You planted the milkweed seed. You watered it. Nothing came up. Same with the serviceberry, the coneflower and the acorns your kids collected.
The seed was probably fine. It was waiting. A large share of North American native plants and almost every temperate tree carries a built-in timer that will not release until the seed has sat cold and damp for weeks. Stratification is how you run that timer on your own schedule instead of the weather's.
What you will find below:
- Why a fridge works and a freezer does not
- The four dormancy types, and how to tell which one you are holding
- Chilling times for named trees, shrubs and prairie natives, with sources
- Three methods that work, including the one that needs no fridge space
- The five ways stratification fails and how to catch each one
What Is Seed Stratification?
Stratification is a period of cold, moist storage that mimics winter and releases a seed from dormancy. It is not a soaking, it is not a temperature shock, and it is not optional for the species that require it.
Inside the seed, the chilling period drives down abscisic acid, the hormone holding germination shut, while gibberellin rises. When the seed finally warms up in spring, the balance has flipped and the embryo can move. University of Illinois Extension puts the working range at 33 to 40 F (0.5 to 4 C), which is ordinary refrigerator territory, and the usual duration at one to three months.
Two details do most of the damage when people get them wrong. Moisture is not optional: a dry seed in a cold fridge is in storage, not stratification, and the clock is not running. And the freezer does not substitute for the fridge. Freezing is a storage tool for dry seed. A fully hydrated seed held below about 23 F (-5 C) for any length of time is usually killed outright.
Which Kind of Dormancy Are You Dealing With?
Cold stratification only fixes one of four problems. Treating the wrong one is why a careful gardener can do everything by the book and still get bare soil.
Physiological dormancy is the common one: the seed can take up water fine, but internal chemistry blocks germination. Cold-moist chilling is the answer. Milkweed, coneflower, black-eyed Susan, redbud and flowering dogwood all sit here.
Physical dormancy means a hard, waterproof seed coat. Chilling does nothing because water never gets in. These need scarification: a nick with a knife, a rub on sandpaper, or a hot water soak. Lupines and many woody legumes are the classic cases.
Morphological and morphophysiological dormancy means the embryo is not finished growing when the seed drops. It needs a warm, moist period first so the embryo can develop, then a cold one. The NC State Extension Gardener Handbook puts warm stratification at 68 to 86 F (20 to 30 C) depending on species. Yews, some ashes, several viburnums and pawpaw belong here.
Combinational dormancy is a hard coat plus internal blocks, and the order matters. NC State is explicit: scarify first, then stratify. Do it the other way round and the seeds will not germinate.
Why This Works: Reading the Signal, Not the Calendar
Dormancy is a plant reading its environment rather than counting days. A seed that fell in September must not sprout in October, because a frost would end it. The chilling requirement is a length-of-winter detector, and the species with the longest requirement are usually the ones from the coldest places. Once you see it that way, you stop asking what a seed needs and start asking what winter it evolved to expect. That habit of designing with natural cycles rather than against them runs through permaculture design generally.
How Long Does Each Species Need?
There is no universal number, and any chart that gives you one is guessing. Here is what named sources actually specify.
| Species | Treatment | Source |
| White oak group | None. Acorns often sprout the same autumn they fall | Mississippi State Extension |
| Red oak group | 4 to 8 weeks cold-moist at 34 to 40 F (1 to 4 C) | Mississippi State Extension |
| Bur oak, northern range | 60 days cold-moist | USDA Woody Plant Seed Manual |
| Black oak group | 4 to 12 weeks at 36 to 41 F (2 to 5 C) | USDA Woody Plant Seed Manual |
| Serviceberry | Autumn sowing, or 60 to 90 days at 41 F (5 C) | University of Alaska Fairbanks |
| Milkweed species | 30 days cold-moist as a starting point | Oklahoma State Extension |
| Joe Pye weed, boneset | 30 days cold-moist | Cornell Cooperative Extension |
| Maples, most species | 45 to 180 days, some need warm then cold | Published ranges via Chen et al. 2015 |
Sources: Mississippi State University Extension, Growing Your Own Oak Seedlings, USDA Woody Plant Seed Manual, Quercus chapter, University of Alaska Fairbanks Bulletin 26, Oklahoma State University Extension, Cornell Cooperative Extension Master Gardeners
Cornell's write-up is worth knowing because it explains the shorthand on native seed packets. A germination code of C(30) means 30 days of cold-moist stratification. C(60), C(90) and C(120) mean exactly what they look like. If a packet gives you a code, it has already answered the question.
Apple and pear are their own conversation. The seeds do chill through a winter or a couple of months in a fridge, but the seedling will not be the variety the fruit came from. Apples do not come true from seed, which is why orchards graft. Grow them for rootstock, for a genetics experiment, or for the pleasure of it, but not to reproduce a Honeycrisp.
Three Methods That Actually Work
Pick by how much fridge space you have and how many seeds you are handling. All three do the same job.
The fridge and bag method
Mix seed into damp sand, vermiculite or milled peat at roughly four parts medium to one part seed, or fold small seed into a damp paper towel. Seal in a bag, label it with the species and the date the chilling started, and put it in the main body of the fridge at 33 to 40 F (0.5 to 4 C). Target damp-sponge moisture: it should feel wet but release no water when squeezed. Check every couple of weeks for mould and for early sprouting.
