Winter Sowing: How to Start Seeds Outdoors in Milk Jugs (and What Not to Sow)

Winter sowing means sowing seeds outdoors, in winter, inside small translucent containers like milk jugs, and then leaving them alone. The containers act as miniature greenhouses: they trap enough warmth on sunny days to help seeds break dormancy when conditions are actually right for that particular species, while the vented top and outdoor placement keep the seedlings exposed to real cold, real wind and real light the whole time. Nothing germinates before its time just because you sowed it in December.

The method is best for hardy annuals, most perennials and native plants that need a period of cold before they will germinate at all, and cool-season vegetables that already tolerate frost as young plants: peas, spinach, lettuce, kale, broccoli and their relatives. It is a poor fit for tender, heat-loving crops with zero frost tolerance, tomatoes, peppers, basil, cucumbers, squash and melons among them, because if those seeds do sprout during a warm spell, an outdoor freeze afterward kills them with nothing to protect them. Save those for indoor seed starting.

Find your zone and last frost date

Everything below is timed backward from your last frost date, so start there.

When to winter sow, by zone

Hardy seeds can go into jugs any time from the winter solstice through roughly 8 weeks before your last frost date, which gives cold-loving species the weeks of chilling they need. Tender-leaning but still frost-tolerant seeds, the half-hardy vegetables and some annual flowers, are better started closer to your last frost, around 4 weeks before it, so they are not sitting through the coldest stretch of winter as tiny seedlings.

Zones 9b through 13b are edge cases worth calling out directly. In zone 9b, the 8-weeks-before-last-frost date lands right at the winter solstice, so the hardy window is short but still real. In zone 10a, the last frost is already so early in the year that the 8-week hardy window would start before the solstice, which means there is little practical advantage over just direct sowing hardy crops in the garden in January. In zone 10b and warmer, the last frost date is in mid January or does not exist at all, and there simply is not a meaningful winter to sow into: the cool season itself, running roughly November through February, does the job winter sowing does further north. If you garden in zones 3 to 7, this whole method sits squarely inside the January to March trough when there is otherwise little to do in the garden, which is a large part of its appeal.

The winter sowing method, step by step

1. Prep the container. A translucent gallon milk jug is the standard: it is free, holds enough soil for a healthy root system before transplanting, and its handle makes it easy to carry and move. Cut it horizontally about 4 inches up from the bottom, leaving a hinge of plastic near the handle so the top folds back like a lid rather than coming off entirely. Remove the cap.
2. Drainage holes. Punch or drill several holes in the bottom, this is the single most skipped step and the most common reason winter sowing fails. Without drainage, the container fills with meltwater and rain and the seed rots before it ever germinates.
3. Fill with 3 to 4 inches of moistened seed starting or potting mix. Not garden soil, which compacts and crusts. Water it until it is evenly moist but not soggy before you sow.
4. Sow the seed at the depth the packet calls for, mist or gently water it in, and close the jug’s lid back up.
5. Tape the seam where you cut the jug, duct tape or packing tape both work, so wind and rain do not pop it back open over the following months.
6. Label with a paint pen or grease pencil, written directly on the jug rather than on a paper tag, since paper labels degrade in a few weeks of winter weather and the ink from a regular marker fades in sun by spring. Include the crop and the sowing date.
7. Leave the cap off. This is the vent that keeps the greenhouse effect from cooking the seedlings on the first warm, sunny day, and it lets rain and snow water the container directly.
8. Set the jugs outside in full sun, somewhere you will walk past regularly, and leave them. Snow sitting on top of the jugs is fine and does not need to be brushed off. It insulates as much as it chills.

From there the containers largely take care of themselves through the rest of winter. Check them occasionally for drainage problems or wind damage, but resist the urge to fuss.

