabbour is a year-round vegetable gardener, coaxing harvests out of every manner of season-extending device imaginable, from cloche to full-on polytunnel. She’s the award-winning author of books that include “Veggie Garden Remix” and “The Year-Round Vegetable Gardener.” And she’s back today to talk cold frames, one of the tactical approaches in “Growing Under Cover” (affiliate link), a book that helps us not just lengthen the season, but also outsmart pests and increase productivity. (Above, at Niki’s, a cold frame full of carrots in fall that will be tucked in to harvest gradually in winter.)
Plus: We’ll have a book giveaway. Enter in the comments at the bottom of the page.
Read along as you listen to the January 4, 2021 edition of my public-radio show and podcast using the player below. You can subscribe to all future editions on Apple Podcasts (iTunes) or Spotify or Stitcher (and browse my archive of podcasts here).
cold-frame 101, with niki jabbour
Margaret Roach: Hi, Niki. I need all of those things in your book. [Laughter.]
Niki Jabbour: Hi, Margaret. Great to talk to you again.
Margaret: Yes. Happy winter. Happy year of madness.
Niki: Indeed. That’s probably the best description. I hope you’re writing a book called that, that we can expect to come out next year.
Margaret: Yes, I’m working on the title. I don’t know how you managed to get this book done in addition to everything else.
Niki: You know what? I don’t know, honestly, but it was really fun to write and sort of revisit the whole season-extending, as well as the many other reasons besides harvesting in cold weather. So yeah, this was a fun project for me for sure. Kept me busy.
Margaret: And it really is… Again, as I said in the introduction, not just season-extending, but it’s like insect prevention. It’s like all of the uses for these sort of barriers and enclosures and so forth, right? And there’s a wide range of gizmos that you have. [Laughter.]
Niki: Gizmos is an underrated word. I’m going to have to steal that from you, Margaret, and start using that one. Yeah, for sure. I mean, I garden in Nova Scotia, which is deer country as is, of course, much of the U.S. and Canada, and I deal with deer every single day. I mean, I went out for a run in my neighborhood yesterday and had to stop to let four deer walk across the road in front of me. And they were probably on their way to my garden. [Above. Niki’s photo of deer int he garden recently, from her Instagram.]
But yet, everything from deer and groundhogs to rabbits, to cabbage worms and slugs and flea beetles and potato beetles and cucumber beetles. There are a lot of reasons to consider covers. I think most people automatically go to season extension and protecting from cold weather. But I use them in so many different ways to not only protect my food from pests and cold weather, but to even grow healthier plants and in the end have a larger harvest, because those plants have been protected and they’re able to produce better.
There are a lot of reasons to sort of consider yourself an under-cover gardener.
Margaret: Right. In the introduction, I also said I was asking questions, ahem, for a friend. [Laughter.] Oh yes, my friend…you know how that is when you ask questions for a friend. Seriously, I actually do have a friend who just built cold frames. This is his first late fall-into-winter with them, and he actually did ask me the question I said, but that is the oldest disguise in the book also.
I have loads of cold-frame questions for me, too, besides the extra extending of the season in them, and we’ll get there–because “build a cold frame” is actually the Number 1 thing on the top of my 2021 garden to-do list, because I had an ancient one that succumbed finally and fell apart for my early years as a gardener here and I never replaced it. And I’m just jealous. Especially this last year, I’ve been jealous of my friends who have them and they’re saying, “Oh, I have my seedlings in the cold frame. Oh, I’m having salad from the cold frame.” All these little extra goodies.
Cold-frame 101: Maybe we should start with like what is a cold frame for? Why would I choose that because there’s, again, all these other gizmos in the book, to use our new favorite word? [Laughter.]
Niki: Yeah, for sure. Well, I’m glad you brought up also starting seedlings in the cold frame, because most of us I think at this point think cold frames are just for harvesting in winter or maybe pushing back spring a little earlier, so that you can start planting in March or even late February, depending where you live. But you can use a cold frame for starting seedlings, which will then be dug up and transplanted to your garden.
