I know I’m not alone in pining for spring and summer, huddling under a blanket and gazing wistfully at my empty garden boxes. February is especially tough because we’re so close—my first plants go into the garden on St. Patrick’s Day, and I’ve already winter-sown a lot of seeds, but they haven’t come up yet. Sitting inside with a mountain of seed catalogs, I can’t help thinking about ways to get started earlier.
Many experienced gardeners extend their growing season using hoop houses or low tunnels. A hoop house is generally a taller structure, large enough for a person to stand up in; a low tunnel is kind of like a “mini hoop house,” a system of fabric or plastic stretched over “ribs” and enclosing a growing bed. Having limited space in my urban backyard, I sadly can’t squeeze in a hoop house of Floret proportions, but my raised veggie beds do make a nice little base for some tunnels.
Here’s the thing: I’ve never done this before. In years past, the pull of growing things hasn’t been strong enough to overcome my aversion to being outside in freezing temperatures. This year, for some reason, it’s different. My seed packets from the usual suspects showed up (I’ll talk about my favorite sources soon) and instead of filing them sadly away for another month or so, I got to thinking, why not now? I ran to Gardenweb to ask about people’s experiences germinating seeds under cold protection in the upper Midwest, and got a range of responses from “it’s too early” to “it’s not too early,” with a common coda of “try it and report back.” Ever the intrepid scientist, I set up a little experiment.
Every two weeks, starting yesterday, I’ll plant half a square of a dozen or so cold-tolerant plant varieties (a few kinds of radish, kale, scallions, orach, chard, spinach, arugula, salad greens, and kohlrabi). I’ll record the air and soil temperatures on that date, the date the seedlings first emerge, the date their first true leaves show up (the first set of “leaves” to come out of a seed are called cotyledons or seed leaves, and they will fall off after feeding the roots—the second set to show up are the first real leaves), and the date they’re ready for harvest. I’m hoping to get a clear picture of which temperatures work for which plants. And of course, I’ll share that with you here.
(A quick note before we jump into the building: you can definitely start cold-hardy plants like kale and spinach indoors and then transplant them out into the low tunnel. This is a pretty well-documented way to have veggies in the winter, and many of them will happily survive a freeze. I’m curious how low temperatures can be and still get good germination, so I’m trying everything from seed outside this year.)
So! Are you ready to grab your tools and get out in the snow? These measurements are for a 4’x4′ raised bed, a pretty standard size (I’ll write up a tutorial for them later—they’re so easy and cheap to build!). If your bed is a different size, just be sure to keep that in mind. For one low tunnel, you’ll need:
- Three 8′ to 10′ long 1/2″ diameter thin-wall PVC pipes (I used 10′)
- One 4′ long 1/2″ diameter thin-wall PVC pipe (optional, but recommended)
- Twelve 1/2″ flat straps (this size bag will do two beds, or most hardware stores sell them in sets of 2)
- 24 decking or exterior screws (length will depend on the thickness of your bed walls—I used 3/4″ and they protruded through a little bit; not ideal, but OK as long as you remember they’re there!)
- A pack of zip ties
- Plastic drop cloth (a thicker one made for outdoor use, not a flimsy one like you lay down to paint—mine was 9×12′, which is a bit small; go at least 10′ in each direction if you can. The roll in the link will make 2 tunnels)
- Large binder clips or other clips to hold the plastic to the ribs
- Optional but recommended: Agribon row cover fabric (I bought the 50′ roll in the link and sewed 10′ lengths together to create a blanket that was wide enough, or you could just overlap them without sewing)
- Also optional but also recommended: a thermometer of some kind (I use this AcuRite multi-sensor thermometer, which allows me to simultaneously monitor the temperatures of both of my low tunnels, the outside air, and the indoor air around my kombucha brewer)
- 4′ lengths of pipe, dowel, or other weights to keep sides down
- Pegs, weights or stakes to keep flaps down (I put grommets in my plastic and used tent pegs through them)
- Optional but also recommended: a friend
I know that seems like a lot, but it doesn’t cost much and this is going to go so fast! You can knock it out in a couple hours tops. Here’s my naked bed, shivering beneath a flimsy blanket of black plastic:
(Forgive the frozen peppers scattered all over the place—Brazilian Starfish peppers are so prolific it’s taken me practically all winter to pick them all up!) Using the screws and metal straps, attach your 3 long pipes to one side of your box. I generally put one strap at the top edge and one at the bottom for each pipe, like so:
Yeah, the straps are a little wonky, but that’s how forgiving this project is. If your strap gets a little twisted, but your pipe is still sticking up pretty straight, just say ¯\_(ツ)_/¯ and move on with your life.
