A Complete Beginner’s Guide to Winter Sowing

Winter sowing is a resourceful and easy way to get a head start on spring planting. Seeds hold over the winter in homemade mini-greenhouses until conditions are just right for their germination. In an insulated environment with plenty of exposure to natural elements, seedlings develop sturdy roots poised for vigorous growth, early flowering and fruiting, and no hardening-off period.

January and February are prime months to winter sow many selections, especially those that are hardy or need cold stratification to germinate. Cold stratification is a period of several weeks at temperatures around 40°F (4°C), depending on the species. These are often native perennials that naturally self-sow in the landscape, but you can winter sow just about anything! Flowering ornamentals and edibles get a jumpstart in the process.

In winter, seeding involves direct sowing, even in snow, for plants that benefit from exposure to cool weather. Here, we’ll review winter sowing as a technique that relies on simple household materials to create an insulated environment for seeds, where seedlings sprout as temperatures warm.

What is Winter Sowing?

Winter sowing exposes seeds to natural conditions while warming the immediate surroundings with an insulative container. When placed outdoors, they receive all they need to germinate (light, water, soil nutrition, temperature variation). 

A mini-greenhouse situation promotes germination with exposure to cold temperatures and garden conditions, with protection. Seedlings emerge when temperatures warm and meet their germination requirements. With exposure, seedlings develop sturdy roots for easy transplanting with no hardening-off period necessary.

The streamlined seed-starting technique uses basic household supplies, making it resourceful, budget-friendly, and accessible to get growing early. It saves indoor growing space and time spent managing seedlings in a controlled environment. Mid to late winter is the time to winter sow, ideal for cold climates.

The Method

To winter sow, the “greenhouses” are repurposed plastic containers. Transparent containers like milk jugs, plastic bottles, bins, or baggies become growing vessels with a few easy modifications (more below).

The little greenhouses sit outside in a sunny spot, exposed to the elements, including snow and rain. The seeds benefit, and seedlings toughen up in the natural conditions as they sprout later in the season. Maintenance includes opening the lid for ventilation on warm days and maintaining soil moisture as needed.

Advantages

The resourceful, nearly effortless way of promoting strong seedlings works with natural conditions and relies on simple household materials. Seed starting is accessible without a lot of supplies or oversight.

When to Winter Sow

As early as December and January, you can winter sow. Climate and temperature affect germination, and starting in freezing conditions ensures seedlings won’t sprout too early and risk freezing again later. Cold weather ensures the stratification period for those that need it while staving off germination until conditions are consistently temperate. Even seeds that don’t require cold stratifying benefit from exposure in “cold storage” until sprouting.

Aim to sow seeds about eight weeks before you would typically transplant them to the garden bed or outdoor container. January is the time to winter sow seeds that benefit from cold stratification (and into February and March in cold climates). In warm zones, the seeds germinate faster.

If your area experiences drastic fluctuations between warm and cold days, wait a month or so to sow tender annuals like tomatoes, peppers, and zinnias. These are frost-sensitive and develop later in the season under warm air and soil temperatures. 

Seeds to Winter Sow

Winter sowing is versatile for beginner home garden seeding, including herbs, vegetables, flowering annuals, and perennials. Hardy perennials are reliable, as are frost-tolerant annuals and vegetables that thrive in the cool season. Petunias, snapdragons, calendula, hollyhock, spinach, broccoli, cabbage, and kale (Brassica species) enjoy cool spring conditions. They handle frost and move easily from the container to the bed or pot. Perennials, annuals, and herbs that self-sow or reseed are good contenders.

Trialing different varieties is part of the fun. Those with sensitive roots at transplanting (root vegetables, cucumbers, larkspur, lupines, poppies) are best direct-sown to avoid disturbance. It’s possible to winter sow some with root sensitivities, though, just like starting them indoors and with care during transplanting.

Germination Times

As the weather is a factor in suitability for germination, so is the average germination length for a given species. Experiment by starting all your warm-season selections at the same time, or hold off until February or March for heat-loving plants (zinnias, tomatoes, peppers, eggplants, squash, ground cherries) that develop later in the season. Some perennials, like milkweed and columbine, have long germination times and may emerge later than others, depending on your climate. 

Those that don’t sprout in time with the others may still be viable, if a little delayed. Leave them in place for two or three more weeks, staying on the lookout for seedlings, moisture control, and ventilation. Make a note of what was successful and what showed lesser germination rates for rounds in future seasons.

Essential Supplies

Winter sowing doesn’t require specialized seed-starting supplies or equipment, making it extremely accessible for beginners. The basics are a clear container, soil, seeds, and water. You can repurpose almost any clear plastic container to become a growing vessel. Get creative on a setup that works for you. The keys are to allow room for about four inches of soil and seedling growth in the container, ample drainage, and stability in the outdoor location.

