By Ken Bezilla, Southern Exposure Seed Exchange
(Opinions current as of July 2009!)
Note: This is based on growing crops in zone 6b (normal winter low ~0 degrees F, low of -5 degrees F one year in ten) here in central Virginia. First fall frost ~10/15. Heavy winter temperatures not kicking in until late Dec/early January (heavy winter = daytime temperatures not getting above freezing, nights below 10 degrees). Last spring frost ~4/15.
Vegetables that die with 32 degrees F frost: basil, beans, corn, cucumbers, eggplants, melons, okra, peanuts, peppers, potato tops (potatoes can handle somewhat lower temperatures), squash, sweet potatoes, tomatillos, tomatoes.
At near-freezing temperatures, run sprinklers at night to keep frost-tender plants (ones too large to cover easily) alive for a couple extra weeks of production. Ideally, when planting, group tall plants like okra and tomatoes close together for running sprinkler.
Persephone dates: when days are too short (less than 10 hours) for growth to happen. Depends on latitude - in central Virginia, ~Nov 20-Jan 20 - but because cold weather doesn't really kick in until mid-December, and then the ground stays cold a long while, the effective dates here are more like Dec 15-Feb 15. During mild winters, some growth may still happen even during these short days, while in cold but sunny weather, crops in greenhouses, coldframes, and under row covers will continue to grow.
Watering: in hot weather, it can be hard to germinate some seed. Ideal would be to plant right before a rain storm, or plant in the evening and give the soil a good soaking. Water soil lightly every day until seeds emerge. Drip irrigation is very helpful for fussy crops like carrots.
Some seeds like cilantro and lettuce have trouble germinating in hot weather, but will grow fine once they've germinated. Helpful technique: chill seed beforehand, plant in evening, soak soil with cold water, and keep soil covered with burlap or boards until seeds emerge (3+ days for lettuce, 5+ days for cilantro). Remove burlap or boards promptly, otherwise grasshoppers may lurk underneath and eat the seedlings!
Some seeds like spinach and beets will germinate in hot weather, but then the seedlings will wilt in the heat and die off within 1-2 weeks; for these, need to wait until September for reliable survival.
Fertilizing/healthier plants: in late summer/fall, don't use fertilizer or compost that's high in nitrogen - plants are more cold-hardy if they grow more slowly. The healthier the soil, the more cold-hardy the plants. (The more nutrients dissolved in sap = the lower the freezing point of the plants, and the sweeter the plants will taste!) For extra protection, spray plants with kelp water (2 tbsps kelp solution/1 gallon water) before frosts to increase plant hardiness.
Raised Beds: To prevent roots from rotting in cold, wet soil, plant into raised beds.
Row cover: standard row cover usually offers about 4 degrees of frost protection per layer at night. (2 layers = 8 degrees, 3 layers = 12 degrees, etc.) Row cover will warm plants an extra 10 degrees during sunny days, greatly helping with growth. Row cover help keeps plants warm at night, but if temperatures don't get above freezing during the day, some plants will eventually get too cold and die.
Row cover can rest directly on plants, but tips of greens may be damaged during hard frosts where they touch row cover. Use bamboo or wire hoops (#9 gauge wire) to elevate row cover, with hoops 4-6' apart.
Put row cover on plants before frost starts - row cover freezes to ground, metal, and itself, and it will tear easily and be hard to unfold without damaging it if it's already frozen.
Row cover will last 2-4 years if stored dry and out of sunlight when not in use. Holes can be patched with duct tape.
To keep row cover from blowing off, use ground staples (U-shaped metal, ~1" x 6"), or heavy sticks, or bury the edges of the row cover. (Buried row cover must be unearthed once a month to keep weeds' roots from growing through it.)
Snow helps to insulate plants from cold. But if there's a lot of snow (4" or more) the snow can smush large greens (6" tall or more) that are under row cover. So better to take row cover off plants if very heavy snow is predicted and let the snow insulate the plants.
Cold frames: advantages = greens won't get crushed by snow, can be warmer than row cover, plants do better out of wind. Disadvantages: can be physically awkward to plant in and weed; may have more problems with critters (voles, mice, etc.); be careful if using glass - plastic is easier/safer to handle.
Mulch helps to insulate root crops and large greens like kale and collards and chard; it's messier around smaller greens like spinach and lettuce and mustards. Mulch to top of roots like beets and turnips - frosts will damage roots sooner than greens - e.g., at 19 degrees turnip greens may look OK even though the roots have already been damaged. Straw or hay mulch is easiest to use.
