Colorado Cold Frames

By Idelle, April 2, 2024

Grow food in the winter months with cold frames!

I’ve always dreamed of getting a greenhouse, but with our backyard landscape’s many trees and irrigation system, it seemed daunting. I was worried that the sunniest spot in our backyard would still get too much shade to support winter growing without additional heat.

Upon prompting from my Grandfather, I looked into taking a greenhouse class to learn more. In January 2017, I found a great Cold Frame Class with Denver Urban Gardens (D.U.G.), taught by Christine and Vince Gallegos – they showed us how to make our own cold fames!

Cold Frames are basically a small, short version of a greenhouse – it’s more or less a raised bed with sloped “lids” that trap solar heat to keep the bed warm. Sometimes called “Dutch Lights,”  cold frames are a much less expensive alternative to Greenhouses to grow in the winter months – and some say they are even more effective for growing winter greens without added heat. Plus, they don’t take up anymore room than a standard vegetable bed, and you can build them easily yourself. You can build cold frames out of old skylights, windows, glass doors, etc. However, if hail is frequent in your area, like it is here, you may want to get the polycarbonate plastic paneling instead as it is stronger and won’t shatter into your garden bed.

The D.U.G. class was very inspiring, plus Christine shared one of their favorite books, The Year-Round Vegetable Gardener: How to Grow Your Own Food 365 Days a Year, No Matter Where You Live by Niki Jabbour, which is packed with information on growing in winter. After the class, I quickly found it at the Denver library (through Prospector) and also fell in love with the book. It has a wealth of information about growing in the fall/winter/spring months, including all types of different types of cold frames and hoop houses that can be easily built. There is a handy diagram in the back for building a cold frame. As a bonus, the book highlights lots of vegetables that you can grow year-round. Niki lives all the way up Halifax, Nova Scotia, so if she can grow year-round, I knew that I could also do so in Colorado! After getting her book at the library, I ending up buying a copy from Tattered Cover because I loved it so much…. and I rarely buy books!

I had a great spot where I could put a cold frame, as we recently had a large old Cherry tree that had died* and we had it removed. This opened some sun and a perfect place to add a cold frame, so I was excited to build one.

In February 2017, with my Dad’s woodworking expertise, tools, and his and my sister’s help, we planned to build just one cold frame, but we ended up building three – one each for my Dad, my sister and myself. My Dad and sister used the scraps of the plastic that we had to make a smaller cold frames by turning the smaller cut pieces into “window panes,” my Dad’s idea… genius. I purchased the 6mm polycarbonate sheets from coloradoplastics.com, which provide insulation and heavy-duty protection. The cold-frame building class instructors Christine and Vince recommended the polycarbonate over glass or reusing windows due to the wicked hailstorms that often frequent the Front Range in the spring and early summer. I’m glad I did, as we have had several hailstorms each spring since I installed the cold frame, a couple with quarter-sized hail that dented our cars, but luckily, did not crack or damage the 6mm polycarbonate panels on my cold frames. Christine said that the idea of having to pick out broken glass from a garden bed was also a motivator to not using old windows if hail is frequent in the area. I’d be curious to find out if anyone who has used windows has had a problem with our hail, some windows may be better than others if they have stronger/thicker glass.  I like the idea of recycling old windows or glass doors, but we do get bad hail these days so I would worry every time it hailed each spring!

Auto-Vents are Crucial for Cold Frame Vegetable Health:
While I didn’t think I had enough sun with only 3-5 hours during the winter months due the shade of the house and a nearby evergreen tree, it actually got quite hot in the cold frame. On sunny days it could get to be 100˚ F very quickly, and if I didn’t go out and open the lids, it would swelter my cool-weather loving vegetables inside. So I decided to get a automatic vent – I bought a Univent brand from acfgreenhouses.com, and it uses a piston that expands as it warms up and opens the cold frame, and when it cools down it closes. This is a pretty neat way to keep your plants from getting too hot on sunny days – and it does’t require any electricity. Lettuce, spinach, mustards, onions, parsley, cilantro, cabbage and other cool-weather veggies don’t like warm temperatures, so venting is definitely essential on warmer sunny days all winter long.

