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All About Seed Stratification

Updated: Mar 15, 2022

Start native plants from seed with these helpful tips.

February 2022

Hopefully, you’ll brighten your garden this year with native plants. Once established, many of these hardy plants require almost no fertilization, minimal amounts of water, and little care. Birds and insects will love you.

So, rush out to buy a handful of native seed packets today. But wait a minute! Many of our native plant seeds will not sprout if planted this spring outdoors. Why not?

Cold Stratification

Many native seeds mature in late summer or in autumn. If the seeds sprouted soon after ripening, the young seedlings might not survive the harshness of winter. Instead, these seeds evolved so that they sprout only after enduring the cold and wet conditions of a Minnesota winter. These conditions are needed to break down chemicals that otherwise prevent the seed from sprouting. They will not grow without this. After time spent in the cold, the baby plant embryos have matured and the seeds are ready to sprout and take off.

You could have planted your native species seeds in late autumn. Mother Nature, working with Fathers Winter and Time, naturally breaks seed dormancy during our cold, wet Minnesota winters. The seed coat disintegrates, chemical changes occur, and the seeds begin to absorb water. Soon, as Spring arrives, the seeds sprout.

In mid-winter, some gardeners sow their wildflower seeds into potting soil inside modified milk jugs. This method is called Winter Sowing. The milk jugs need to have drainage holes punched in the bottom and the caps left off for air circulation. The jugs are left outside to undergo freeze-wet-cold-thaw cycles. Again, seeds will sprout in spring.

Imitating Mother Nature

If you missed those windows of opportunity to plant natives, you can still copy the natural process in your refrigerator. Mix seeds in a small container of moist planting medium and let winter occur in the refrigerator. This process is called stratification. The media could be a 50:50 mixture of sand and peat moss, washed sand (rinse sand thoroughly with water to remove any salts), vermiculite, coconut coir, perlite, or fluffy soil mix. Vermiculite works well because it is sterile and holds moisture.

Using a mixture of media and seeds is ideal for larger seeds.

Mix seeds with moistened media in a ratio of about 1-part seeds to 3-parts media; a total volume of roughly ½ cup of media and seeds should be enough per container. Containers may be small plastic or glass jars, small cottage cheese containers, or any small, resealable plastic container. The containers are only used to sprout the seeds, but not for the further growth of the seedlings. Punch a few holes in the lids to provide air circulation.

It is very important to add just enough water to make the medium moist but not wet. Drain off or squeeze out any excess water. Too much water will cause rot; too little water will cause sprouting seeds to die.

Some seeds with thick coats (like Baptisia) may need to be scarified before stratification. Use a nail clipper or small file to nick the seed coat, opposite the dimple on the seed coat where the baby plant will emerge. Rolling seeds between sandpaper enough to scar the coat will also allow water to enter the seed. Try not to crush the seeds in the process.

Make sure to write the name of the plant, date started, and expected end date on the container. Otherwise, you may get very confused and end up planting asparagus into your wildflower bed.

Leave the mixed seeds and media in the refrigerator for several weeks or months. Each species requires a different length of time before sprouting occurs.

Seedling Germination

Stratifying seeds in plastic bags works well with smaller-sized seeds. Fold a sheet of paper towel into quarters (or use a coffee filter). Moisten slowly and carefully. Wring out excess water. Partially unfold the paper towel.

Scatter seeds on a quarter portion of the towel. Fold the towel over the seeds and insert into a zip-lock bag. Close the bag, leaving a corner open for air circulation. Place the labeled bags or other containers into the coolest part of the refrigerator, but not the freezer. For adequate air circulation, do not crowd the bags or containers.

Check the bags or containers a few times each week to make sure that they have not dried out, that fungus is not present, and to note if any seeds have sprouted. Add a tiny bit of water if necessary. If you see fungus, try cutting back on the water; some gardeners apply a fungicide at the time of seed preparation. You can also remove the infected seeds, wash them thoroughly, and try placing them into fresh media or a clean paper towel. As soon as you see a radicle (tiny white tip of the seed root), carefully pick out all the seedlings taking care not to damage the tiny roots. Waiting too long, especially if using the paper towel method can result in a tangle of small roots in the paper. Transfer the seedlings carefully from the containers or plastic bags into a commercial seed starting mix and grow them out in a very well-lit place in your house. Or you may plant into expanded peat pellets. Do not use garden soil as it may contain organisms harmful to young plants. Garden soil also tends to compact severely in containers, squeezing out air and water that roots need.

Times for seed stratification can vary from a few weeks to a few months, or longer. Remember seeds have not yet learned to read recommended refrigeration times, so in some cases, germination may occur sooner or later than stated in references. The times for refrigeration may be listed on the packet label or can be discovered using a Google search. Prairie Moon Nursery ( has an excellent description of stratification and recommended refrigeration times.

Start now, and you should have beautiful plants to set out in late spring to early summer.

Happy Gardening.

Joe Baltrukonis

Ramsey County Master Gardener Volunteer &

Garden Sage

Additional Reading

Photo credits

  • Markus-Spiske on Unsplash

  • Deon Haider (Ramsey County Master Gardener Volunteer)

  • Deon Haider (Ramsey County Master Gardener Volunteer)


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