But you can also use them for overwintering half-hardy perennials, or if you love to force bulbs inside, and oftentimes, of course, many need a cold period, you can pot them up and put them in a cold frame for a couple of months, until that cold period is up.
So it’s not just about growing vegetables. There are many uses for cold frames. If you’re into alpine plants or different things like that, you can use them to shelter those over winter, too. Lots of applications.
But yeah, for the most part, I do use mine to get a jumpstart, an extra-early one in spring, as well as go later into the fall, and throughout winter we harvest. A cold frame really, its whole job is to shelter plants from ice, snow, winter winds. And even though it’s a small little gizmo or device, it really does take in a lot of solar energy and it heats up. Like today here, it is freezing. It’s minus-20 Celsius. I don’t know what that is in Fahrenheit [note: It’s equivalent to minus-4].
I just know it’s really cold, but the temperature in my cold frame out there now, I checked this morning, it’s just above freezing. I mean, that’s amazing to me, that it’s sunny here today and therefore the temperature inside the frame is just above freezing. So that’s the whole job of a cold frame: to create a microclimate around your plants and start those seeds earlier and harvest earlier and harvest out of season. That’s the main, I think, way most of us are going to use a cold frame.
Margaret: In this 101, maybe we start with where is a good place to site a cold frame relative to sun and buildings and blah, blah. Where’s a good place? And do I dig it into the ground? You know what I mean? Like how big? The basic sort of engineering before we get started building, scoping it out?
Niki: I think planning is definitely going to work in your favor. Before you just either order a cold frame online, or you decide to go out and grab the materials, start by thinking about it. There’s two main parts, of course: the box or the frame, and then the top, which is also called the sash. Those are the two main components of a cold frame, and they can be made from lots of different materials. And I’ll talk about that in just a sec. But I think as you mentioned, siting it is important.
When I first put my cold frames in, oh my gosh, probably 18 years ago, the first frames I had, I put them in the part of my backyard where the snow always melted first, because I thought, well, obviously that’s a bit of a microclimate. It was a very slight south-facing slope and the snow just always melted first there, so that’s where I sunk my cold frames. And I did sink them down into the soil, and I built them from untreated local hemlock. They were each 3 by 6 feet, and I had three of them at the time. And my gosh, I got a lot of use out of those three cold frames.
They were just my winter food factories—that’s how I’ve always kind of thought of them. And that was one of my introductions into really using different season-extenders.
I love cold frames. I think they deserve a place in every garden, even my garden, which is a big garden. And even though I have a polytunnel now, I still use my cold frames. I have portable ones, and permanent ones. I use them for lots of things.
Materials you asked about as well. Wood I think is probably the most common material if you’re going to build a cold frame. I use untreated hemlock, as I mentioned, but you can use cedar or other types of lumber.
You can also use bricks or cinder blocks. There’s so many different types of things you can use for them, but also polycarbonate. A lot of people are buying cold frames, and they’re made from polycarbonate, the box. And while it’s not as insulating as wood, they certainly have their place. And you can do things, a few tricks, to make them maybe a little more insulating, even straw bales. My gosh, can any gardener have enough straw? [Laughter.]
No. I often buy like 20 or 30 or more straw bales in the fall, and I pile them in my shed and in the back of my polytunnel for winter. But I also use some of them for straw-bale cold frames. Super-quick. In the spring, that straw is maybe a little bit partially starting to rot, and then I can use it to make straw-bale beds for squash and zucchini and things like that. You can make the frame from lots of things. [Photo below of straw-bale frame by Joseph DeSciose, from Niki’s earlier book “The Year-Round Vegetable Gardener.”]
