Winter Sowing: When to Start and What to Sow Where You Live

Winter sowing means sowing seeds in covered, vented containers such as milk jugs and leaving them outdoors over winter, so cold, wet weather breaks their dormancy and they sprout on their own in spring. Extension guides sow seeds that need a cold period, hardy annuals and the toughest greens from December to February, broccoli and lettuce in March or April, and tomatoes and other tender crops in March or April.^5,4,6

The date that matters most is the last one. A seed labeled for 60 days of cold needs about 60 cold nights before spring warms up. We counted nights at 40°F or colder in NOAA’s 1991–2020 normals for every ZIP code: that deadline falls around Feb 12 in Columbus, Ohio, Jan 25 in Washington, DC, and Jan 2 in Atlanta.¹ A January 31 sowing still gets 60 cold nights in 95% of zone 6b ZIP codes, but in only 44% of zone 7b and 6% of 8a.²

What winter sowing is, and how it differs from starting seeds indoors

Penn State Extension describes it as planting seeds outdoors in a container in winter and leaving it there until seedlings come up in spring; the container keeps out animals and the worst weather while cold and moisture break dormancy.⁴ Trudi Davidoff introduced the method on GardenWeb in 2000, according to University of Maryland Extension Master Gardeners.⁵

It replaces grow lights, heat mats and hardening off, because seedlings grow up outside.^4,5 It doesn’t change when plants can go into the garden. For crops you start under lights, our guide to when to start seeds indoors counts back from each crop’s planting-out date; this page uses the same frost dates.

When to start winter sowing: three waves

Extension and Master Gardener guides split winter sowing into waves by how much cold each seed can take:

Sources: Maryland’s Charles County Master Gardeners,⁵ Illinois Extension,⁶ Penn State⁴ and Maine’s Wild Seed Project.¹² As Penn State puts it:

Tender plants including annuals and vegetables can be sown later in spring (March or April)

Don’t start in fall. Illinois Extension waits until January or February, and explains why:

If there is a warm spell, the seeds can germinate but won’t be hardy enough

Google’s AI Overview, citing a YouTube channel, said to start once nights stay below 50°F, but normal lows reach 50°F by Sep 24 in Minneapolis and Oct 19 in Atlanta, months before any extension start date.¹

The real deadline: enough cold nights before spring

Many perennial and native seeds stay dormant until they have spent weeks cold and moist, a process called cold stratification. The seed packet should say how many days a species needs, notes UW–Madison Extension’s Lisa Johnson in Dane County.⁷ Prairie Moon Nursery writes it as a code: C(30) or C(60) for 30 or 60 days of cold, moist treatment, which it gives at 33–40°F in a refrigerator.¹⁰ A jug outside does the same job on cold nights, so its sowing date has to leave enough of them.

We counted, for the NOAA station nearest each ZIP code, the nights whose normal low is 40°F or colder, the top of Prairie Moon’s range. Frozen nights count too, since the guides treat freezing and thawing as part of the process.¹¹ The last start for a 60-day seed is the date that still leaves 60 such nights before spring.¹

The deadline runs from late February in Minneapolis, Madison and Denver to early January in Atlanta and Portland. Dallas averages 48 cold nights, enough only for 30-day seeds sown by Jan 2; Houston’s normal nights never reach 40°F. Gardens far from a station can run a week or more off, so treat these as a guide and look up your own frost dates.

How we counted cold nights

We spread NOAA’s monthly normal lows into daily values, as our daffodil and tulip guides do, and counted nights from July to June. These are station air temperatures in a typical winter, not the soil in a jug. We dropped two other measures: a daily mean of 40°F or colder gave Atlanta zero cold nights though its January lows average about 33°F, and leaving out fully frozen days pushed Minneapolis’s deadline into November, against Hennepin County Master Gardeners, who winter sow from January on.⁸

Does your hardiness zone tell you when to winter sow? We tested it

Google’s AI Overview told us that cold zones 3 to 5 should sow perennials in January and February, and zones 6 to 8 from late December through January. We checked the end of each window for 39,894 contiguous-US ZIP codes in PRISM’s 2023 zone table (look up your zone).^2,1

The direction is right: warmer zones run out of cold nights sooner. The end dates are not. A seed needing 60 days of cold sown on January 31 gets enough nights in 95% of zone 6b ZIP codes, 78% of 7a, 44% of 7b and 6% of 8a. A February 28 sowing works in 64% of zone 5a. Inside one half-zone, the middle 80% of ZIP codes reach the deadline over a span of 22 to 29 days from 4b to 8a, and 53 days in 8b, which takes in both the Gulf Coast and the Pacific Northwest.

