Winter Sowing: How to Start Seeds in Milk Jugs (and When)

Updated November 3, 2026

Winter sowing is a way to start seeds outdoors in winter, in clear containers such as milk jugs, so winter cold and moisture prepare the seeds to sprout, with no grow lights or heat mats needed indoors. It suits perennials and native wildflowers whose seeds need a cold period to sprout. Here is when to start, which seeds to sow, how to set up a jug and how to care for the seedlings in spring, from university extension guides.

Jump to: What it is When to start Frost dates by zone What seeds to sow How to set up a jug Other containers Spring care Mild winters

What is winter sowing?

Penn State Extension defines it as planting seeds outdoors in a container during the winter months; the container stays outside until seedlings come up in spring. The container keeps out animals and harsh weather, while the cold breaks the seeds’ dormancy. University of Missouri Extension compares the covered jug to a mini greenhouse.

The cold, moist period is called cold stratification. University of Minnesota Extension says the jugs “sit outside all winter, giving seeds the cold stratification they need to break dormancy”, with no indoor lights or heat mats needed. Illinois Extension adds that the seedlings harden off naturally and can be planted out once the soil is warm enough for them.

When to start winter sowing

No extension source gives a start date for each USDA zone. The sources give a calendar window instead, written for cold-winter states (Pennsylvania, Illinois, Minnesota, South Dakota and Missouri):

Why not November? Illinois Extension says to “Wait to begin winter-sowing until January or February.” If a warm spell follows, seeds sown too early can sprout and then be too tender to survive the next cold snap. The University of Arkansas System Division of Agriculture describes setting pots of cold-needing perennial seed outdoors in late fall or winter, and Missouri lists cool-season annuals such as snapdragon, pansy and nasturtium to start in fall or winter. For milk jugs of perennials, the other guides start in December (Penn State, SDSU) or January (Illinois, Minnesota).

Seeds that need cold need enough of it: SDSU notes that native perennials sown as late as March may germinate less well if their required chilling time isn’t met.

Last frost dates by zone, for planting seedlings out

The sowing date is a calendar window, but the planting-out date follows your last spring frost. University of Maryland Extension transplants winter-sown seedlings after frost. For vegetables, NDSU Extension’s seed-starting chart sets broccoli, cabbage and kale transplants out about 3 weeks before the last frost, lettuce about 4 weeks before, and tomatoes and peppers about 2 weeks after. Typical last frost dates and those offsets:

About this table: For each ZIP code we use the nearest NOAA NCEI 1991–2020 U.S. Climate Normals station’s 50%-probability last spring date for 32°F (half of years have a later frost), summarized by USDA zone across ZIP codes (median ZIP, plus the earliest and latest 10% of ZIPs). The two right columns are our calculation: the median minus 3 weeks and plus 2 weeks, using NDSU H1139’s set-out offsets. Source: NOAA NCEI U.S. Climate Normals. Enter your ZIP in the planner for the dates at your nearest NOAA station.

Zones 8 to 11: winters are short or mild, and none of the sources gives a winter-sowing schedule for them. Where winters don’t stay cold long enough, see the refrigerator method below.

What seeds can be winter sown?

The best candidates are seeds that need a cold period. Penn State Extension suggests checking the seed packet or catalog for words like “self-sows,” “reseeds,” “hardy seeds,” “sow outdoors in late autumn or early winter,” “needs pre-chilling” or “requires stratification.” SDSU Extension says cold stratification is usually shown as a germination code on the seed packet; a code such as C30 “means the seeds should be stratified at least 30 days before planting.”

Vegetables: cool-season crops can be winter sown in late winter (Missouri). SDSU cautions that although brassicas are cold-hardy, their seeds germinate better in warm conditions, so they shouldn’t go outside too early, and in South Dakota it doesn’t recommend winter sowing heat-loving crops such as the tomato and squash families at all. Tomatoes, peppers, zucchini and cucumbers wait until April (Missouri); Illinois says the same for frost-sensitive zinnias, tomatoes and squash (March or April).

More on two popular natives: when to plant milkweed seeds, growing coneflowers and black-eyed Susan.

How to winter sow in milk jugs, step by step

1. Clean the jug. Wash it in warm, soapy water and throw away the cap; the open top lets rain and melting snow in (Penn State).
2. Cut a hinge. Cut around the jug, leaving a strip uncut under the handle so the top hinges open: about 1.5 inches (Penn State) or about half an inch (Illinois). SDSU cuts about 4 inches up from the bottom.
3. Poke drainage holes in the bottom. Penn State’s warning: “if you don't make drainage holes, your seeds will drown!”
4. Add the mix. Use 2 to 4 inches of soilless seed-starting mix (Penn State, SDSU) or about 3 inches of well-draining potting mix (Missouri, Minnesota). Avoid garden soil, which may carry disease organisms and weed seeds (Penn State).
5. Moisten it and let it drain. It should feel like a damp sponge (Illinois).
6. Sow at the depth on the seed packet. Small seeds can sit on the surface (Illinois). UMD plants one type of seed per container and mixes tiny seeds with coarse sand or vermiculite to spread them.
7. Tape it shut with duct tape, leaving the top open (Penn State, Missouri). Pipe cleaners or zip ties through holes work too, and open more easily (Penn State, SDSU).
8. Label it with the seed and the date. Penn State prefers a laundry marker to a permanent marker; Minnesota and UMD label the inside and outside.
9. Set it outside where it gets winter sun plus rain and snow, out of strong wind. Not on a covered porch: the seeds need the moisture (Penn State). A milk crate keeps jugs from blowing over (Penn State, SDSU).

Winter sowing where winters are mild

If your winters don’t stay cold long enough, you can give seeds their cold period indoors. University of Maryland Extension describes putting seeds in a container with a moist paper towel in the refrigerator (about 40°F), checking the moisture, and leaving them 1 to 3 months depending on the seed, then sowing them in pots or outside. The University of Arkansas System Division of Agriculture gives 35 to 45°F as the best range for chilling pots of seed. For more on this method with milkweed, see when to plant milkweed seeds.

More guides

When to plant milkweed seeds What vegetables to plant in fall and winter Coneflowers Frost dates by ZIP ZIP planner Planting tips