Newly built cold frame does wonders warming soil

Gazette Gardener Natalie Walsh spent last weekend building a cold frame in her back yard.

Last weekend, my husband and I built a cold frame. George calls it Natalie’s clubhouse.

It’s not fancy, but it has its advantages.

First, the length of time I normally shuttle seedling vegetables and tiny tropicals into the house at night and out to the garden by day to harden off prior to planting will be lessened.

But most importantly, having the frame warmed the soil rapidly much to my delight. Tuesday morning, the soil temperature inside the frame was 68 degrees — a significant increase from the day before when the soil temperature inside the frame was 58 degrees and out in the garden the soil temperature was 50 degrees.


Another benefit is that critters won’t get into this vegetable patch, as I intend to remove the plastic in the weeks to come and replace it with chicken wire to keep squirrels and cats out.

Come fall, when I am looking to extend the season, I will put the plastic on over the wire and be able to harvest the last of the tomatoes without concern for frost.

The idea for the cold frame came after a visit to the home of one of my readers, Bob M. in Schenectady. He created a greenhouse from a metal carport frame covered in plastic and made adjustments including wooden ends with a door for easy access, fans to ventilate and a small heater for chilly nights. I was inspired not only by the greenhouse, but the production going on.

The space inside was full of robust heirloom tomatoes ready for transplanting, peppers of all varieties, eggplants, annuals and herbs. It was a little bit of heaven, and his results were impressive.

I was hoping to plant last weekend, but on Saturday morning the soil temperature was a chilly 45 degrees — too cool for the tomatoes, peppers and tender herbs.

Those require warmth. The optimum growing temperature for tomatoes is between 75 and 95 degrees. Soil temperature is just as important if not more. The minimum soil temperature for tomato growth is 50 to 55 degrees.

Planting too soon can lead to problems with disease, delayed growth and failure to set blossoms. Or worst, they set blossoms and then drop them. Blossom drop can occur when daytime temperatures are warm, but night temperatures fall below 55 degrees. In my experience, tomatoes transplanted when the soil and air temperatures are warm catch up with, grow and produce just as well as those planted earlier.


So I resisted the temptation to plant on Saturday. Instead, we built our science project — a cold frame 6 by 8 feet approximately — right up against a wooden fence and over the ground where I will plant this weekend now that the soil is warm.

We constructed the frame with 3⁄4 inch metal conduit ($30 for more than we actually needed), heavy plastic sheeting that we already had in the garage from another project, and two clever devices that I had stored from previous gardens: Build-a-Ball connectors and plastic clips that snap snuggly over the conduit holding the plastic in place.

The Build-a-Balls remind me of Tinkertoys, but instead of wooden connectors they are rubbery balls with holes at different angles in which you insert the conduit to build a structure. I tried to find a source online, but could only find them in Great Britain. If anyone knows a source closer to home, let me know and I will pass the word along. Same with the clips, which I originally purchased through Charley’s Greenhouse, but these clips are no longer available in the online catalog.

Essentially, we used the existing wooden fence as the back support and attached the conduit to the fence with braces, and then used the Build-a-Balls, to create a simple rectangular frame to support the plastic. The cold frame is one inch taller than I am — 5 feet 5 inches — which is ideal for me working inside.

THE FINAL results

I took air and soil temperature readings in the cold frame and outside of the cold frame Sunday, Monday and Tuesday. Here are the results:

On Sunday at 8 a.m. — the first day the frame was complete — the soil temperature was 45 degrees outside, 47 degrees inside the cold frame. Air temperature reached 95 degrees inside the cold frame in the afternoon and I opened space to vent.

On Monday at 8 a.m., the soil temperature in the garden was barely 50 degrees. Inside the frame, the soil temperature was 58 degrees.

On Tuesday at 7 a.m., the soil temperature outside the frame was 56 degrees. Inside the frame, it was 68 degrees.

Why didn’t I do this sooner?

Happy gardening.

Categories: Life and Arts

Leave a Reply