How to Extend Your Garden Season With Cold Frames

Farm tour participants admiring the new flush of transplants started in cold frames

Up here in Cape Breton, spring is just a tease. Cold frames are an excellent backyard strategy for gaining a few weeks on food production, either in the spring or late fall.

Last fall, this handy tip sheet was compiled to support a cold frame building workshop hosted by Leonard Vassalo at Blue Heron Farm in Gardiner Mines. We built three cold frames, which were donated to the Glace Bay Food Bank garden.



  • Materials: You can build them out of wood, bricks, or straw bales, and you can top them with old windows, screen doors, shower doors, greenhouse plastic (best in two layers to create an insulated pocket), polycarbonate, etc.
  • Type: What type suits your purpose? You can use a moveable cold frame (to put over crops that you have started in field), you can build a permanent cold frame (that is dug into the ground to offer more insulation and protection), or you can use temporary cold frames (that you construct around crops, out of things like straw bales).
  • Placement: ensure your cold frame will get adequate sunlight and aim for southern exposure. It should also be easily accessible in the winter. Consider snow load – i.e. if you put it up against a building that sheds a lot of snow, you will be doing a lot of shoveling to keep it clear, and your covering material may break under the load.
  • Adaptation: Do you have raised garden boxes already? Consider building an angled “roof” to use with your box in the winter. Alternatively, you could fix hoops to the box for winter protection with floating row cover.
  • Heating: Consider using thermal mass, like a jug of water, to capture the sun’s heat for release at night. Len uses a light bulb to add some warmth when needed to protect less hardy transplants. You can also increase insulation around cold frame with: wool blankets, Styrofoam, leaves, straw bales, or even windbreak.
  • Venting: Cold frames need to be vented almost every day in spring and fall. If left unvented, the plants can cook. Venting also prevents disease problems, and promotes hardiness. In summer, remove the tops completely. In the deep of winter you can leave them shut, but when the days get longer again in late February, you will have to keep an eye on them and start venting as needed. Rule of thumb: if the outside temperature rises above 4 deg. C. the cold frame needs to be vented a crack. If the outside temperature is 8-10 deg.C. the lid can be opened completely. If this sounds like too much babysitting, consider investing in an automatic vent opener, which go for around $80.
  • Watering: Cold frame contents will need to be watered in spring, summer and fall (either by opening during a rain or hand watering. Little to no water is required in the winter, because the plants are not growing, and the closed lid keeps water loss by evaporation to a minimum. When you do water in the cold months, wait until late morning when the coldframe warms up, but leave time for plants to dry of before the cold night returns.
  • Fertility: If you have a fixed location for your coldframe, consider dedicating it to cover crops during hot months. Work crop into soil in late summer and add a generous addition of compost.
  • Snow Removal: light snow can be removed with a broom. Shovels can easily damage cold frame covering. During the depths of winter a bit of snow can be left on the cold frame for insulation, but a heavy or wet load can cause damage, or ice over.

Len Vassallo showing us the many stages of his fall planted spinach



  • Prep in fall! Compost, mulch, etc. A properly prepared bed can be planted with cool-loving crops as soon as snow has melted. There are few pests (besides slugs and deer) at this time.
  • The “cool” vegetables can handle frost, but they will grow faster with the help of season extenders: low tunnels, floating row cover, cold frames.
  • Cold loving veg to direct seed: lettuce, spinach, mache, mustard greens, arugula, radish, green onions, carrots, beets, endive, kohlrabi, tatsoi (can be seeded in cold frame as early as March).
  • Cold loving veg to transplant: broccoli, cauliflower, cabbage, leeks, choi, Swiss chard, kale, bulb onions, spinach, lettuce, green onions


  • Warm-season vegetable time! In the early summer, season extension tools (low tunnels and cold frames) can be used to protect young frost-sensitive transplants (tomatoes, eggplants, peppers, basil, cucumbers, squash).
  • Remember to start thinking about fall and winter. Many crops for overwintering must be planted in mid-summer. For example, if you want to overwinter carrots in a coldframe, they must be planted by late July in order to size up.


  • In late summer and early fall, we can start direct seeding and transplanting out our cold loving greens again: spinach, lettuce, arugula, etc. There is also still time for a crop of radishes. These can be planted directly into a cold frame, or in the field for later covering with a low tunnel.