No Heat Needed!
It’s hard to believe, but with a little cover, cool-weather veggies thrive even during sub-freezing temperatures. See the temperature gauge above from Weather Underground for some examples of cold November weather. The veggies shown below in my cold frame, including lettuce, cabbage, onion, parsley seedlings all survived a super cold week of cold weather including 1˚ F nights, along with some of the shortest days. They don’t grow fast during cold weather, but they say alive. And this is just from a little solar heat and cover, there is no added heat or insulation, just the cold frame lids were closed.

How do the cold frames not freeze overnight?
I have cool-weather veggies growing in it all winter long, even in super cold weather. I believe it’s the combination of trapping the solar heat which heats the soil during the day, which then radiates heat overnight. The cold frames’s lids keep that warm air in, and also prevent frosty winds from damaging the veggies. It’s really a small miracle, but it does work! I’ve had winter greens survive 1˚ F overnight lows without a problem. And I’ve noticed that it does get below freezing inside the cold frame, but the added wind protection and warmer soil keeps cold-weather veggies alive. 　

Watering: 
I do water my cold frame during the winter months, but it’s not that often – if we have a couple weeks of warm weather I may water it every week and a half to two weeks, but when it’s cold, I typically don’t have to water it at all.  With warmer weather it dries out much quicker as I typically open both lids on nice sunny days in the winter. The fresh air and breezes help cut down on pests and mold/dampening off, so if it’s over 45-50˚ F I will open the cold frame lids to let the sun and breeze work their magic. If you open the cold frame often, the soil will dry out more quickly, so just check the soil and if it starts to get dry in the first 1 inch, grab a watering bucket and fill indoors (if you’re outside spigots are off for the winter), and drench the bed.

Mulching:
I’ve also read that leaves and twigs can help insulate the seedlings and plants, so I add a light layer of leaves and sticks throughout my bed in the winter. It helps feed the soil and the worms (yes, they’re active all winter in my cold frame!), and it helps insulates the plants. Just don’t smother your seedlings, they can’t grow if they’re completely covered by leaves. You’ll discover over the winter the leaves will “disappear” – that’s the doing of the worms, they pull the leaves down and munch on them all winter long, and leave fertilizing worm castings as thanks. 🙂

What To Do With Cold Frame Vegetables:
Cook! Enjoy fresh green onions, Chinese cabbage, lettuce, herbs and celery all winter long by growing it in a cold frame. Fresh greens are great for salads, and you can also use a lot of these vegetables and herbs in soups, stir fries, and other dishes.

Growing Your Own Food Year-Round Reduces Plastic & Pollution:
One of the ways we can cut back on plastic and emissions is to grow own own food! Just think about it, when you buy vegetables at the grocery store, especially organic vegetables, it seems that plastic is often part of the equation. Organic mini-bell peppers, brussel sprouts, lettuce, and tomatoes, for example, often come packaged in plastic containers or bags. With my cold frame for winter growing, I can pick fresh lettuce, cabbages, spinaches, green onions, parsley, cilantro and more in my backyard – no trip to the store or plastic packaging required.

Growing your own food cuts back on emissions too as all of these vegetables at the store have to be transported by trucks, and some come from very far away! Not to mention, if you drive to the store, that also uses gas and produces emissions. Riding a bike with a backpack or basket is a great way to cut down on air pollution. But if you grow your own greens and vegetables, you can cut out transportation all-together and pick your veggies right in your own yard! Nothing beats freshly picked lettuce – it’s a heck of a lot better than the wilted days-old lettuce you can get at the grocery store!

Start Your Cold Frame Seeds in August/September:
I learned that it’s important to start your winter crop early – ideally in August. This way you can get them growing quickly before the days start to get shorter and colder in autumn. But that’s not to say that you can’t start seeds later than that, the seedlings shown above sprouted in November after I scattered them in October from an old spent lettuce seed-head. They’re now growing strong in January and we’ve been snacking on the leaves daily!

