How to stratify seeds indoors
- Jeanne McRight
- 1 day ago
- 2 min read
by Jeanne McRight
Stratify your native seeds indoors if you are trying to get a head start with a spring or early summer (before June 15th) plant installation. Indoor stratification is like giving seeds a simulated winter in your fridge—a cold, moist period (usually 2 to 3 months) that breaks their natural dormancy, mimicking how they'd overwinter in soil, so they wake up ready to sprout in spring, ensuring better survival and germination for many native plants.
Two basic methods
You sprinkle the seeds in a single layer on a moist coffee filter or paper towel, cover with another moist coffee filter, fold into quarters, seal in a ziplock baggie, and store in a tightly-lidded plastic container in the refrigerator for 60 days (or according to specifications for your particular species)
You mix seeds with a damp medium (like peat or sand) in a sealed bag, label it, and refrigerate for the species-specific time before planting.
Key concepts
Mimics Nature: Recreates winter conditions (cold + moisture) for seeds that fall in autumn.
Breaks Dormancy: A survival mechanism; prevents premature sprouting in fall warmth.
Triggers Germination: Tells the seed it's spring, increasing sprouting success.
Cold & Moist: Essential; dry or warm conditions keep them dormant.
How to do it, step-by-step
Gather: Seeds, ziplock plastic bags, tightly-lidded container to hold ziplock bags,, coffee filters OR peat moss/vermiculite/sand, spray bottle filled with water, permanent marker.
Mix: Dampen coffee filters OR your medium (moist, not soaking) and sprinkle on/mix in seeds.
Bag & Label: Put mix in bag, seal, label with seed type & dates.
Refrigerate: Place in the fridge (not freezer) for recommended days (e.g., 30-90 days for many).
Check: Ensure it stays moist; plant immediately if sprouts appear.
Germination Codes
The North American Native Plant Society (NANPS) uses germination codes (like
A, B, C, etc.) to tell growers what seed treatments are needed, such as cold stratification (Code B), light (Code C/H), or scarification (Code I), but these codes aren't universal and vary by supplier, so you must check the specific guide, with common treatments simulating winter (cold/moist) or requiring light for temperate natives. Key codes often mean: A (no treatment), B (cold stratification), C (light needed/multicycle), D (warm-cold-warm cycles), H (light to germinate), and I (scarification for hard coats).
Common NANPS & Related Codes Explained
Code A: No special treatment needed; sow outdoors fall/spring or indoors at warm temps (around 70°F).
Code B: Needs cold, moist stratification (winter simulation) for 30-90+ days (e.g., B(30)).
Code C: Light needed to germinate, or multiple warm/cold cycles (e.g., 40-70-40-70).
Code D: Warm, then cold, then warm stratification (e.g., 70-40-70).
Code G: For fleshy fruits (pulp/skin inhibitors).
Code H: Requires light to germinate (surface sow).
Code I: Needs scarification (scratching/sanding) due to a hard seed coat.
Code K: Hemiparasitic species needing a host plant.
Check the Source: Codes aren't universal; always refer to the specific guide from NANPS, Prairie Moon Nursery, or other native plant nurseries.
Resources:
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