If you're in SW Indiana and you want to start planting as soon as it's finished in January, you're not going to have any good results without a heater. It gets to be -20 sometimes in January and it'll only be about 10 degrees warmer inside that greenhouse no matter how well you insulate at night. You might want to wait until closer to spring (March) and put a small heater in there.

Welcome...
I'm not sure what zone Southwest IN is in, but what you can grow during winter will depend upon the temperatures you're able to maintain in the greenhouse. I noticed you mention having no heaters in the GH, which will probably limit what you can and cannot grow. A tarp may help insulate the GH, but depending on the material, will also block light from entering your GH. I currently have my GH covered with a clear solar pool cover which insulates and lets light in. I'm currently using an electric heater (5000 BTU) in the late fall/early winter and very soon we will be turning on our 30,000 BTU propane heater for the dead of winter. We're still testing, but we currently have tomatoes, cukes and salad greens growing and doing well so far. You'll also find that what you can grow is limited by the amount of daylight available. With some vegetables you'll have to add supplemental light to extend the daylight in order fo rthem to grow. We' planning to add a strawberry tower, but probably not until spring.

Hope this helps

I'm keeping an eye out for a free or cheap used solar pool cover. I don't think I can afford to buy a heater.

Can you tell me what temp I should be keeping the greenhouse at for planting at day and at nighttime? just above 32F? or maybe in the 50's?

If I don't have a cover or a heater by the time it's built at least I'll know what temps to be looking for in there before I plant stuff.

I might be able to house some of my poultry in there as an alternative. Their breathing and body heat is significant in their coop. I don't need to heat their coop at all to keep their water from freezing. So that could generate free heat for me.

Thanks.

I don't heat my greenhouse either, and I let the conditions dictate what I plant. Last winter we grew spinach in February and by March I had all kinds of stuff growing happily. I get the seedlings started inside the house, then bring them out to the GH once they are growing a bit.

As for what to grow, my suggestion is to experiment. Our GH is only on its 2nd year so I still have plenty to learn. We've had great luck with salad greens, basil, broccoli, green onions and carrots in the cooler weather. In summer we grow tomatoes, peppers and cucumbers because our summer weather is too cool to ensure good crops of these heat-lovers. So far I have been impressed with the GH carrot crop. The ground never freezes in there so I can pull carrots as needed all winter.

I've learned lots from two books: Greenhouse Gardener's Companion and Four Season Harvest. They include helpful info about what grows in what temperatures and conditions.

But mostly I just buy seeds that seem like they may work and I just try it to see what works. Thankfully seeds are cheap!

Up here in Zone 4/5 if I didn't heat or insulate, my GH stays a few degrees above the outside temps, except perhaps on a sunny day. Not much grows well in those temps so we heat and insulate. I'd test as Karin suggested and try some cold season crops. Get yourself a cheap thermometer (about $8 on eBay) that records inside and outside temps and records min and max temps and experiment. If you have any power in the GH you may want to try seeding/heat mats just to get the seeds started. Beyond that some experimentation is the best bet.

gibbsgirl,

You definitely need to look into "Four Season Harvest" by Eliot Coleman. Also, you can look at "Winter Harvest Handbook" by Eliot Coleman.

Basically, he says this:

#1 - the goal is economically and biologically feasible, if you try to extend the harvest, rather than extend the growing.

#2 - the goal also can be accomplished simply if you are choosing the right plants.

You need cold hardy veggies. In general, these are the plants that do not FRUIT! Anything that you eat the leaves or root of are much better options than fruiting plants. Next, certain types of plants are far more cold-tolerant than others. If you time it correctly, and grow these types in late summer - mid-fall, they can get sufficient growth before the sunlight drops off drastically. From that point on, the plants will go semi-dormant (hibernate, as he calls it). The plants can handle freezing temps, but don't harvest, unless the temp is above freezing.

A few of the veggies he mentions:
Carrots, spinach, arugula, endive/escarole, beets, turnip greens, raddichio, kale and mache. Mache is the hardiest plant he says, and can form a nice base for a winter salad.

As you may have noticed most of these are leafy greens. I guess mostly what you will get is salad fixings, with a few others.

I wish I could speak from experience with these, but I am in the very infancy of this myself. I have just learned about these books, and wasn't able to plant early enough to have the plants for this year.

The key is to HARVEST throughout the calendar, not to GROW throughout the calendar. Choosing correctly, as you seem to understand by your opening question, is the key.

I just wanted to post a thank you to all the good answers. It's very helpful. I'm always amazed at how much great info people post here.

I think I'm going to see what kind of difference moving some of my poultry in the GH will make for raising the temp. Has anyone else tried this? Or any other unconventional heating besides the solar pool cover.

In case you haven't guessed, my family is part of the "brotherhood of use it up, make your own, or do without" club. My husband and I were both raised to "poor boy it". :)

Nothing wrong with that! When I was younger, my parents qualified for food stamps, but we didn't take them. My dad just took another job, and we got used to less. One day my dad came home with Cheez Whiz. My mom cried! She was so happy!