Creating garden beds to grow winter vegetables is very similar to creating summer gardens, but – in addition to setting up a cold frame – there are a few other special issues that you need to consider.
1) Plant your winter crops in raised beds.
Soil that is just 2-3 inches higher than level ground will drain better and warm up faster. This is very important for winter gardens. In my zone 6b climate, I've had good luck growing winter crops in beds up to 8 inches high, but if you have very cold winters you'll probably want to keep your beds a bit shorter, as higher beds are more exposed to the bitter cold. Another option would be to put some insulation around your raised beds during winter, such as mulch or shredded leaves.
Some northern gardeners even sink their cold frames down into the soil to give more protection from winter weather, but you need well-drained soil for that to work. The Year-Round Vegetable Gardener: How to Grow Your Own Food 365 Days a Year, No Matter Where You Live describes how to do that, and provides clear photos of the process. Eliot Coleman, author of Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long, lives in Maine, and has had great success by simply setting his wooden cold frames on top of his slightly raised garden beds.
2) Grow your winter crops in fertile, moist soil rich in organic matter.
Summer crops are often a bit more tolerant of mediocre soil, but many winter crops need excellent soil to thrive. You can add compost, well-rotted manure, peat moss, or other sources of organic matter, but don't use raw manure. If you have poor quality soil, mix about 2 inches of organic matter into the top 6 inches of the soil in your raised bed. Most winter crops like fertile soil, but don't add too much nitrogen to your root crops, or you will harvest pitiful roots. Here is an example of poor quality, compacted soil:
Poor quality soil doesn't drain well (unless it's sandy soil), doesn't allow much fresh air or rain into the soil, makes it difficult for seeds to germinate through the hard crust, and stays colder and wetter during the winter. The following photo is the same soil one year later, after adding compost and keeping the soil lightly mulched. The soil is softer and more open, allowing roots, rain, and fresh air to easily penetrate the soil. This soil drains very well and warms up much faster.
Other gardeners choose to use a soil mix in their garden beds, instead of real soil. There are some advantages and disadvantages to this, as I've mentioned in my review of All New Square Foot Gardening, Second Edition: The Revolutionary Way to Grow More In Less Space. I've noticed very few carrot maggots when using a soil mix instead of real soil, but top-heavy plants such as broccoli need good support, as soil mix is too loose to keep those larger plants upright.
3) The best place for your winter crops, if possible, is a gentle south slope.
You have probably noticed that snow melts faster on south-facing hillsides (anywhere from southeast to southwest). Any soil that tilts towards the winter sun low in the sky absorbs a lot more solar energy and stays warmer. Northern slopes can stay frozen for several weeks after southern slopes have thawed in the spring.
If you don't have a south slope to work with, at least try to level your raised beds by digging a level base when you create your beds. This is not important for your summer crops, but it can really help for your winter ones.
I don't bother to level garden beds that are on gentle south slopes, but if the slope is too steep I will make the beds a bit more level. An easy way to decide if a slope is too steep is if you really notice when you are walking up the hill. You will barely notice when you are walking around a gentle slope.
Avoid putting any of your garden beds in a frost pocket. Cold air travels downhill and collects at the bottoms of hillsides, or on the uphill sides of buildings, solid fences, hedgerows or woodlands.
If you are in a very cold climate, one of the warmest places to tuck your cold frames is next to the south side of your home, garage, or shed. It can also help to have a wind break on the upwind side of your garden. The book, Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long, goes into great detail on what makes the best micro-climate for your winter garden.
4) Plan on using 30-60 square feet of garden space per person for your winter crops (2 to 4 small 4'x4′ beds).
Start small with just one or two cold frames on your garden beds, and gradually add more cold frames as you learn how much you eat and how to grow your winter crops. Winter vegetables spend weeks or months staying pretty dormant – not growing, but remaining available for you to harvest. That is why you would need a larger winter salad garden than you do a summer one.
As an example, if you find that you need to plant 4 square feet of garden space to grow enough salad plants to feed you for 2 weeks, you could grow all of your salad plants from spring to fall by rotating your crops in just 16-24 square feet of garden beds.
Because your salad plants won't grow much, if at all, from mid-October to mid-March (in my climate), you would need to plant about 40 square feet of winter garden space to harvest fresh crops for those 5 months (4 square feet x 10 2-week periods from Oct-Mar).
When you design your winter vegetable garden, remember that it will still need some maintenance – how much depends on your climate, and if you use cold frames or plastic mini tunnels that need regular ventilating on sunny days. The need to take care of and harvest your winter garden, especially if you have to wade through deep snow, are excellent reasons to put your garden beds close to the entrance to your home.