Summertime Growing & Starting your Winter Veggies:
I grow tomatoes and peppers from sandiaseed.com in the summertime in my cold frame bed – I leave the lids open or I can also remove them entirely thanks to my Dad’s handy engineered detachable hinges he made from some old hinges he had.  So in August, I started seeds in small pots for lettuce, chinese cabbage, arugula and green onions to get them going to transplant into the cold frame once the summer vegetables were removed. It was great to get them started early as they were able to grow some nice roots before the cold weather set in. I transplanted them in late-October and sprinkled more lettuce seeds that I had collected from bolted summer lettuce. To my surprise, with the steamy heat trapped by the lids during November quickly sprouted all the lettuce seedlings so I now have a lush (albeit, over-planted) bed of green lettuce and greens.

Above: January 2020 Cold Frame Veggies are growing nicely! I have been “thinning” by eating them.

Colorado Cold Frames & Greenhouses Group:
If you live in Colorado, join our Facebook group called Colorado Cold Frames & Greenhouses. I created the group to share photos, successes, tips, etc about growing in winter in Colorado. Lots of people have joined, and it’s really interesting seeing how everyone is growing in the wintertime.

More Gardening Classes:
While they are not currently offering a cold frame class, make sure to check out more classes offered by Denver Urban Gardens here:
dug.org

Treating Wood Cold Frame with Stains or Oils:
We used cedar wood for the cold frame, which is naturally pest and rot-resistant. However, I didn’t treat the wood immediately after building and installing it, so it weathered a bit the first couple winters/summers, I wish I had treated it sooner as the wood is starting to dry and crack in places. My sister and Dad both treated theirs, and also disassemble and store theirs in the winter months, so theirs look a little nicer for sure! My husband and I did oil the cold frame this past spring with some Boiled Linseed oil, however, I found this to be very stinky for a couple weeks. Not ideal for our vegetables, wildlife or us.

So, this year I used Vermont Natural Coatings company’s semi-transparent PolyWhey® Exterior Penetrating Wood Stain in the color: Lakeside Cedar. They also have a Penetrating Water Proofer Infused With Juniper that I have not tried. These Vermont Natural Coating products use whey proteins instead of toxic ingredients found in traditional wood finish. I found that their penetrating wood stain went on really easily, it has a very thin consistency and doesn’t gunk up your brush. Best of all, it hardly smells at all compared to other stains. I also learned that it provided a nice waterproof finish for the summer. It was so easy to use, I’m not afraid to put on another coat sometime in the year or two to come. My sister also used the PolyWhey stain on her cold frame and raised garden beds, and was also happy with the results. It is supposed to be safe for people, pets and the environment. Their products also have ultra-low VOC levels, and are also free from carcinogens, mutagens and reprotoxins that can still be found in many products that claim to have low or no VOCs.  Before I purchased their PolyWhey Penetrating Wood stain, I called Vermont Natural Coatings, and was told by their support department that since I already used Boiled Linseed oil on my coldframe, that the PolyWhey stain may not work as well, as it is best used on unstained, untreated wood. However, I tried it anyway as I really hated the long-term smell of the Boiled Linseed Oil and I’m sure the plants did, too. Despite having been treated with Boiled Linseed Oil, the PolyWhey stain went on very nicely, the wood absorbed it well. I think because the Linseed oil, while toxic smelling, didn’t last that long in the outdoors, and with a light cleaning with a spray nozzle and a hose, the wood soaked up the PolyWhey stain without a problem. I would recommend it.

I’ve also heard from a local greenhouse builder that they use beeswax to protect their wood greenhouse frames, which is an interesting idea!! Beeswax has long been used as a natural wood treatment and waterproofing sealant throughout history. Beekeepers sometimes use beeswax to protect their hive’s wood – with a combination of beeswax, walnut oil, and sometimes ground wood charcoal that they heat it up to mix, cool, and then apply to wood. I may just have to try that someday!

I’d Build a Taller Cold Frame Next Time:
If I were to build another cold frame, I would make it a little taller – mine is about 14″ high in the back, but shorter in the front, so in early spring the veggies start to get taller and they touch the lid when closed. So a little more height would be nice. The extra height would also be nice for storing summer vegetable seedlings during the hardening off period in May/June. Right now the solution is to dig down a little to make some headroom for potted plants, which also works.