What to sow and what not to

Good candidates. Hardy perennials and native wildflowers that need cold stratification to break dormancy, which is most of them. Hardy annual flowers like poppies, larkspur, bachelor’s buttons and calendula, which self-seed outdoors anyway and simply do better with a head start in a container. Cool-season, frost-tolerant vegetables: peas, spinach, lettuce, kale, broccoli, carrots, beets and onions from seed. All of these either need the cold or tolerate it as young seedlings, which is exactly the environment a winter-sown jug provides.

Poor candidates. Tender, heat-loving vegetables and flowers with no frost tolerance at all: tomatoes, peppers, eggplant, cucumbers, summer and winter squash, melons, and basil. If a warm stretch triggers germination in February or March, the seedlings have no defense against the freeze that follows, and you lose the whole jug. These crops are better started indoors under lights on a schedule tied to your last frost date, covered in the guide to starting seeds indoors by zone, and transplanted out once it is genuinely warm.

Common mistakes

No drainage holes. Covered above, and still the most frequent reason a jug of seed never comes up.

Opening the jug during a warm spell and forgetting to reseal it. A few 60 degree days in late winter tempt people to peek or vent, and then a hard freeze follows a few days later while the lid is still cracked open, killing whatever had started to grow.

Forgetting to vent once it warms up. Once daytime temperatures are regularly climbing past 50 degrees inside the jug, usually a few weeks before your last frost, propping the lid open or cutting slits in the top keeps seedlings from cooking on a sunny afternoon. A sealed jug in direct sun in April can get surprisingly hot surprisingly fast.

Sowing tender crops too early, discussed above: the jug does not know a crop is tender, it only responds to temperature, so the mistake is choosing the wrong seed for the method rather than a timing error.

Unlabeled jugs. By the time seedlings are a few inches tall, most seedlings of unrelated species look similar enough that an unlabeled jug becomes a guessing game. A paint marker directly on the plastic survives the winter; masking tape and ballpoint pen usually do not.

Can you winter sow directly into the ground

Yes, for a specific short list: peas, spinach, and hardy self-seeding flowers like poppies and larkspur can all be sown directly into garden soil in fall and left to come up on their own the following spring, which is the same underlying idea as winter sowing without a container.

The jug earns its keep for everything else for a few practical reasons. It protects seed from birds and mice, which will happily clean out a bed of exposed seed over a long winter. It stops heavy winter rain from washing small seed downhill or burying it under runoff sediment. And it lets you control exactly where the seedlings end up rather than wherever the seed happened to land, which matters if you are winter sowing dozens of varieties and want to keep them organized until you are ready to place them in the garden.

Ziplock bags and other containers

The gallon jug is standard because it is free, sized right, and stands up on its own outdoors, but it is not the only option. Two-liter soda bottles work the same way at a smaller scale. Clear plastic storage totes with the lid propped for ventilation handle a larger volume of seedlings at once. Even a large ziplock bag, propped upright with a couple of inches of moist soil in the bottom and a few holes poked for drainage and air, has been used successfully by winter sowers who are working with limited container supply. The requirements are the same regardless of container: it needs to let light through, hold a few inches of soil, drain excess water, and be easy to vent once the weather warms.

Transplanting winter-sown seedlings

When seedlings inside a jug have several sets of true leaves and the weather has settled into your normal transplant window, cut or tear the whole block of soil and roots out of the container as one piece, sometimes called the hunk-o-seedlings method, and gently tease individual plants apart at the roots rather than pulling them by the stems. Set them into the garden the same day.

There is no hardening off period needed first, which is the real time saver of this whole method. A seedling that spent its entire life outdoors in a vented jug has already been exposed to real wind, real temperature swings and direct sun, so it skips straight past the one to two week hardening off process that seedlings started indoors under lights require. The hardening off schedule guide covers that process in detail for anything you did start indoors instead.

Where to go next

• Last frost date by zone: what you can plant before it
• When to start seeds indoors by zone
• What to plant in October by zone
• USDA planting zones explained