A little protection goes a long way!
Since every regions weather is a little different, it will take some experimenting to know when to start your seeds for your climate. I have noticed that most experienced gardeners are usually willing to share their knowledge, so don’t be afraid to ask the neighbor down the street with the explosive garden every year for some tips.
Growing through the winter in a cold frame, in my experience, is all about timing. I set my cold frame up late summer/early fall in an area of the garden which will have good sun exposure throughout the winter months. Once the temperatures start cooling in the evening to 50* - 55*, I plant my cold hardy seeds and either leave the lid off or have it in the fully vented position.
Fall in the cold frame
For this time of year we plant kale, chard, parsley, cilantro, spinach, lettuce (romaine is a great cold hardy variety), kohlrabi, tatsoi, bakchoi, carrots, parsnips, rutabaga, turnips, beets, radish, and peas. I have tried cabbage, brussel sprouts, broccoli, and cauliflower too, however, these plants get fairly large before you get a harvest. Three or four of these brassicas will take up the whole cold frame. Potatoes are another great plant for winter gardening in a cold frame. Since their foliage takes up so much space they too get their own cold frame.
As soon as the temperatures begin falling below 40* or as the fall rains come in, it’s time to put the lid on and start monitoring the temperatures inside the cold frame. A small thermometer inside is a good idea. When I have missed getting the lid on before the hard fall rains my seedlings ended up beaten, bruised, and the soil was over saturated essentially sending them into a period of shock, slowing their growth. During this time I will keep the cold frame vented, at least slightly, since overheating on a sunny day at 50+* can happen very fast when the lid is closed. If you have the automatic vent on the back panel, the cold frame will regulate the temperature even with the lid closed. In an un-vented cold frame the seedlings get too warm, making them grow long, spindly, with smaller leaves. In the end, a cold frame that is kept too warm in the fall makes for a weaker plant that can’t adapt to colder temperatures.
I have always enjoyed over seeding with cold hardy varieties. Thinning “the carpet” as they fill in. Feeding my tender seedlings each time I thin with a compost tea or some worm castings. The thinned seedlings are consumed in salads and sandwiches (think micro greens).
As the temperatures become colder, dropping below 35* in the evening, its time to start closing the lid at night. If day time temperatures don’t get over 40* it isn’t necessary to vent.
Winter in the cold frame
We usually get a cold snap in the fall before any snow flies with temperatures dropping below 20*. During this time, for those really cold nights when it can get into the single digits, we drape a wool blanket over the cold frame for the night. Having a few stones or garden gargoyles can help retain some warmth in a cold frame, but for the most part, the warmth is retained in the moisture of the soil. Watering once or maybe twice a week is plenty, remembering that dry soil will freeze plants faster than damp soil. When watering, it is important to try not to get the foliage of the plants wet and tending to your plants in mid-morning is best. That way your cold frame has the rest of the day to retain heat for the night.