Some seed will not germinate until it has been through a cold, damp spell first. Here is the dormancy that causes it, why a winter breaks it, and how to give a seed that winter on purpose.
Photo: Matt Lavin from Bozeman, Montana, USA · CC BY-SA 2.0, via Wikimedia Commons.
A seed that refuses to come up is often doing its job
Sow a packet of native wildflower or a hard-coated perennial seed in warm, damp mix and you may get nothing. The seed is probably dormant, and the dormancy is a feature. A seed that germinated the moment it was warm and wet would sprout in the first mild spell of autumn and die in the first hard freeze. Dormancy holds it back until the cold has come and gone, so it wakes to spring.
Some seed will not germinate straight from the packet. It carries a built-in dormancy that a pre-sow treatment has to break first. A hard-coated seed is scarified or soaked so water can get in; a seed with internal dormancy is cold-moist stratified (chilled damp for weeks) before it will come up. The treatment has to be started far enough ahead of the sowing date to finish in time.
If you skip this, it still grows, just less well. The planner suggests it.
How it works: A hard, water-impermeable seed coat holds the seed dormant by blocking imbibition, so nicking, abrading, or soaking the coat is what lets water reach the embryo; a seed with physiological dormancy stays shut until a period of cold, moist chilling lets its germination-inhibiting hormones dissipate. So each treatment takes its own span of time, and starting it late leaves the seed still dormant at sowing.
Evidence: checked against 2 sources in the research-based literature.
A tough seed coat prevents water uptake and seed germination
Remedy
Check the plant's page for the treatment it needs. Scarifying (nick, abrade, or a hot-water/acid treatment) and soaking are done right before sowing; cold-moist stratification takes weeks, so start it well ahead. About six weeks before the intended sowing date is a safe general lead, though the exact period is species-specific and is on each plant's page. Fall-sowing outdoors lets winter do the chilling for hardy natives.
Dormancy comes in two common forms, and they need different handling.
A hard, water-tight seed coat locks the seed by keeping water out entirely, so it can’t even begin. The key is physical: nick the coat with a knife, rub it on sandpaper, or soak it, so water can finally reach the embryo. Do this just before you sow.
Internal (physiological) dormancy is chemical. The seed holds germination-inhibiting hormones that only dissipate after a long, cold, damp period, which is to say a winter. The key is to imitate that winter, and that’s what “cold-moist stratification” means.
Giving a seed its winter
Stratifying is unglamorous. Mix the seed with damp sand or vermiculite, seal it in a bag or tub, and leave it in the refrigerator, a little above freezing, for the weeks the species needs. Some seed wants a month, some three; the plant’s page carries the figure. The catch is timing. It takes weeks, so you have to start weeks before you mean to sow, and it’s best planned ahead so you aren’t remembering it in spring.
For hardy plants, the low-effort alternative is to let the real winter do it. Sow outdoors in autumn, or set the pots outside, and the seed gets its cold spell for free and comes up when the ground warms. It’s the same mechanism, with the season doing the fridge’s job.