Hoop Houses Have More Headroom:
Hoop houses are also great for winter growing, though most have to be manually vented, and they do provide more headroom for growing taller vegetables and storing seedlings and potted plants during cold spells or storms. I was lucky to get a hoop house from Earth Love Gardens last spring, see pictures below. This simple garden bed hoop house is made from metal conduit hoops and long-lasting Agtec Super Strength Clear Woven Greenhouse Film fabric (12mm) that should last 7+ years. I’ve found that it withstands heavy snowstorms and wind very well (when closed) and it grows taller plants including pots in the winter months. They really take off in the early spring

Never enough gardening space:
Even in the hoop house I ran out of room quickly as I filled it with vegetables and seedlings, but I do love that it has a lot more headroom for seedlings, large containers, and plants like Kale and Mustard. However, the more space the better, so the more hoop house space you can add, the more you vegetables can grow in the spring/fall/winter months!

Put your Seedlings in Cold Frames or Hoop Houses in early Spring
In March, April and May, I like to put some of my indoor-started seedlings out into my cold frame and hoop house IF I have any room. First, I put a “test subject” or two in the cold frame or hoop house to see how they fair with cold nights.

Cool weather veggies like kale, lettuce, onions, cilantro, parsley, cabbage will thrive even in below-freezing temps in a cold frame or hoop house, so feel free to put them in the cold frame whenever you want all winter long (pick a warm spell to put them outdoors if started inside). I also have big pots of lettuce that I start seedlings in grow and bring into the hoop house in so they really take off fast in early spring. Below are some lettuce seedlings and a few volunteer sunflowers that were started from seed in the hoop house last winter.

Summer Veggie Seedlings in Cold Frames & Hoop Houses
Hoop houses and cold frames are great for hardening off seedlings when it gets closer to transplanting time in April and May in Colorado.

If you’re bringing out your summer veggie seedlings in the early spring when night-time temperatures are still getting cold but not below freezing, you can first test a plant – for example, you could use a spare basil plant as an indicator plant as it wilts at the first chance of frost. I also put an extra pepper or tomato seedling out into the cold frame or hoop house to test for a few nights before moving more in.

I typically have lots of extra plants because I have extra seeds sprout and pull out the extras and transplant them, so one of the many extras can be used to test out the temps in the cold frame or hoop house. As long as it doesn’t drop below freezing overnight, the pepper seedlings I had in the cold frame and hoop house seemed to hold up well. If a deep cold spell is expected I’d bring them back into the house. I even had some spring snowstorms with my peppers outside and they faired quite well.

NOTE: Always VENT cold frames and hoop houses during the day by opening them up or installing auto-vents. Do not let the plants “steam” inside, that will make them grow greener weaker leaves that are tender and more prone to pests, cold sensitivity and ill-health.

Keeping summer veggies warm in the spring:
Of course, peppers and tomatoes and other summer veggies like basil and eggplants, really DO like warmer temperatures, so if you have room indoors with bright light(s) you can keep them inside until nighttime temperatures have gotten over 50-60˚ regularly (which is now typically in late May and early June here in Denver), that would be best for fastest growth.

But as the springtime temperatures warm up and the days get longer, and/or if you run out of space for the growing seedlings, a cold frame or hoop house can provide a protected space outside to acclimate them to the outdoor weather and protect them from the cool overnight temperatures until you transplant them into the garden. I also put my larger pepper plant containers inside my hoophouse during late spring cold spells and rainstorms/hailstorms to protect them.

Another tip with Peppers and Tomatoes:
Transplant them to larger pots as they grow. Tomato seedlings or pepper seedlings kept in the same small pot will not grow anywhere near as fast as a seedling that has been transplanted to larger pots as it grows. Especially with tomatoes, you can remove the bottom leaves and bury the stems, leaving about 2-3″ of the plant above ground as you transplant them, which gives them a deeper and deeper root system. You’d be amazed how much bigger they’ll grow in the same amount of time if they have deeper roots and more growing room.