Winter sowing in jugs
Cut a translucent gallon jug most of the way around, leaving a hinge. Fill the base with 3 to 4 in (8 to 10 cm) of moist potting mix, sow, tape it shut, remove the cap for ventilation, and set it outside where it gets rain and snow. Nature runs the chilling, the jug stops the seed washing away, and you get hardened-off seedlings in spring. It costs nothing and uses no fridge space.
Direct autumn sowing
Sow into a prepared outdoor bed or nursery row in autumn and let winter do the work. This is what happens in the wild, and Illinois Extension recommends it as the simplest option for anything needing cold-moist treatment. The trade-off is losses to rodents, washout and frost heave, so sow generously and mark the bed clearly.
Key Takeaway
Count backwards from your last frost date. A species needing 90 days of chilling plus 2 weeks to germinate has to go into the fridge by roughly Christmas for an April sowing. Missing that window by three weeks costs you a whole season.
Why Stratification Fails
Five failure modes cover almost everything.
Mould. Too much water, or a medium that was not clean. Wring the medium out harder, open the bag to air it when you check, and discard visibly rotten seed. A light fuzz is often surface fungus on the medium rather than the seed itself.
Drying out. The quiet killer. Paper towel dries fastest, sand slowest. If the towel is crisp when you check, the chilling clock stopped whenever it dried, not when you noticed.
Sprouting in the fridge. This is success arriving early, not failure. Pot the sprouted ones up immediately and keep them under light. It is why you check every two weeks rather than opening the bag on day 90.
Not enough chilling. Half the required period does not give you half the germination. Many species behave as though nothing happened at all until the requirement is met.
Double dormancy. Some species, including a number of viburnums and lilies, need a warm period then a cold one, so seed sown outdoors in autumn may show nothing the first spring and come up the second. Label those beds and leave them alone for two years before writing them off.
Test Before You Blame the Method
Old or badly stored seed will fail no matter how perfect your chilling. Before you repeat a failed batch, run a simple germination test: 10 seeds on a damp paper towel in a bag, kept warm after their cold treatment. If two of ten sprout, the seed is the problem, not the technique. Most native perennial seed is best used within two to three years, stored cool, dry and dark.
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Send Me the ChartFrequently Asked Questions
What is stratification of seeds?
Stratification is a period of cold, moist storage that breaks seed dormancy by mimicking winter. During the chilling period the hormone holding germination shut declines and the growth-promoting hormone rises, so when the seed warms up it can germinate. It applies mainly to temperate woody plants and herbaceous perennials, and it requires both cold and moisture at the same time.
How long do you cold stratify seeds?
Most garden perennials need 30 to 60 days, and Illinois Extension gives one to three months as the general range. Named requirements vary widely: milkweed and Joe Pye weed manage on 30 days, serviceberry wants 60 to 90 at 41 F (5 C), red oak acorns need 4 to 8 weeks, and some maples run to 180 days. Check the species before you start, because guessing short is the same as not doing it.
How do you cold stratify seeds in the fridge?
Mix the seed into damp sand or vermiculite, or fold it into a damp paper towel, seal it in a labelled bag, and keep it in the fridge at 33 to 40 F (0.5 to 4 C) for the required period. Aim for damp-sponge moisture, no free water. Check every two weeks for mould and for seeds that have already sprouted, and pot up anything that has.
Should I stratify seeds in the fridge or the freezer?
The fridge. Stratification is a cold-moist process, and freezing a fully hydrated seed usually kills it. Freezers are for dry storage of seed that is not being chilled. If your fridge runs very cold, keep the bag in the door rather than the back.
What happens if you do not stratify seeds that need it?
Usually nothing at all, which is the confusing part. The seed sits in the soil in perfect condition and does not germinate, so it looks like poor seed or bad technique. Some species will produce a scatter of seedlings over a season or two, but germination is low and uneven compared with a properly chilled batch.
Do vegetable seeds need cold stratification?
Almost none of the common ones do. Tomatoes, beans, squash, lettuce and brassicas were selected for fast, uniform germination and go straight into warm soil. Stratification is a native plant, prairie perennial and woody plant concern. A handful of perennial herbs and alliums are exceptions worth checking individually.
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Read the Free GuideGrowing your own trees and shrubs from seed is the cheapest way to fill a planting. Thirty acorns and a bag of sand will stock a boundary that would cost hundreds in nursery stock, whether you are starting a food forest or filling out a hedgerow. It just runs on winter's schedule, so the work starts in autumn.
Resources
- University of Illinois Extension: Seed stratification, what seeds require cold treatment
- University of Illinois Extension: How to get seeds from native perennials to grow
- NC State Extension Gardener Handbook, Chapter 13: Propagation
- Mississippi State University Extension: Growing Your Own Oak Seedlings
- USDA Forest Service Woody Plant Seed Manual: Quercus chapter
- Oklahoma State University Extension: Native Milkweed Germination Guide
- Cornell Cooperative Extension: Some native seeds can be planted in the spring
- University of Alaska Fairbanks Bulletin 26: Propagation of Woody Plants by Seed
- Center for Plant Conservation: Seed dormancy explained
