Building and using a cold frame for season extension

Late last year I decided to try a season extension stratagy.

Our last frost date in my zone 5B garden is October 20th but I wanted to continue harvesting into the late fall and winter. I had been watching a bunch of YouTube videos about how to use cold frames and decided to build one myself. With the help of my friend Dave I acquired two old glass patio doors. A local glass installation company had just installed new doors and needed to get rid of the old ones. So the best part was they were free! My plan was to build wooden frames and use hinges to attach the glass doors for 2 large cold frame boxes.  


The concept behind cold frames is to create a mini greenhouse for your cold hardy plants. This cover will prevent frost from killing the plants off. On sunny days this mini greenhouse will use the power of the sun to keep the ground and plants much warmer than the outside cold temps. I have heard that a cold frame can give you a 1 ½ zone advantage in cold months. This will turn my zone 5B Ohio garden into a zone 7 garden similar to the climate in parts of North Carolina.

Using a cold frame I would be able to not only enjoy fresh food later in the year but also start plants earlier in the spring.

I headed to Lowes and picked up lumber and 2 large piano style hinges. I already had screws on hand in garage.

I used treated 2x12’s for the bottom frame and 2x8”s for the top layer. Connecting the 2 together with 2x4”s in each corner. I then attached the large hinge first to the glass doors and then to the wooden frame. This was by far the most difficult part of construction since the repurposed patio doors weighed a bunch.


Once this was completed I attached a piece of metal to the edge of the glass doors and cut an extra board I had laying around to prop the door open. This was a very important safety feature to use when working in the cold frame. The last thing I wanted was to be smashed by the door when I was picking arugula!

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The final step was transplanting onions, lettuce, spinach, arugula, kale, and carrots from my uncovered garden beds to the cold frame. This was the fun part!


Being the digital nerd that I am I also ordered a remote thermometer so I could monitor the temperature and humidity of the cold frame from inside the house. This was helpful to see exactly how warm or cold the plants were and if I needed to prop open the glass doors to vent when the temp was over 70F. I was amazed to see that the cold frame was able to keep the plants 10-20 degrees warmer than the outside temperature. Even when we had some below zero extremely cold nights the cold frames never dropped below 10F. And if the sun was out during the day it would warm back up into the 40’s in no time.


The best part is I still have plants growing in January. In fact I just used some green onions in an omelet I made Friday.

Omelet with smoked salmon, goat cheese, and fresh, cold-frame green onions


The few carrots I transplanted where only a few inches long when I first put them in the cold frame but the last one I pulled was over 8 inches. I was very surprised that even with limited sun these carrots continued to grow.

After our last frost date in late April I will remove the top section of the cold frame including the glass doors. Next season around September I will sow plants directly in the uncovered cold frame and cover them before the first frost. This will give them a head start and with any luck I will be enjoying an even larger amount of garden fresh veggies long into the winter.


This was my first experience with building and using a cold frame. Below are some lessons learned that will help you be more successful if you decide to build one.

  1. Patio glass doors are super heavy which make them tough to open and move around. For my next cold frame I will be using something much lighter like storm windows.

  2. Check the sun hours in the winter. The spot I used for my cold frames gets a lot of sun in the summer but only has 6 or less hours of sun in the winter months due to shadows from my neighbor’s house. If I knew this in advance I would have picked a better spot with more hours of sunlight per day.

  3. More slope. I did not slope my cold frames at a very large angle. This prevented it from catching more sun and also made removing snow, ice and leaves a bit more challenging. Next time I will give it a better slope.


Have you ever used a cold frame?

How did it work?


John Lemmon