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Stratify Seeds to Improve Germination

Refrigerate seeds before planting to improve germination.

Garden SeedsIt’s not always so simple as just sticking seeds in the ground. There are a number of techniques and treatments that encourage seeds to germinate. We’ve all soaked wrinkled-skinned pea and other big seeds to help loosen those skins and make water absorption easier. Or we’ve nicked hard skin seeds with a sharp blade or even a fingernail (scarification) for the same purpose.

Then there’s stratification, the act of simulating winter conditions — cold and moist — to prep seeds for their usual germination temperatures come spring. This can involve placing them in the refrigerator, usually in some kind of moist potting soil. Or it can mean storing seeds outside during winter in a sealed plastic bag or covered container, again with grow mix.

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Seeds that benefit from stratification are often small and tiny perennials. Delphinium and violets, even when started indoors, have higher germination rates after stratification. Lettuce seed, which goes dormant at higher temperatures, takes to stratification. Some tree seed, including maple, walnut, and apple, require some kind of stratification.

Many perennial herb seeds do better after getting the cold, stratification treatment.

Kevin Jacobs over at A Garden for the House, has a long list of perennials that benefit from seed stratification.

Consider tiny ornamental poppy seed. They benefit from a chill period before planting. Blend seed and mix in a 1:3 ratio in a plastic bag or a small container like a margarine tub and put them in your refrigerator.

Keep them refrigerated for several weeks before planting in the garden. Three months is not too long if the soil mix isn’t too moist. Don’t be afraid to pull them out of the refrigerator every once in a while to check their condition. If you only have a couple weeks before planting, that’s still going to give you better germination rates that no stratification at all.

Seeds can also be stored outside if your winter is cold enough. Use the 1:3, seed-to-mix ratio and be sure to seal bags and containers tightly to prevent them from drying out. Putting the containers away from the harshest conditions, including wind and direct sunlight, will help keep the seed from drying out. Often the north side of a house or garage, out of the wind, is a good choice.

Don’t be afraid to put them out when it’s good and cold in January and December and leave them until just before planting. You might bury the packets under some mulch, or even move them into an unheated shed, if you expect an extreme cold snap. Again, check them for moisture as is convenient.

Of course, sometimes it is as simple as just sticking seeds in the ground. We’ve had surprising spring germination of greens, including bok choy, using natural stratification, planting the seeds in the late fall or early winter at proper depth and mulching with hay.

We suspected that our shredded leaf mulch might have kept one fall-planted kale patch too moist one year, allowing the seeds to rot during a thaw. But as always other factors might have been involved as well. Seeds planted outdoors in autumn can fall prey to many problems, including hungry mice. They might be safer in the refrigerator.

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One Response to “Stratify Seeds to Improve Germination”

  1. William Emmanuel on July 19th, 2018 at 3:53 am #

    Hi I’m in Ireland and I’m really struggling to get Delphinium seeds to germinate and I have now tried twice and nothing not a single seed germinated in the trays. I now have some in the bottom of the fridge (third time lucky) wrapped in damp paper towel and they have been there for about 2-3 weeks. I need to get them out soon and into a tray so I can bring them on before the weather changes for the worse. The intention is and if they germinate to pot them on and to overwinter them in a cold greenhouse and plant them in the borders in the spring. I was going to sow them in a starter compost in my utility room and put the tray on a heat mat and I am just wondering what is the best temperature to set the thermostat at?

    All advice appreciated.

    Regards

    Bill

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