Cover crops may be seeded between rows of roots crops in Oct/Nov (rye grows fastest) once plants have a head start. (At least 5 weeks head start for carrots and others, but only 2 weeks for small radishes.) Cover crops will be ready to take over beds when roots get harvested!
Weather forecasts - use an outside recording thermometer to compare the weather forecast for your area with the actual low temperature. Cloudy nights are warmer, clear nights are colder. Dry, windy nights exacerbate frosts.
Harvesting notes: The morning after a frost, don't harvest plants while they're still frozen - wait for the temperature to rise above 32 degrees and for the plants to thaw out before harvesting.
Roots and greens storage:
- Grasshoppers - sow 2-4 times as much seed as usual in late summer - grasshoppers will thin out plants while they're less than 2" tall!
- Aphids - especially like lettuce, kale, turnips, rutabagas, and mustards. Protection: keep plants under row cover from beginning, or spray plants with Safer soap as soon as you notice aphids. MALBs (Multicolored Asian Lady Beetles) if present in fall will help keep aphids under control.
- Slugs - may do minor damage to greens, especially those inside a greenhouse. Protection: look for them at night with flashlight; or leave boards in garden for slugs to hide under during the day, then turn over the boards to collect the slugs.
- Voles - may eat root crops or greens under heavy plastic. Protection: traps and cats may keep down their numbers, but easiest to harvest root crops to safety of a refrigerator or root cellar if noticing damage.
- Deer - if it's a hard winter, deer may cruise your garden for food. Row cover usually confuses the deer and hides the crops from them, especially if crops are covered before deer notice them.
Break off tops when harvesting root crops (so that the greens don't regrow and suck nutrients out of the roots). Rinse off dirt before storing. Store in crisper or fridge for best life - root crops should be good for 2+ months.
Try to have as many greens and roots mature as possible by ~Dec 15 before growth stops. From Dec 15-Feb 15, what you see is what you can eat; don't count on any new growth happening. If there's limited garden space, greens are the most productive. Sow seeds thickly and harvest thinnings as plants grow. Once plants have matured to their final spacing, only harvest outer leaves, leave small leaves to become larger.
Most productive/cold-hardy greens: spinach, kale, collards, some mustards, Even' Star Winter Arugula, cilantro, sorrel, salad burnet, curly parsley.
Best root crops: carrots are first priority - wonderfully sweet when maturing in cold weather!
Arugula and mustards bolt (go to flower and seed) in January/February. Other greens bolt in early April.
Root crops that overwinter should get harvested in March before plants bolt in April.
- dates of last planting for fall, cold hardiness, and some good/favorite varieties. Particulars: this is based on experience of growing crops in zone 6b (winter low ~0 degrees F) here in central Virginia, first fall frost ~10/15, heavy winter temperatures not kicking in until late Dec/early January (i.e., daytime temperatures not getting above freezing, nights below 10 degrees F). Cold hardiness also varies with varieties, the health of the garden soil (healthier and more nutritious the soil = the hardier the plants), wind chill factor, etc.
- Broccoli (6/1-7/1): 28 degrees - leaves can handle to 15 degrees, but heads are more tender
- Cabbage (6/1-7/1): 20-25 degrees - if damaged by frost, harvest and peel off any damaged layers before storing.
- Chinese Cabbage (7/31): 25 degrees
- Cauliflower (6/1-7/1): 32 degrees - leaves can handle to 15 degrees, but heads damage easily.
- Kohlrabi (8/15) (Early Purple Vienna): 20 degrees? not sure.
- Green onions (8/1) (Evergreen Hardy White, Deep Purple): 25 degrees (purple types have more color in cold weather)
- Garlic (10/1-/11/15): 5 degrees if not too much topgrowth (i.e., started ~mid-Oct-early Nov); frost-burned plants will survive, but wont produce as large a bulb.
- Leeks (7/15): 10 degrees (probably much lower) (American Flag) ('summer' types are much more tender, don't try to overwinter them)
- Collards (9/15): 12 degrees - start a fall crop, since young collards are shorter/easier to cover.
- Kale (9/15) (Red Russian): 15 degrees
- Scotch types (Squire, Vates, Siberian): 12 degrees
- Swiss chard (9/1): variable - smaller-leaved varieties are the most cold-hardy: 25 degrees?