And then the sash, people use old windows, which is great, but they can break. I had a neighbor that always used windows for her cold frames. Invariably, one would break over the winter and you’d be picking glass out of the soil, which is just not fun. Especially when your lettuce is peppered with little glass splinters, you don’t really necessarily want to eat that. I prefer using polycarbonate. I’ve also turned old windows… I have knocked the glass out of them and then stapled thin polyethylene material to the top and bottom to make an instant sort of cold frame top that’s unbreakable. You can do that, too. Then it’s double-insulated, but polycarbonate is probably my favorite material for the top.
Margaret: The sash, the lid, whatever we want to call it, should it be tilted? In other words, should the back of the frame where the hinged part of the stash is, should that be higher than the front part, maybe where there’s a fastener or something like that? Do you know what I mean?
Niki: Yeah, it totally should. You should be angling your cold frame. The wooden ones I started with were 18 inches tall at the back, 12 inches tall at the front.
Niki: It had that little 6-inch differential, and I sunk them in the ground about 6 inches as well just for a little bit of extra installation. And I thought those were fantastic. Basically you do want to have an angle in your cold frame, because it’s going to capture more solar energy. And you want them to face, as I mentioned, towards the south so they do get a lot of sunlight, even in winter when the winter sun is so low in the sky.
Like right now up here in Nova Scotia, it just grazes the horizon, but it’s still enough that my cold frames are warming up in the day. You just need to find a place where they get that sun.
Definitely have them at an angle. And I’ve seen some where they’re 2 feet tall at the back and 1 foot at the front. It’s quite a steep angle. It doesn’t have to be super-steep, but you definitely want to have some angle so you do capture as much light as possible.
Margaret: O.K. I would imagine also that the snow melts and drips off–do you know what I mean, it might help with that, like the way that a roof is pitched. Do you know what I mean?
Niki: Yes. Snow shedding and rain shedding and all that. Because, of course, we get rain and then overnight it’s going to freeze. If the roof is sloped, it’s going to run off much easier for sure.
And same with snow. If it’s a light snow, it’ll slip off easier. I do go up there. I have like a little broom and I brush it off when it snows. I try to do that. At this point, we don’t get snow as much as we used to 20 years ago. But if we do get a snow out there, I knock it off the tops.
Now, sometimes in February, when we’re in the middle of that deep freeze of winter, and I just have something like carrots in a cold frame, I don’t even bother to take the snow off because really, they’re not growing. They’re just sitting in there waiting for me to harvest them. So it’s not as important for me to make sure the light is getting into that cold frame for the root crops versus the salad greens, like the spinach or the lettuce or the arugula, where they’re still slowly growing and they still need that solar energy to warm up the interior of the frame during the day.
I always make sure I brush the snow off those frames. But if I just have a carrot cold frame, the snow is extra insulation, so I don’t mind that sometimes.
Margaret: You just said extra installation, and sometimes, and this is where my friend, my neighbor Paul, who actually did get in touch with me to ask me…
Niki: Hi, Paul.
Margaret: Yeah. Hi, Paul. This was his first year with these wonderful new cold frames that he built, and he’s been enjoying, enjoying, enjoying. And as you said a couple minutes ago, we have a different weather pattern. It doesn’t get cold as early as it did, and it’s not as severe and so forth.
We’re just about to have our first serious cold this week while we’re taping, which is mid- to late December. He wanted to stretch it and stretch it a little more.
And he was like, “What can I do? Do I cover it? What can I do? And do I put heating cables in it? Oh my goodness, what should I do?” A light bulb, he read about somewhere. You know what I mean? What about that? Do you cover them? You said snow insulation.
Niki: There’s lots of options. I mean, some people do put heating cables in the soil of their cold frame, usually before they’re planted, of course, because you want that to be sunk down at the soil, but that is something you can totally do.
You can also hang little incandescent lights in there as well. I’ve done that in my mini hoop tunnels. In a cold frame, it’s a little harder, where it’s a lower profile. You don’t want the lights hitting… If it’s a polycarbonate top, you don’t want them bumping up against that.
You also don’t want them touching the food crops. If there’s not a lot of space in there, it can be kind of hard to do.
