I love winter. Yes, it may seem odd to hear this coming from a horticulturist as with it comes the end of the main growing season, but there is something about the cold, the starkness, and the frost (and potential for snow!!!) that enchants me about this time of year. In Tennessee where I grew up, this shift in seasons was a bit harsher than it is here in Texas. But, winter can still have its moments here in zone 8b, too. A few years ago, we experienced 11F, 12F, and 14F three nights in a row.
Even though much of our plant activities are on pause, I like to cheat the these colder months out of a little more with season extension. Extending the growing season for cool season crops relies on two principles, simple, unheated structures + cool-season crops.
My first forays were simple straw bale cold frames that I saw in gardening books. I didn't have the means to get glass panes made, so I would get old windows from my family and neighbors nearby to use as a covering. The straw was good insulation. It was in these structures that I started my tomatoes and other vegetable transplants.
One of my first cold frames. You may be wondering about the hog wire. That was used to keep the dogs out.
Eliot Coleman really helped to open my eyes on season extension with his book The New Organic Grower. In 2014 I built my first ever plastic covered low tunnel in my garden. I didn't know much about what I was doing. I used PVC pipe and painters plastic from the hardware store. Within a couple of months, the cover split right down the middle. But, I was delighted to see that what I was growing in this structure actually worked and that you could continue gardening through the winter in Tennessee.
My first two low tunnels in my garden in 2004.
A rare eight-inch snow event showed me just how sturdy these structures were.
I buried the edges before I realized that sand bags were a thing. It was hard to take the plastic off, and I got down in them and crawled around. It's fun thinking back now about how I got under those covers and scurried around working in my covered beds.
Fun as it was, the strain on my neck made me want to build something larger. So, I experimented around next with an A-frame made out of electrical pipe from the local hardware store. Armed also with more weather resistant plastic, this structure could be walked into, and I did not have to worry about the cover degrading. I learned about wiggle wire, this simple wire-channel combo that helped to hold plastic down, and I used it on the A-frame end for quick access to my plants.
My A-frame in 2006 after another snowfall.
Inside, mizuna, beets, and spinach are happily growing in the cold.
As my love for season extension grew, I convinced my parents to let me build a small 16 x 24 hoop house at our home in 2008. Just like the other structures, this greenhouse was unheated and relied on natural heating from the sun. But, it allowed for more experimentation. Inside it was a cold frame; putting protection within protection helped to increase the season extension effect.
Yay! My first greenhouse!!! This snow event was a rare sight in March. You can see the Narcissus foliage on the side poking through the white.
Inside the hoop house, spinach, cabbage, beets, carrots, and more are visible.
Spinach, onions, kale (left, front to back), corn salad, and mache (right, front to back) all survived multiple nights of below freezing temperatures during the 2007-2008 winter.
I decided to track night low temperatures using thermometers over two weeks in January 2008 outside the hoop house, in the hoop house, and under cover, just like the young scientist I was. You can really see the effect of this cold frame in the unheated hoop house in the figure below. While the outdoor low and hoop house low were close to each other most nights, the cold frame temperature registered consistently higher than both of them. One other thing that I’d like to point out is that even with temperatures as low as 8F, crops like spinach still were able to survive in the hoop house with one layer of cover.
The outdoor, hoop house, and cold frame low give a sense of just how cold these plants can tolerate.
However, after winter that I left for grad school, and I had to put my season extension experiments on pause while I lived in an apartment. Now that I have a garden again, I’m back to covering crops during the winter to see just how much I can squeeze out of the growing season. I’ll add that in the six year hiatus the technology has advanced so much. Now with pipe benders you can make hoops out of electrical conduit pipe for around $3 a pipe, and then cover these arcs with floating row cover that lasts for a few seasons if you treat it right. These permanent hoops are great. Not only do I use them in the winter, but I also cover squash in the summer to prevent squash vine borer.
My latest iteration on season extension uses bent electrical conduit and floating row cover.
To me, it’s fun to be able to go out in the middle of winter and harvest a salad grown right in my backyard. It is so simple I believe that everyone can do it. So, experiment around with season extension to see just how much can be grown by pushing the limits beyond frost. You’ll be pleasantly surprised