- Arugula (10/10): 22 degrees (will bolt in Jan/Feb as days lengthen)
- Even' Star Winter Arugula: 6 degrees
- Mustards (9/15) (Red Giant, Southern Curled): 25 degrees (will bolt in Jan/Feb as days lengthen)
- Tat Soi Mustard (up to 10/15 - do succession plantings 1-2 weeks apart): 22 degrees (good mustard to overwinter - hardy, close to ground, easy to cover - but will bolt in Jan/Feb as days lengthen)
- New Star Mustard, Chinese Thick-Stem Mustard - Even' Star winter-hardy varieties: 6-12 degrees
- Parsley - flat leaf (7/15): 20 degrees (Flat Leaf Italian) (best for flavor and drying, but less hardy)
- Curly leaf (7/15): 15 degrees (Moss Curled) (not as good flavor, but prettier, more hardy)
- Cilantro (9/15-10/1): 15 degrees (plant earlier for fall harvests, later for overwintering crops - younger/smaller plants overwinter best). In hot soil, cilantro can have trouble germinating - see notes under Watering.
- Dill (9/15): 25 degrees (Fernleaf)
- Spinach (Long Standing Bloomsdale, Winter Bloomsdale) (9/10-9/25): 10 degrees (large leaves), 5 degrees (small leaves). Wait until cool weather to seed - doesn't germinate/survive well in hot soil.
- Sorrel (Broad Leaved) (9/1): 12 degrees (large leaves), 5 degrees (small leaves)
- Salad Burnet (9/1): 0 degrees (leaves have strong cucumber flavor - use small amounts for salads)
- Endive, escarole: 25 degrees and lower (similar to lettuce)
- Radicchio: 25 degrees and lower
- Lettuce: 25 degrees (large leaves), 15 degrees and lower (small leaves) (Red Salad Bowl, Bronze Arrow, Winter Density, Rouge d'Hiver, Red Sails) (Red lettuces are best for cold weather - the cold makes the red colors more intense, whereas in cold weather green lettuces look more yellow and sickly.) Lettuce can have a hard time germinating in hot soil - see notes under Watering.
- Large lettuce: 9/15. (Large heads don't handle very cold weather well - usually rot and decline fast ~mid-December.)
- Small lettuce to overwinter: 10/1-/15; get it to 4-10 leaves before winter - it'll slow down, then mature in February/March.
- Beets (9/15) (Lutz Green Leaf, Chioggia, Bulls Blood): 20 degrees (roots), 16 degrees (leaves). For fall crop, either seed by 6/15, or wait until cool weather to try again - doesn't germinate/survive well in hot soil.
- Radishes (11/1): 20 degrees (roots), 16 degrees (leaves) (Cherry Belle)
- Daikon radishes, fall radishes (8/1-8/15): 20 degrees (roots), 16 degrees (leaves) (Misato Rose, China Rose, Black Spanish Round, Miyashige White Daikon)
- Parsnips (6/1): 0 degrees (get very sweet in cold weather!!) (Harris Model, Hollow Crown)
- Carrots (8/31): 12 degrees (Danvers, Oxheart)
- Rutabagas (8/15): 20 degrees (roots), 16 degrees (leaves) (American Purple Top Yellow)
- Turnips (9/30): 20 degrees (roots), 16 degrees (leaves) (Purple Top White Globe) (Seven Top - greens only)
- Salsify (6/15) (Sandwich Island Mammoth): 20 degrees? Not sure
- Jerusalem artichokes (sunchokes) - leave in ground, harvest anytime after frost and until the following April
- Other plants not covered here which I don't have enough experience with to mention: corn salad (mache), cress, minutina, claytonia; fall crops of peas, calendula, borage
- Eliot Coleman, Four-Season Harvest (for home gardeners and market growers); The Winter Harvest Handbook (for market growers)
- Alison and Paul Wiedeger, Walking to Spring
- Steve Solomon, Gardening When It Counts
- Charles Siegchrist, Building and Using Cold Frames
- Binda Colebrook, Winter Gardening in the Maritime Northwest (emphasis on the NW, but generally interesting)
ATTRA publication on season extension: http://www.attra.org/attra-pub/seasonext.html
Oregon State University publication: http://extension.oregonstate.edu/lane/sites/default/files/documents/LC322WinterGardeningrev.pdf