Often people use heat sinks. They will take milk jugs or water jugs, 1-gallon jugs, or even… Well, I would call it a 2-liter pop bottle, but I guess I’m not sure what that would be in the imperial system, probably like half-gallon pop bottle or soda bottle. And you can paint that black, or your milk jugs black, and fill them with water and put them inside. And during the day when it’s sunny, they’re going to absorb heat. And then during the night, they will release some of that heat, just preventing huge temperature swings. It just more moderates the temperature inside your cold frame at night.
For my polycarbonate cold frames, I do have one that’s top and sides polycarbonate. It’s not nearly as insulating as my wooden cold frames. But I take those polycarbonate cold frames and I put leaves or straw around the perimeter of them just to bulk up.
In winter, the top is still open and clear, so the sun can still get inside, but you can also put evergreen boughs around the outside of them, too, for that extra insulation. You’re taking something that maybe is more like a three-season cold frame and turning it into a four-season cold frame.
Some people paint the interiors of their cold frames white to reflect extra light. Some people put heavy-duty aluminum foil in the inside of them along the sides to help, again, reflect more light in. So there’s lots of tricks you can do.
But for the most part, I will use some heat sinks in mine. And I will use insulating materials, like if I have a carrot cold frame. Right now one of my polycarbonate cold frames is filled with carrots. And I went up yesterday before the temperature dipped and I filled it with some shredded leaves and straw just for extra insulation. I mean, again, those carrots aren’t growing now, so having the insulating leaves and straw on top just helps prevent the soil from deep-freezing. [Above, a polycarbonate frame at Niki’s with evegreen boughs and straw at the edges for added insulation.]
Margaret: On the other end of the season, you got a box with a lid and it gets hot in there in the summer. [Laughter.] We have to have a venting plan right now, just like a greenhouse has venting. In the case of greenhouse, there’s usually fans and so forth to move out excess heat, not just keep heat in. You know what I mean? Not just a heating system to prevent cold. What do we do about that in a cold frame, the venting? Are there automated hinges and stuff that you use? Or what do you do?
Niki: You could totally get automated hinges. But since I work from home, I just generally… If the temperature is going to be over 40 or about 4 degrees Celsius, I just go up and I prop them open. Some of the polycarbonate ones you buy, they have the top you can kind of slide and then it’s a screen. It’s very easy and convenient. You can do that. Or you can prop it open with like a little wood prop, or you can buy automatic vent openers that once a temperature reaches a certain threshold, they open a couple inches or more.
You can do that as well. And I fully expect, even though I know it’s mid-late December, I’ll probably still vent from time to time, because our winters don’t just stay cold all the time now. If it’s going to be late December and all of a sudden it’s above freezing and we’re getting a rain, I’ll open those cold frames just to give them a watering. It’s super-convenient to do as well. Now, some of my old frames that I had, the wooden ones that I would use, I would often unscrew the hinges and store them in the summertime.
But this past summer, I left a couple on and I kept them half propped open, and I grew melons in my cold frames in the summer, thinking a little bit of extra heat might help. And it did. I had so many amazing melons the summer, and it’s purely because I grew them in cold frames. [Laughter.]
Margaret: That’s so funny. I just was talking to a friend who is an estate gardener for a big place in… It’s kind of near West Point, New York. And he was saying that the property has all these old cold frames, but they’re no longer in serviceable condition, the glazing, the lids, the glass. When he came to the property, he used one row to plant asparagus in. It’s hilarious. It’s still this sense of the old architecture of a classical estate garden and with these wonderful, ferny asparagus coming out of it.
But like you say, you could use them for something else as an extra bed, so to speak, without the lid.
Niki: It doesn’t seem like just last year, it seems like 10 years ago now, but I was at Mount Vernon just outside Washington last year. They have cold frames there as well that are not really in serviceable condition right now, usable condition I should say. But they are beautiful structures. And you can still see how useful they are, even though the tops are a little broken and stuff. There’s still things growing inside and it was coming up like crazy in early spring. It was quite beautiful.
