Our greenhouse is bigger than that so has more ground heat to call on. For the last 3 winters we have planted our winter greens in Oct and ate on them until April. I plant Swiss Chard, spinach, lettuce, (3 or 4 kinds) bok choy, miner's lettuce, kale, carrots, beets, radishes, (French Breakfast matured, Winter radishes just hold all winter and then bolt to seed without making roots.) We lost a few lettuce plants last winter, and the bok choy didn't last all winter--it went to seed during the winter, but everything else did well without heat. We especially like Swiss Chard, eating young leaves in salad and older ones steamed. Our house will hold 10-15 degrees above the outdoor temp, unless there is a lot of wind, so we leave the veggies uncovered until it is going to get below 20 outside. Covered up with old sheets last year the temp at ground level went down to 18 degrees on a night that was -4 outside and 11 degrees inside the house. Last year was a year without any heat at all. Good luck with it. Let us know how it does for you.

I'm glad to hear that greens did well. I am putting out broccoli, brussells sprouts, carrots and beets in a bed we will cover with a hoop house. I am starting with tulle to keep out the little white butterflies. I may have greens in another hoop house over a bed a bit later. I am worried about wind for the hoop houses and the green house.

Ours also is larger than yours, and if you are not already aware, the smaller a greenhouse is, the harder it is to regulate the temperature. Before we built the greenhouse, I assumed it would be easier in the cool season to keep it cool during the day (via shade cloth, opening doors and vents, etc)., and harder to keep it warm at night. I had it backwards. Like Dorothy, even without heat in the greenhouse, we have had most plants survive overnight low temps as low as the upper teens. During the daytime, though? The greenhouse can hit 145 degrees on a sunny winter morning if I forget to go outside before sunrise (I actually try to do it right after sunrise as that is when the really rapid warm-up begins) and open the 2 doors and 4 vents. At temperatures that hot, plants will not be happy for long.

We grow much of the same stuff Dorothy mentioned....I usually have several varieties of lettuce, a mesclun mix, spinach, kale, ornamental cabbage and kale just for fun, Swiss Chard, beets, carrots, radishes, scallions, chives, parsley, and sometimes collards (although I usually put them out in the ground in the garden). Some years I grow the exact same crops out in the garden in the ground, in a bed with hoops over it so I can drag frost blankets over the hoops in the evening if we are expecting lows in the teens. It is fun to see which ones last longer (sometimes the ones in the ground outlast some of the things in the greenhouse).

You can plant your fall crops of broccoli and other cole crops in containers and drag them into the greenhouse when it cools off, and that can extend the harvest period for broccoli side shoots, for example.

I usually have some peppers and tomatoes in containers in the greenhouse, but they usually aren't fresh plants for winter--just some left over from summer. I'm not really trying to get new fruit set in fall for a winter harvest, but rather just keeping them in there until all the small and immature fruit that were on them when I moved them into the greenhouse has a chance to size up and mature. Once the nights are pretty cold, the fruit is pretty slow to enlarge and to color up, so it probably isn't really worth bothering with them---but even a small, slow-to-ripen tomato from the greenhouse is better in winter than anything from the grocery store.

Some years I grow golden purslane or flower boxes of chicken treats in the greenhouse, and sometimes I grow cat grass (rye grass, wheat grass, etc.) so the kitties have some winter greens too.

I also overwinter some cold-tender containerized plants in the greenhouse---generally a fig tree, lime tree, lemon tree and orange tree.

In the years when I have ornamental sweet potatoes in containers, I carry them into the greenhouse, usually in October, to overwinter there. This year I put them all in the ground, so I'll likely just dig the potatoes and overwinter them as tubers in the cool back corner of the walk-in pantry.

It is fun to experiment with different things to see how they do, but you have to remember that cool-season plants and warm-season plants thrive under different conditions. If I have both types in the greenhouse, I need to separate them at opposite ends of the greenhouse and then try to keep the western end (where I keep the warm-season plants) warmer at night than the eastern end (where I have the cool-season plants). If you're trying to keep a smallish greenhouse hot enough to keep winter tomatoes and peppers producing and happy, it is going to be too hot for your lettuce and your lettuce may bolt. If you're trying to keep a smallish greenhouse cool enough so the lettuce and other winter greens don't bolt, the tomato and pepper plants will be so cool that not much growth will occur. You kinda have to choose which one is most important to you, especially in a small greenhouse.

Last year I kept my two containerized brugmansias, which are about 10 years old, in the greenhouse and had blooms almost all winter. Prior to having the greenhouse, I kept them in the garage and would drag them out into the sun during the day and into the garage on cold nights. It worked well, but dragging them in and out got old after a few years.

We keep a Min-Max thermometer in the greenhouse so I can keep track of the temperature fluctuations. I've never had heat in there, but have filled many cat litter buckets and molasses feed tubs with water to serve as solar collectors. They collect heat all day and release it at night. My greenhouse is just a wood and PVC pipe frame covered with 6 mm greenhouse plastic and Aluminet shade cloth so I am pretty sure it doesn't hold heat as well as Dorothy's. We've never been down to -4 outside. About the coldest it ever gets here in 0 or 1 degree, and some years we don't even make it into the single digits. When we drop below 5 degrees outdoors, just about everything in the greenhouse freezes or at least suffers damage and drops leaves.

My main intent when we built the greenhouse was partly to provide a spot to overwinter a few things but mostly to have a place to raise seedlings in winter/spring and hold them until it was time to transplant them into the garden. For that purpose, it is absolutely superb. Some winters I play around with stuff in it more than others---it just depends on how busy we are with other activities. In a bad winter fire season, the greenhouse is completely ignored because I just don't have any time to deal with it.

You have to keep an eye on any outdoor plants you move into the greenhouse. If you aren't careful, you'll find you are providing a place for undesirable pests to overwinter.

Dawn

You mentioned cold frames, which have worked well for me.
I have 3 which have a size of 23x48x12rear & 6 in front. One 8x4 exterior plywood sheet will make 2 cold frames. They are located on the south side of the house against the brick house exterior. The winter sun heats them directly and
it is mandatory that a thermometer be placed inside, as on a normal winter day of 60 or 70 f, it will easily be 90 to 100 inside the cold frame. Monitoring the days weather and
adjusting the plastic cover daily is necessary. It is easy to grow lettuce, spinach, swiss chard, etc. and it is wonderful to have fresh greens in January.
Daleok