The winter solstice, another common start date, leaves 60 cold nights in every half-zone through 8a, but in 72% of 8b and 22% of 9a ZIP codes. Hardiness zones measure the coldest night of the year, not how many cold nights there are.³ Seattle is zone 9a, yet it averages 125 cold nights; Dallas, in 8b, has 48. See planting zone vs. frost date and what changed on the 2023 zone map.

Which seeds to winter sow

Penn State suggests looking for catalog terms such as “self-sows”, “hardy” or “requires stratification”.⁴ Many native perennials need cold, but not all. Prairie Moon Nursery’s germination codes for common species:

Brandywine Conservancy in Pennsylvania gives the same 60 days for nodding onion and 30 for milkweeds.¹¹ Purple coneflower is a useful surprise: Google’s AI Overview and the Maryland Master Gardeners’ list group it with seeds that need winter’s chill, but Prairie Moon gives it code A. It still winter sows well; the Wild Seed Project sows no-cold seeds outdoors from November to April.¹²

Hardy annuals such as poppies, snapdragons, calendula and bachelor’s buttons do well too, according to Seed Savers Exchange.¹³ Most vegetables are a different case:

The majority of garden crops do not need nor benefit from cold stratification.

For vegetables, winter sowing is about an early outdoor start, not cold. Root crops are poor candidates because they dislike transplanting, says Hennepin County Master Gardener Theresa Rooney.⁸

How to winter sow in a milk jug

1. 1

   Prepare the container

   Wash a clear or translucent jug, cut it around below the handle, leaving a hinge, and throw away the cap. Make 4 to 6 drainage holes in the bottom.⁵ Penn State warns that without drainage holes your seeds will drown.⁴ Freezer bags, salad-green boxes, takeout trays and 2-liter bottles also work.^5,9

2. 2

   Add moist mix and sow

   Fill the base with 3 to 4 inches of damp potting mix, not garden soil; natives and perennials need at least 4 inches for roots.^5,11 Sow one kind of seed per container at the depth on the packet. Press tiny seeds, and those that need light, onto the surface.^7,10 Space seeds about 2 inches apart if you want to separate them later.⁹

3. 3

   Label, tape and set outside

   Label the jug outside and inside, since markers fade over winter. Tape it shut and set it in winter sun, out of strong wind, where rain and snow can reach it, not on a covered porch.^4,5,6 Snow around the jugs helps insulate them.⁷

   

Leave it, checking occasionally for condensation inside; water only if the mix dries out.^5,9

Spring care and planting out

Once seedlings appear, open the containers on sunny days and close them at dusk, Illinois Extension advises.⁶ Maryland’s Master Gardeners move them to shade in a sudden heat spell and cover them with a sheet or row cover if a freeze is forecast. Transplant after at least two true leaves, opening the containers for two or three days first; thick sowings can be cut apart “brownie-style”.⁵

Winter sowing does not change your planting dates. Cool crops such as broccoli go out about two weeks before your median last frost. Tender crops wait until frost risk is down to one year in ten and nights average 50°F, the rule our tomato planting guide tested.¹

Maryland’s Master Gardeners told one tomato story: sown in a jug on March 3, 2021, planted out April 12 and covered with cloches on April 21 when a freeze was forecast.⁵ For La Plata, the nearest NOAA station, 15 miles away, puts the median last frost at Apr 17, the 10% date at May 2 and 50°F nights at May 9, so that planting was early and needed the cover. See what the 10% and 50% frost dates mean.

Expect tender crops to lag. A Fairfax Master Gardener’s January-sown tomatoes sprouted only in May, smaller than indoor ones, though they caught up.⁹ A Mother Earth News reader contributor in Virginia starts summer crops indoors because winter-sown tomatoes stay “too small”, but sows cool crops straight into straw-covered beds in February.¹⁴

Winter sowing where winters are mild

Across the lower 48 states, 12.5% of ZIP codes average fewer than 30 cold nights a winter. Most are in zones 9 and 10: 55% of zone 9a ZIP codes and 72% of 9b.^1,2 There, seeds that need cold won’t get it in a jug.

Seed Savers Exchange suggests the refrigerator: seeds in a damp medium in a sealed, labeled bag for 4 to 12 weeks before you would normally sow.¹³ Prairie Moon gives 33–40°F for this step.¹⁰ Seeds that need no cold can go in at their usual local time; for fall and winter crops, see our fall vegetable planting dates.

Next step: find your ZIP code’s frost dates in our frost date lookup, then count back for anything you start indoors with our seed-starting guides.