The Ongoing Mini Greenhouse Experiment

For Christmas, my husband got me a mini greenhouse. It's not too big, about 5 feet tall, 2 feet wide and 18 inches deep. It has a plastic shell over it, and a metal frame. Each shelf holds about 40 pounds. It is not meant to be a year round greenhouse in our climate (read: frigid arctic tundra from December through February), but it is supposed to help aid in seed starting in the early spring, which I intend to try. I'm also going to try growing lettuce, peas, radishes, and other cold weather veggies I can think of  once I figure out how to seal off the bottom so at night it doesn't get down to the outside temperatures. 

So, I put it together, secured a thermometer in there with a twisty tie, and started running my ongoing testing of temperatures. 

Test 1: 

Snow day, with a tiny bit of sun. Local Daytime Temp: 36°F (2.2°C) 

As Is - No insulated bottom. 

In the morning when it was cloudy, overcast, and snowing, the greenhouse was still 5-10 degrees warmer than the patio, which happens to usually run 5-10 degrees warmer than the actual climate in my area because of the small, fenced-in, south-facing, cement patio effect. (When it's too cloudy for too long, the temperature on the patio will match the local temperature.) 

In the afternoon, the sun came out for an hour, and the temperature shot up to 80°F (26.6°C) inside the greenhouse. I was getting really excited here! Could tomato seedlings be in my immediate future?

An hour later, when the sun was setting, and the clouds were back, the temperature was back down to 40°F (4.4°C). So, all in all, not a totally successful day, but I did learn something. When it's sunny, the greenhouse is going to be way too effective. If it got up to 80 on a frigid day with sun, it's going to be a little rough if sealed off on a warmer day, even if it's too cold to plant outside. It does have the front zipper door for ventilation, which I may have to start opening on spring days for my seedlings.

Thoughts from this test: If I could only figure out how to regulate the temperatures inside from severe fluctuations? 

Test 2: 

Overcast day. Local Daytime Temp: 34°F (1.1°C) 

Put plastic on bottom shelf, but didn't seal off to allow for subtle air flow. Also put in pots of soil to thaw them.

I put in a folded up tarp on the bottom shelf to block the major air flow coming from underneath, and then kind of ran it up the sides a little, but didn't seal it off. I was hoping to block most of the drafty air, but not completely seal off the greenhouse. When I went out to check it in the morning, it was definitely a little warmer than the patio, but still not at the temperature I'd like for seed starting. They say lettuce can germinate at 40°F, but this is still pretty risky. 

I also noticed condensation all over the inside of the plastic cover from the pots of soil. It looks likes they are definitely thawing. 

Thoughts from this test: How to keep it warm in there on sunless days?

Test 3: 

Coming soon. I'll keep you posted on what I try. 

++ Has anyone grown in one of these mini greenhouses? How did you regulate the temperature? 

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