Winter Seeding – A Shortcut to Spring?

Some gardeners have found that winter sowing cool-season annuals and vegetables in protected plastic jugs or other transparent, covered containers can give them a slight jump on producing seedlings for spring. The term winter sowing was coined by Trudi Davidoff (WinterSown.Org) (1) and is defined by USDA as “A propagation method used throughout the winter where temperate climate seeds are sown into protective vented containers and placed outdoors to foster a naturally timed, high percentage germination of climate tolerant seedlings” (2). Winter seeding, on the other hand, refers to techniques including frost and snow seeding that are typically done on a larger scale including for wildflower meadows and pastures.

The winter-sown containers are moistened then placed outside in a cold but protected location (for access and maintenance). If you do this in February-March (or earlier), seeds of cold tolerant crops such as beets, chard and cabbage will be ready to germinate as soon as soil temperatures reach minimum suitable levels (35-40º F). Like a mini greenhouse, the transparent covers capture solar energy that warms the soil or medium in the containers earlier than exposed soil in the spring (1, 3). Be sure to provide slits or holes for ventilation to avoid over-heating on sunny days, and holes for drainage. Once you have vigorous seedlings, transplant them as soon as possible.

Mid-winter sowing or seeding also allows for natural stratification to occur, exposing seeds to cold moist conditions needed by many perennials and some annuals to break seed dormancy. For many herbaceous and woody plant species, 60 to 90 days of cold temperatures (32-50º F) are required, but some plants have more specific requirements (4, 5). There may be other seed dormancy mechanisms in play as well, such as hard seed coats on lupines and baptisia. Seed companies are often a good source of specific information.

Fall planting is another effective way to expose seeds to natural cold moist stratification. Herbaceous perennial seeds that require stratification include milkweed, gentian, geranium, bee balm and purple coneflower, as well as many others commonly found in wildflower seed mixes. By planting a wildflower patch in mid- to late fall, after carefully preparing the site throughout the previous growing season (6), the dormant seeds experience natural cold stratification that results in enhanced germination the following spring. If you missed the window for fall seeding, sowing in January or February still allows adequate time to meet the cold requirements of many seeds. Simply scatter the seed by hand over exposed ground or atop a minimal layer of soft snow so that it can work downwards into the soil. Some of the risks include seed loss to hungry birds and other wildlife, or seed movement in runoff water especially if on a sloped site.

You can moist-stratify seeds in your refrigerator instead of winter sowing or seeding. Mix seeds with a small amount of moist (not wet) sand, vermiculite or sphagnum peat in a plastic bag or covered container, label it and leave it to chill until spring. Once the seed has been stratified it should be planted before it germinates.

Similar terms -- frost seeding, snow seeding, and dormant seeding -- all refer to broadcasting seed over top of frozen or snow-covered ground during the winter. These methods are strategies used for enhancing pastures, forage or wildlife food plots (7,8) and even over-seeding athletic turf for early spring germination (9). Most of the grass and forage species do not require stratification per se but are tolerant of cold temperatures and germinate early in the spring.

After frost seeding, the repeated freeze-thaw cycle works the seed down into soil cracks, providing good soil-seed contact -- very important when not burying the seed. If it snows after scattering the seed, all the better, because melting snow helps work the seed downward as well. Nestled in the soil matrix it lies dormant until spring, ready to germinate when temperatures reach suitable temperatures. You may get a two-to-three-week head start on your cool-season crop and you don’t have to worry about working the soil in early spring when it is probably quite wet.

Winter seeding or sowing is not recommended for home lawn grasses or warm-season annuals and vegetables. These plants require warm soil temperatures before they will germinate, typically not until late May or even June in New Hampshire. Cooler temperatures and moisture may encourage damping off and other diseases and/or result in poor germination and vigor.

If gardeners want a shortcut to spring, winter sowing or seeding may do the trick, but be sure to choose appropriate species and pay attention to timing and other details.

References

1. WinterSown Educational website, accessed 2.11.2021. http://www.wintersown.org/index.html
2. United States Dept. of Agriculture, National Agricultural Library Thesaurus and Glossary, accessed 11.21. https://agclass.nal.usda.gov/whats-new
3. Penn State Extension, 2021. Successful Winter Seed Sowing. https://extension.psu.edu/successful-winter-seed-sowing
4. Dirr, M. and C. Heuser. The Reference Manual of Woody Plant Propagation. 1987. Varsity Press, Athens GA.
5. Cullina, W. Wildflowers, A Guide to Growing and Propagating Native Flowers of North America. 2000. New England Wild Flower Society, Framingham MA. Houghton Mifflin, New York NY.
6. UNH Extension, 2019. Establishing a Wildflower Meadow from Seed. https://extension.unh.edu/resource/planting-pollinators-establishing-wildflower-meadow-seed-fact-sheet
7. UNH Extension, 2017 Improving Pastures and Hayfields. https://extension.unh.edu/resource/improving-pastures-and-hayfields-fact-sheet-0
8. Cornell Small Farms Program post, 2014. March is Frost Seeding Time. https://smallfarms.cornell.edu/2014/01/march-is-frost-seeding-time/
9. NYS IPM post, 2015. Snow, Frost a Big Help for Head Start on Quality Turf – or Crops. https://blogs.cornell.edu/nysipm?s=dormant+seeding

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