Next, take those pipes, arch them gently over the bed, and affix them in place on the opposite side. You should end up with this:
Hey, you’re like 2/3 of the way done! I promised this was easy.
The next step is optional, but it’ll help your bed stay strong under snow, wind, rain, and accidentally tripping onto it. Take a 4′ length of pipe and run it perpendicular to the ribs you just installed, on the very top. This will be the “spine” of your tunnel. Secure it with zip ties, like so:
You can trim the zip ties down if they bother you, or ignore them because they’re not in anybody’s way and you have better things to do with your weekend. Up to you.
The next step is also optional, but may be helpful if you’re new to this kind of thing. Remember that multi-sensor thermometer station I mentioned up top? This is a good time to hang a sensor in your new tunnel, before you cover everything up. Nothing fancy required here, just get it working and hang it from the spine with some string. I generally aim for about the middle of the tunnel, high enough not to interfere with growing plants, low enough to get an average temperature.
Now it’s time to cover up these naked ribs! When I first built mine, I used the plastic sheeting alone. This is a fine option if you’re not trying to extend the season by that much, or if your climate isn’t that cold. I draped the sheeting long-ways over the ribs, popped a couple of binder clips on the spine to hold it in place, and then rolled up some heavy 1.5″ pipe I had lying around in the excess, to help weigh the sides down. When I need to open the sides on a warm day, I can roll the sheeting around this pipe and then clip it to the spine to keep it all neat and tidy. Don’t feel like you have to buy pipe for this—anything long and rigid is fine. A dowel, a scrap of wood, etc. Or you can use stakes all the way around. Or you can just pile flower pots and rocks on the edges. This step is pretty much freestyle.
On the ends, this is where it would be great to have more plastic to work with than I did. If you have enough, you can just pull it down to the ground and either stake or weight it in place, which is way easier. My sheeting is a bit small, because I had intended to use 8′ pipes rather than 10′ ones, but my local hardware store had other ideas. I went with it and it works fine, I just have to clip the sides shut in a couple places to ensure no air gets out.
Is it the prettiest thing in the world? No. Will it do the job for a couple of months until it’s warm enough that the air doesn’t hurt your face anymore? Probably. And when it snows, it does look pretty cozy and charming.
I was concerned that the plastic alone wasn’t going to keep my plants warm enough, so I went back and added a layer of Agribon 19 landscape cloth underneath it. Agribon is good stuff—it adds a couple degrees of cold protection, plus it provides a barrier between the plastic and your plants’ tender leaves, which can get frost-burned if they’re up against the plastic for extended periods of time. Agribon comes in different weights; 19 is pretty light, only offering 4-6 degrees of protection, but it allows 85% of the light to come through so you shouldn’t have to remove it frequently.
So, do they work? Well, yeah! They won’t create heat from nothing, so don’t be alarmed if your temperatures drop like normal at night. During a sunny day, though, you should see temperatures warmer than outside. This is my monitoring station on a partly-sunny day last week—the temperature in one of the tunnels is on the left, the outdoor temperature on the right. (Ignore the temperature at the bottom, that’s the temp in my kitchen.)
This warmth comes with a caveat, though. If you’ve got a nice crop of happy plants, adjusted to growing in the cold, and then a warm sunny day appears and blesses you with a 60°F day in February—your babies are gonna cook! In full sun on a warm day, the temperature in the tunnels can get 30, 40 degrees warmer than the temperature outside. Fortunately, you can fix this! Just roll up the sides of your tunnel during the day if it’s above 50 or so outside, and unroll them as it begins to get dark at night. (If you’re worried about remembering to do this, you might try using just the Agribon, not the plastic. It’s probably not enough in Iowa, but in warmer regions it just might be.)
The other thing to remember is: wilted plants aren’t necessarily dead plants! Many cold-season plants wilt intentionally when the temperature drops below freezing, as a survival mechanism—it’s ice crystals forming in their cells that kills them, so they release their moisture and wilt. They will perk up once the temperatures rise, provided you’ve chosen hardy varieties. Don’t try to grow peppers and tomatoes in a low tunnel in Iowa in February. It’s going to be a seedling bloodbath, assuming you ever get them to sprout in the first place. So sad.
So that’s it! The first batch of my seeds went out into the tunnels yesterday, and I’m checking them every day to see if/when they germinate. I’ve got a spreadsheet and everything. When I know more, I’ll come back and share. In the meantime, go have your own winter gardening adventure! (Or stay inside and drink hot toddies until May. I won’t judge you.)
P.S. This post has some Amazon affiliate links in it. I won’t ever link you to something I wouldn’t use myself—in fact, almost everything in this post is something I bought from Amazon within the last few weeks—but if you buy something from one of those links, I may get a cut. Just FYI!