Winter Vegetables: What Actually Grows, and Where

Winter means three different things in this country. Start by working out which one you have.

Most winter gardening advice is written for somewhere specific and then repeated everywhere. It is why a gardener in Minnesota reads that winter is a productive season and concludes that gardening is not for them. Across this country winter is three separate propositions, and almost everything else follows from which one you are in.

The ten-hour rule, which explains the rest

Below about ten hours of daylight, most vegetables stop putting on meaningful growth. They do not die and they do not bolt. They sit. The market grower Eliot Coleman named this stretch the Persephone period, and it is the single most useful thing to understand about winter growing.

What it means in practice is that a winter harvest is not something you sow in winter. It is something you sow in late summer or fall, bring to nearly full size, and then hold. Aim to have a crop at about three quarters of its final size when the window closes, and you can pick from it for months. Sow it a month late and it will still be a seedling in March.

The window depends entirely on how far north you are:

Work out roughly where you sit between those and you have your own date. It moves by about a week for every couple of degrees of latitude.

Which winter you have

Frost-free: South Florida, Hawaii, the low desert, coastal Southern California

Winter is your main season, and the summer is the hard part. This is when tomatoes, peppers, beans, squash, cucumbers and corn actually work, because the heat and the humidity and the pest pressure have all dropped away. Plant the warm-season list from October onwards and treat June to September as the season to rest beds or grow cover crops.

The cool-season crops grow here too, but many of them run to seed quickly once it warms in February, so sow them early and pick them young.

Mild winter: the Gulf South, Texas, the Southern Plains, the Mid-South, California, the Pacific Northwest

This is the closest thing to a classic winter vegetable garden. Frost comes, but it is light and occasional, and the ground rarely freezes hard. Cool-season crops grow slowly right through and taste better for it, because cold converts starch to sugar in carrots, parsnips, kale and Brussels sprouts.

The Pacific Northwest is its own case: mild and wet rather than mild and dry. Drainage and slug pressure matter more there than cold does, so raised beds earn their keep.

Hard winter: the Northeast, the Midwest, the Northern Plains, the Mountain West, Alaska

Nothing grows outdoors unprotected in January and pretending otherwise wastes seed. What does work is different and still worth doing:

• Overwintering under cover. Spinach, mache, kale and leeks established in September will sit under a low tunnel all winter and give you the earliest harvest of the following year.
• Garlic. Planted in fall, it wants the cold. See below.
• Cover crops. Winter rye and hairy vetch sown in fall hold your soil, feed it, and beat the spring weeds to the ground.
• Storage. Carrots, parsnips, leeks and salsify can be left in the ground under a deep straw mulch and dug as needed, which is a working root cellar you already own.

The crops, ranked by how much cold they take

These are unprotected figures. A row cover moves every one of them.

• Takes hard freezing, to around 10F and lower. Kale, collards, mache (corn salad), spinach, leeks, parsnips, salsify, Jerusalem artichoke, garlic. Kale and parsnips are genuinely better after a hard frost.
• Takes real frost, to the low 20s. Brussels sprouts, cabbage, savoy cabbage, turnips, rutabaga, arugula, mustard greens, tatsoi, mizuna, Swiss chard, carrots, onions, radicchio.
• Takes light frost, high 20s. Broccoli, cauliflower, lettuce, bok choy, beets, radishes, cilantro, parsley, peas.
• No frost at all. Tomatoes, peppers, beans, squash, cucumbers, basil. These belong in a frost-free winter garden and nowhere else at this time of year.

A rule that holds almost everywhere: if you eat the leaf or the root it probably takes cold, and if you eat the fruit it does not.

What a row cover is actually worth

Row cover fabric is sold by weight, and the weight is the whole trade-off. Heavier fabric holds more warmth and lets less light through, which matters a great deal in the darkest part of the year.

• Light, around 0.55 ounces per square yard. Passes roughly 85 per cent of light and holds frost off to about 28F. The right choice for extending fall and for insect exclusion.
• Heavy, 1.5 to 2.2 ounces. Buys up to 8F, but passes only 30 to 50 per cent of light. Good for getting a crop through a cold snap, poor as a permanent winter roof because the plants are already short of light.

Two covers beat one heavy one. A light fabric inside a low tunnel of clear plastic gives you the warmth of the heavy option and keeps most of the light, and it is the standard winter arrangement for good reason. Lay the fabric over hoops rather than directly on leaves, since anything touching frozen fabric burns.

Garlic, which is its own calendar

Garlic goes in during fall almost everywhere in this country, four to six weeks before the ground freezes hard. That is generally October in the north, November through December in the south. It needs a cold period to split into cloves, which is why spring-planted garlic so often comes up as one round bulb.

Plant cloves point up, two inches deep, six inches apart, and mulch heavily once the ground has chilled. Harvest when the lower third of the leaves have browned, usually June or July. Hardneck varieties suit the north and give you scapes in June; softneck suits the south and stores longer.

Getting the timing right

1. Find your first frost date and your ten-hour date. The second one matters more for winter than the first.
2. Count back the days to maturity, then add some. Fall sowings grow more slowly than the seed packet assumes, because the packet assumes lengthening days. Add roughly two weeks for anything sown after the equinox.
3. Sow so the crop is about three quarters grown when the window closes. That is the whole trick.
4. Get the cover on before the first hard freeze, not after. Plants that have hardened off gradually take far more cold than plants hit suddenly.
5. Harvest in the afternoon in winter, once the leaves have thawed. Frozen leaves shatter when handled and turn to mush later.

Questions worth answering

Can I really grow vegetables outdoors in winter in the north? You can harvest them, which is not quite the same thing. Establish the crop in late summer and it will hold under cover through the winter and start again in February. What does not work is sowing in November and expecting anything.

Why has my kale stopped growing? Almost certainly day length rather than anything you did. Below ten hours it holds rather than grows. It will start again on its own.

Is frost bad for winter vegetables? For the ones on this list it is usually good. Cold converts stored starch into sugar, which is why parsnips, kale, carrots and Brussels sprouts are all noticeably sweeter after a freeze.

What about Florida and Hawaii? Your winter is everyone else's spring. Grow the warm-season crops now and use the summer for cover crops, because summer is the season that will beat you.

Do I need a greenhouse? No. A low tunnel of hoops and plastic over a bed costs a fraction of one and does most of the same work for winter crops that are holding rather than growing.

See also: Growing garlic, which is the one crop that wants to go in as everything else finishes, Green manure and cover crops for the beds you are not using, and Planning a vegetable garden for working backwards from a harvest date.