Margaret: I think for some of the uses that you were… So we were talking about dimensions earlier and I’m just going to backtrack for a second. We were talking about like yours are 12 on the low end and 18 on the back.
Niki: Three feet by 6 feet and then 18 at the back and 12 of the front. And then I also have polycarbonate ones that are 3 feet by 2 feet.
Margaret: Right. So obviously you would go taller on both the front and the back sides. If you were going to make them very deep in the ground—some people like to go very deep in the ground. Or if you are going to, like you talked about, forcing bulbs or whatever, or overwintering tender things, sometimes if those are woody things and those things are going to be… Do you know what I mean? If you want to tuck away stuff that’s 2 feet tall or 3 feet tall.
Another design variation is if you’re going to use them for storage of things, as opposed to some of these other uses we’ve been talking about, you might want them deeper, if you want the versatility.
Niki: Yeah, for sure.
Margaret: But for general purposes like you’ve been talking about… What have you been harvesting recently from them?
Niki: Oh my gosh. Well, my sort of frames now, there’s like four scattered around the garden, I like to grow… Because again, they’re not super-tall my frames, you’ve got about, depending on the frame, 12 to 18 inches height inside. For the most part, I like to grow compact crops, so ‘Vates’ blue curled Scotch kale is so delicious and it’s so beautiful. And it only grows about 12 or 14 inches tall, so it’s the perfect cold frame salad crop for winter.
But also, my favorite mache. I love mache and mizuna and mustards, all the different colors and textures. Winter lettuces, ‘Winter Density,’ ‘North Pole,’ ‘Winter Wonderland.’ It’s wonderful how many varieties of winter-hardy lettuces there are now, the Salanova types. I would say if you’re interested in growing some of them in the winter, don’t just buy the traditional ones you’d have for summer. Look for some of these more winter-hardy lettuces.
Last year, I did a test of them and most of them went all winter long with no extra protection other than the cold frame. It was great. [Above, Niki prepares one polycarbonate frame for summer planting.]
Margaret: Did you sow those direct into the frames, or did you start them somewhere and transplant them?
Niki: For winter lettuces, I do start them indoors under my grow lights, and then I transplant them out when they’re like about 2 inches tall. It really gives them a nice headstart indoors. It’s still hot in September when they’re transplanted, in mid- to late September, so I like to give them that little headstart indoors and it helps. But things like mache and mizuna is direct-sown. Spinach is direct-sown and arugula, which I’m harvesting now, all that’s direct-sown, tatsoi, claytonia.
Generally, I really only give the kales and the winter lettuces a headstart inside and the other salad crops are direct-sown. And of course, so are the root crops: carrots, beets, parsnips, winter radishes, they’re all direct-sown, too—scallions, things like that.
Margaret: You just said claytonia. I think the common name of that I think is miner’s lettuce. Is that right?
Margaret: What does it taste like? I’ve never grown it,
Niki: It tastes like spring. [Laughter.] That’s how I like to describe it. It’s a bit succulent. It’s got these small beautiful leaves that are kind of almost diamond-shaped, but mature so that they’re almost like a waterlily. They encircle the stem with a tiny little flower as well towards the end of winter, but they have that fresh taste of spring greens, which I think why it’s called miner’s lettuce. During the California Gold Rush, it was one of the first greens to come up that was then, of course, harvested and eaten as a salad green and helped ward off scurvy. It’s a lovely… [Laughter.]
Margaret: This has been a bad enough year. We can’t deal with scurvy right now. O.K? Let’s be clear.
Niki: Just say no to scurvy. Eat your greens.
Margaret: I wanted to just double back to the book. You have everything. I mean, you’re just the woman with everything up there. You’ve tried all these tools, again, I said in the intro, from cloches to full-on polytunnels. I mean, where do you find… It sounds like the cold frames are some of your tried-and-true companions that help you, but where else? What other devices you’re really recommending or excited about or whatever?
