Cold Stratification

Starting seeds indoors can be a rewarding process for home gardeners. It provides a welcome activity as we anticipate the arrival of spring and can be more cost-effective than purchasing plants. However, not all seeds are ready to sow directly from the packet and may require some extra preparation and time before sowing. Understanding the germination needs for specific plants is very important. By knowing these requirements, gardeners can successfully start seeds indoors and get a head start on the growing season. This article to explains the process of cold stratification for the plants that need it.

Certain seeds, such as Milkweed, Purple Coneflower and Black-Eyed Susan, benefit from a period of cold stratification before planting. Cold stratification is a process where seeds are exposed to moisture and cold temperatures. This helps break their dormancy and promotes successful germination. 

In Minnesota, this process occurs naturally outdoors. Plants produce seeds in late summer, which are then exposed to snow and low temperatures during the winter. As the snow melts and the weather warms in spring, these seeds sprout. This adaptation ensures seeds and plants in areas with cold winters germinate in spring when conditions are optimal for growth instead of prematurely in the fall when new seedlings would be too fragile to survive the winter. Many Minnesota native plants require this cold stratification process to germinate. 

Home gardeners can mimic these cold, moist conditions indoors using a refrigerator. The length of time required for cold stratification varies by plant species, typically ranging from one to three months. For example, Purple Coneflower seeds require about 60 days of cold treatment. Always consult the seed packet for specific germination instructions, if stratification is needed and recommended time to start seed indoors. If seeds are collected and saved without packaging, be sure to research germination needs for the plant species. 

The beginning months of the year are an ideal time to begin cold stratification for seeds that require longer periods of cold. Starting early allows seeds to be grown indoors and planted outside in spring after danger of last frost.

Artificial Cold Stratification at Home

Refrigerator Cold/Moist Stratification

Materials needed: Refrigerator, Paper Towel, Resealable Plastic Bag, Permanent Marker, Thermometer (if needed) 

Moisten a paper towel so it is damp but not soggy. Wring the paper towel out until no excess water drips out. Using paper towel that is too wet can lead to mold. Next, place the seeds in a single layer on half of the paper towel and then fold over other half so that it covers the seeds. 

Use a permanent marker to label a bag with the plant name, the date started and date to remove from the refrigerator after the amount of time indicated on the seed packet. Then transfer the paper towel and seeds into the bag and seal. 

Place the bag in the refrigerator at a temperature between 33°F and 40°F. Set a reminder on your phone or calendar for the future date when the seeds should be removed. Check the seeds weekly to monitor the moisture, ensuring that the paper towel does not dry out or has become too wet. If the paper towel is too dry, mist with a spray bottle until damp. If the paper towel is too wet, it could be replaced with a new damp paper towel. If any seeds have sprouted, they can be planted right away into small pots with a seed-starting mix indoors. If the seeds have not sprouted yet, they can be sown into pots once the cold moist stratification period is complete. Visit the University of Minnesota Yard and Garden site for useful tips for starting seeds indoors: https://extension.umn.edu/planting-and-growing-guides/starting-seeds-indoors#when-to-plant-seeds-1178964

By following these steps and taking the time for artificial cold stratification, you’ll set up your seeds for successful germination.

Sources and for more in-depth reading:

1. Seed stratification: What seeds require cold treatment. (2022, January 27). Illinois Extension. https://extension.illinois.edu/blogs/good-growing/2022-01-27-seed-stratification-what-seeds-require-cold-treatment

2. Capon, B. (2005) Botany for Gardeners 

3. Unknown. (n.d.). Native seed propagation methods. https://www.missouribotanicalgarden.org/Portals/0/Shaw%20Nature%20Reserve/PDFs/horticulture/Propagation.pdf

4. USDA NRCS Elsberry Plant Materials Center, Britton, N., 1913, Brown, N., Henry, J., Bruckerhoff, S., & Kaiser, J. (2003). Plant fact sheet. https://plants.usda.gov/DocumentLibrary/factsheet/pdf/fs_ecpa.pdf

Photo Credits: University of Minnesota Extension (1,4), Jessie Thompson (2,3)