  • Once there is less than 10 hours of sunlight per day, vegetables pretty much stop growing. Root vegetables can be “held” in the field under protection. This means that they are staying alive (and fresh) but not continuing to mature. The same is true of hardy greens – like spinach. The key is not to pick them when they appear frosty. Wait for then to defrost before cutting leaves. The exception is mache. It won’t turn to mush if picked frozen.
Sample Planting Cycle for a Cold Frame:

Direct seed hardy veggies like lettuce, Asian greens, arugula, spinach, potatoes, beets for greens, arugula, and radishes.


Sow things like Swiss chard, kale, broccoli and summer cabbage to be moved to main garden area at the end of May.


Harvest all of those beautiful greens and onions! You will be enjoying those baby potatoes in the later part of the month.


Sow cover crops (like buckwheat) and remove lid of frame.


Grow on cover crop


Work in cover crop and 3-4” of compost. Transplant hardy veg into box (like leeks) and direct seed things like carrots and beets.


FALL Sept:

Transplant crops like Swiss chard, kale, spinach, green onions, parsley and thyme into cold frame. Mulch around the plants with leaves, straw, or eel grass. All cold frame planting should be done by the end of the month.


First frosts. Time to close the cold frames at night. There are still some hot days in October, so venting is a major concern.


Continue to vent when needed. Add mulch around established plants. Start insulating around frame.


Consider adding extra insulation (like straw or wool blanket).

Keep wet and heavy snow off.


Continue to insulate and manage snow.


Mid to late month, work in compost into available area in cold frame and seed with crops like green onions, spinach, arugula, Asian greens, and mache.




Why is spinach Len’s favourite crop to overwinter?

  • As a commercial grower spinach is a high demand crop – he just can’t grow enough of it!
  • Spinach gets sweet and delicious in the winter months
  • Spinach is made for the cool months – it is poor at germinating when soil temperatures rise above 15 deg.C. and it goes to seed quite quickly even once you do get it going. The short days of winter keep it from “bolting”.
  • Because Len has a root cellar! Things like potatoes, beets, carrots, turnips, leeks, onions, and cabbage can all be stowed away in the root cellar, which makes it more sensible to focus on greens in his tunnels and cold frames.
  • Because spinach is super hardy and he prefers it over mache, which can be lower yielding, harder to sell, and fussier to harvest.

Len ensures a steady supply through a succession (every week or two) of direct seeded and transplanted spinach. Start transplants 3-6 weeks before last frost (in succession from mid-August to early September). Start direct seeding in early September. You don’t want to direct seed it sooner or it will bolt before overwintering. Once direct seeded spinach is up, fill in gaps with transplants. Len aims for 6” spacing.

Every season is different. Succession allows you to hit the sweet spot in terms of crop establishment and size gained before the dormant winter period.


  • Choose cold tolerant varieties, with shorter days to maturity. Choose bolt resistant varieties meant for overwintering (lettuce, spinach).
  • It is not just a temperature thing! Plants stop growing when we have less than 10 hours of sunlight in a day. Beyond actual daylight hours, light quality changes over the winter. On December 22nd, plants receive 5% of the light they receive on June 22nd.
  • Succession planting is key. There is no telling what each season will bring. A planting date that worked last year for your spinach, might not work for you this year.
  • Floating row cover is your best -friend. It can: create a barrier to pests, warm up beds for faster and more successful germination (due to moisture retention), offer frost protection (of varying degrees depending on the weight purchased), and protect plants from wind damage.

Len Vassallo sharing his transplanting strategy


Sources for Season Extension Materials

  • William Dam
  • Halifax Seed
  • Veseys
  • DuBois (commercial scale)
  • Scotia Gold (commercial scale)



Len’s Favourite Reference Books:

Best Local, Back-Yard Reference: The Year Round Vegetable Gardener, by Niki Jabbour (This book has detailed instructions on how to build a cold frame).

Commercial (but small) Scale: Anything by Eliot Coleman, but the Four Season Harvest is the most relevant to our topic. He also has one called the Winter Harvest Handbook.



Jody Nelson is the Community Food Coordinator for Cape Breton with the Our Food Project of the Ecology Action Centre.

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