Niki: I mean, it can be super-simple. You don’t have to go and buy anything. If somebody has a bed of carrots or parsnips or beets or other types of hardy crops like leeks, I mean, in late fall, you can dump shredded leaves or straw right on top of that bed and harvest all winter. You don’t have to spend money to extend your season and use garden covers like this.
2020 has brought a lot of things, including a garden boom. And here in Nova Scotia and beyond, I’m hearing from so many gardeners, they did put in a bigger structure this year.
For years, they’ve been saying, “Oh, I wish I had a greenhouse or a polytunnel.” And this is the year they said, “I’m doing it.” And honestly, hundreds of letters and texts I’ve gotten from people—polytunnels, greenhouses, even geodesic domes. And in the book, I talk about how to use those.
Maybe you want to grow vining cucumbers or melons in those in the summer. I’ve got lots of info on how you do that. Or maybe you’re going to grow peppers in a northern climate and tomatoes in your structure and pruning them or training them to one or two stems—it can be so much more productive for you. I talk a lot about the little tricks like that as well. It’s not just “let’s grow tomatoes in our greenhouse.” Well, how can we get the best crop, reduce diseases and pests, and just harvest a bumper crop of delicious tomatoes?
I think there’s a lot of considerations we often don’t think about when we’re growing under cover, and I wanted to take the guesswork out of that for people for sure.
Margaret: Across the Hudson River from me, Lee Reich, another longtime garden writer, he has a greenhouse, but it’s a polyhouse, and he keeps it at 36 in the winter degrees Fahrenheit because he loves figs and he has fig trees in there. And that’s all it takes for him in the very cold Zone 5, whatever, to have a reliable crop. And then on the floor, in the ground beneath them and around them, he grows what you just talked about, all the winter greens and so on and so forth.
I mean, if you have a passion for something like you want a fig tree and you’re not in a place… well, you could do that.
Niki: Figs are big here. We have a Canadian fig expert for cold climates, Steven Biggs, and he has inspired me and he also has enabled me, because he keeps sending me cuttings. I have put figs in my polytunnel. It’s not heated, though, but I inflate them with straw and stuff. And I mean, I have artichokes in there and they over winter for me fine, but I deep-mulch them with straw in late fall.
But it’s very tempting, Margaret, to think about maybe getting another polytunnel and marginally heating it so I can have a fig forest. Because again, my husband’s Lebanese and so many of the vegetables and things I grow are for his family, and figs are a big part of that. [Laughter.]
Margaret: Well, Niki Jabbour, the new book is “Growing Under Cover.” I am so appreciative of this cold frame 101. Thank you.
Photographs except as noted by Jeff Cooke/Cooked from “Growing Under Cover,” copyright 2020 by Niki Jabbour. Used with permission from Storey Publishing.
enter to win a copy of ‘growing under cover’
I’LL BUY A COPY of Niki Jabbour’s “Growing Under Cover” for one lucky reader. All you have to do to enter is answer this question in the comments box at the very bottom of the page:
Do you harvest anything from your garden either extra early or extra late, and what tactic do you use? (Tell us where you garden, too, for climate context.)
No answer or feeling shy? Just say something like “count me in” and I will, but a reply is even better. I’ll pick a random winner after entries close at midnight Tuesday, January 12. Good luck to all.
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prefer the podcast version of the show?
MY WEEKLY public-radio show, rated a “top-5 garden podcast” by “The Guardian” newspaper in the UK, began its 11th year in March 2020. In 2016, the show won three silver medals for excellence from the Garden Writers Association. It’s produced at Robin Hood Radio, the smallest NPR station in the nation. Listen locally in the Hudson Valley (NY)-Berkshires (MA)-Litchfield Hills (CT) Mondays at 8:30 AM Eastern, rerun at 8:30 Saturdays. Or play the January 4, 2021 show using the player near the top of this transcript. You can subscribe to all future editions on iTunes/Apple Podcasts or Spotify or Stitcher (and browse my archive of podcasts here).