The “Greenhouses”

Start with clear or very transparent containers like milk jugs, plastic bottles, bins, or baggies. Anything that allows light to penetrate with the capacity for drainage, ventilation, several inches of soil, and room for seedling growth works well. Clean the containers in warm, soapy water to sanitize them.

While stored outside, the vessels collect seasonal moisture from snow and rainfall. Drainage holes are essential in preventing rot and other fungal problems. Make four to six holes in the base. Use a permanent marker to label and date the seeds by container.

The containers need simple modifications to account for temperature variation and sprouting. We’ll cover a few basic containers (milk jugs, bottles, and freezer bags) that allow for opening and closing for airflow and temperature regulation.

Milk Jugs

For milk jugs, remove the cap and poke the drainage holes in the bottom. Cut the jug nearly in half just below the handle, almost all the way around (about three-quarters). Leave the handle portion attached as a means to lift the lid during the season. The open cap allows airflow and moisture in the container, while the cut makes a hinge at the handle to open it further on warm days.

Add three to four inches of potting mix to the base. Sow seeds according to the depth and spacing of the packet guidelines, covering them lightly or gently pressing them for soil contact as indicated. Water them in, and tape the jug’s cut seam.

Plastic Bottles

Two-liter soda or water bottles make protective little greenhouses, too. Prep them as you would a milk jug by removing the caps and making a cut three-quarters of the way around. Leave four inches of space at the bottom for soil and seedling growth. 

With drainage holes in place, add three to four inches of soil. Sow the seeds according to packet directions and moisten the media. Tape the cut seam and place the bottle outside with the cap off.

Freezer Bags

Gallon-sized resealable freezer bags become seedling pouches by clipping the bottom corners and cutting slits or holes for drainage. Add several inches of soil and sow accordingly. Water the seeds in for evenly moist potting media.

Leave an inch or so of the seal open at the top of the bag for ventilation. To adjust airflow and regulate temperature on warm days, open the seam more. In spring, roll the seal down like a cuff for full exposure, closing it if necessary on chilly nights.

To stabilize the bags, string them along a skewer by piercing the tops. The bottom should rest on a tray, crate, or bin.

Potting Mix

A high-quality potting mix is best. No special seed mixes or formulated growing mediums are necessary. Avoid regular garden soil or straight compost, both of which are too dense. The mix should be light and fluffy with good aeration, drainage, and moisture retention. Don’t use any with moisture control or added fertilizers – a regular potting mix works well. 

To mimic the seed’s garden environment, providing good drainage and even moisture are the keys to success.

Tending

Often, and especially to start, natural moisture from snow, rain, and condensation is enough to maintain soil moisture. Check containers once a week or so, especially as conditions become drier or warmer as the season progresses.

With regular rain or snow, the vessels likely need only supplemental water or misting every few weeks. Watering needs may increase as temperatures warm and seedlings sprout. If the soil surface shows signs of drying, mist or sprinkle with water to prevent seeds or shallow roots from drying out.

Look for condensation as a sign that moisture is available. If it lessens, make sure soil moisture is adequate and that rain can enter the top. If not, add more openings. Keep tape in place at cut seams to prevent evaporation early in the process.

Vent the lid on warm, sunny days to allow natural transpiration and airflow to stave off diseases. Close it again before temperatures drop in the evening. As seedlings develop, this allows natural hardening-off that makes for easy transplanting without shock.

Outdoor Placement

Place the containers in the sun, where they’ll also receive moisture. Expose them to the elements, even snow. The seeds benefit, and seedlings harden off in the natural conditions as they sprout later in the season.

A southeast location is ideal for sunlight and warmth. If this becomes too sunny and warm toward spring or in warmer climates, move them to an east-facing spot with good morning exposure.

In windy areas, secure the vessels to keep them from blowing over. Nestle them against a foundation or wall, or place them in a milk crate, tray, or tub. Breezes help develop strong stems, but toppling over can disrupt the situation.

Transplanting

Not only is winter sowing resourceful and easy for beginners, it’s also highly productive, with good germination rates. Seedlings that develop in these situations have strong roots and upper growth. They’re ready to transplant after exposure to warm days in their open containers. Look for true leaves (fully developed) and a few inches of height as indicators of readiness.

Follow regular transplant guidelines on what to look for and for the best air and soil temperatures. Quick growth follows as winter-sown seedlings have a headstart on indoor and direct-sown seeds.

To transplant, gently divide the seedlings. Reach into the open container or cut side flaps to slide them out. With the right spacing, you can plant the entire block without dividing. Enjoy the next growing phase on the way to flowering and fruiting!