Build Your Own Cold Frame:
You can build a wood-frame cold frame, or if you have access to bricks, you can even build a simple brick back wall, and then drape plastic or a window to create a warm protected area. Don’t forget to vent these on warmer sunny winter days or the trapped solar heat can “cook” your winter greens.

If you don’t have a budget at all, you can also use old (even cracked) plastic saucers, food containers with clear lids, or old windows – and place them on the ground to germinate and keep cool-weather veggies like lettuce warm enough to sprout and grow even in the middle of winter. You can dig down a bit and cover the area for a protected spot for seeds to germinate. You can find lots of free windows / shower doors / doors on Craigslist and Next Door. You can even use old wood beams as a back wall, then use that to prop up an old window, skylight or other clear material.

See these seedlings in the photos on right and below, I scattered seed that I collected from an old lettuce that had bolted and gone to seed last summer,  and covered it with a 16″ saucer, and voila, baby lettuce! Totally free.

I water it from time to time, and as it gets closer to spring I may pull off the saucer and let it really start to grow, possibly separating it and planting elsewhere in the garden.  Lettuce in Colorado is at it’s peak in early spring months (March/April/May.) It’s amazing what a little protection from the wind and winter weather can do, this lettuce survived well-below freezing temperatures for the past month.

So you don’t need to build a fancy cold frame to grow in the winter time, any amount of protection can help some cool-weather veggies thrive in the snowy, below-freezing weather of winter.

So… Should You Grow in a Cold Frame? Yes!
If you love gardening, you should definitely add some winter-growing options like a cold frame in your backyard. Cold frames are best for slow harvests over the deep winter months (December/January) and then things really pickup in February/March/April, where all your small lettuces, green onions, cabbages, spinaches, and other cool-weather veggies really start to take off with the longer days. They’re also great for keeping indoor-started summer vegetable seedlings warm during cooler nights while you harden them off to prepare for planting.

Locally-Made Colorado Cold Frames:
I met Aaron Michael from Boulder-based EarthLoveGardens.com at the Home and Garden Show in January 2020, his was one of the few actual garden-related booths in a sea of window, roofing, spa and other non-garden-related businesses. Aaron was showcasing their beautiful locally-made cold-frames that have polycarbonate sides and lids for maximum sun. Their mini-greenhouses are stained and come with auto-vents so they’re ready to place in your garden bed and plant! They also offer hoop houses as well, which is what I ended up getting for our garden. I love the hoop houses as they are quite tall so you can fit larger potted plants and grow taller plants in them than cold frames. So if you’re not super handy, this is a great alternative to get some locally-made, well-built cold frames or hoop houses for your Colorado garden. See some of their mini-greenhouses below or visit their website for more mini-greenhouse info and pricing.

Also at the Home and Garden Show was Solar Gem Greenhouses, shown above, who are based out of Tacoma, Washington. They build backyard gothic-arch greenhouses from one-piece fiberglass greenhouse construction, which diffuses light and are more air-tight for better heat-retention in the winter months. I’ve dreamed of one of these greenhouses for years, so I was happy to meet John at the show and see one up close. They do feel quite sturdy, and I love that they have no framework to rust or rot, nor panels to get loose from gusty winds and storms. He said they also withstand hail. One of these days I will hopefully be a proud gardener in one of these gems. 🙂 For now, I’m quite happy to have a cold frame!

For more cold frame ideas, check out these Colorado cold frames I saw during the 2019 Denver Garden Bloggers Fling:

I hope this blog post helps pass on the inspiration to build cold frames and grow your own food year-round!

Happy growing!

*Sadly, our beautiful old Cherry Tree died due to a hard early frost in fall of 2014. In November temperatures had stayed quite warm in the 60s and 70s, then on November 10th, temperatures plummeted from 58˚F at 8am to just 16˚ F by 11pm, staying frigid for nearly a week. This deep, instant freeze resulted in the death of many fruit trees (especially Cherry trees) as well as the suffering of many other trees and shrubs across the Front Range. The one thing I learned from the experience? Change is inevitable, but don’t worry, it often means something exciting and new – in this case it meant a new gardening space! 🙂