Garden · Seed starting
Seed cold stratification calculator
Many native and perennial seeds won't germinate at all straight out of the packet. They need a period of cold, moist storage first, a process called stratification, which mimics the winter the seed would experience outdoors before naturally sprouting in spring. Skip it or cut it short and you can end up with a tray of seeds that simply never come up, not because anything went wrong, but because the seed's dormancy was never broken. This calculator turns a species' rough stratification window into an actual start and end date you can plan around.
Stratification length
30–70 days
Check seeds periodically for mold and keep the medium moist but not wet the whole time. If a few seeds have already sprouted before the earliest date, sow those right away.
What to actually buy
HEALCO Digital Fridge Thermometer with Max/Min Memory
This calculator assumes you're actually holding 34-41°F the whole time, but home fridges run colder or warmer than their dial suggests, especially near the door. A dedicated min/max thermometer confirms your real range instead of trusting the fridge's own setting.
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How this is calculated
There's no single formula for stratification length, it comes down to what the seed evolved to survive. Extension services and native seed growers group species into rough bands instead of one universal number, and this calculator uses those bands: 10-15 days for most warm-season grasses and legumes, 21-42 days for milkweed and many prairie wildflowers, 30-70 days for perennial wildflowers and coneflowers, 60-90 days for shrubs and woody perennials, and 90-120 days for fruit trees and other double-dormant seeds.
The calculator adds that range to your start date so you get an actual calendar window instead of just a day count. Keep seeds at 34-41°F in a moist (not soaked) medium like a damp paper towel or sand for the whole period.
If you know the exact requirement for your specific species, trust that over this general band. This tool is meant for seeds where you only know the general type, not an exact published requirement.
Sources
Frequently asked questions
Can I stratify seeds in the refrigerator instead of outdoors?
Yes, and it's the more controlled option. Seal the seeds in a damp (not wet) paper towel or moistened sand inside a plastic bag and store it in the fridge, ideally 34-41°F, checking weekly that the medium hasn't dried out or gone moldy. Outdoor winter sowing works too and is lower-effort, but the fridge gives you a more precise, repeatable temperature.
What happens if I stratify a seed too long?
Most seeds tolerate stratification somewhat past the minimum without harm, since in nature winters vary in length too. The bigger risk with an overly long stratification is mold or rot in the storage medium if it's kept too wet, not the cold itself, so check on the seeds periodically rather than sealing the bag and forgetting it.
My seed packet gives a specific number of days, should I use that instead?
Yes, always defer to a specific, published requirement for your exact species over this calculator's general bands. This tool exists for the common case where you know the general seed type (a prairie wildflower, a shrub, a fruit tree pit) but don't have a species-specific number to work from.
Do all seeds need stratification?
No. Many vegetable and annual flower seeds germinate fine straight from the packet with no cold treatment at all. Stratification is mainly relevant for seeds from plants native to climates with a real winter, where the cold requirement evolved as a signal that it's safe to sprout.