What is Plant Seed Cold Stratification?3 min read

Cold stratification is a process of pre-treating seeds before they are planted in the ground or in pots. In nature, some species of plants native to cold areas require cold stratification to break dormancy. Stratification mimics the cold, moist winter condition and triggers the seed to germinate when the soil warms up in spring.

The name stratification was used because seeds used to be layered (stratified) between two layers of moist soil and then exposed to cold. Some gardeners refer to stratification as cold treatment.

Why do some seeds require stratification?

Seed dormancy is a state in which the seed is unable to germinate to prevent it from germinating during the harsh winter season. This is a survival tactic that increases its chances of survival by preventing the seed from germinating until conditions are right.  Seeds requiring stratification possess a hard outer shell and remain in the soil until the cold, moist winter environment softens the coat, which allows the seed to absorb water and triggers germination.

When is the best time to stratify seeds?

Wintertime is the ideal time to begin stratification, and then the seeds will be ready to plant when spring arrives.

  • Northern hemisphere: January – February
  • Southern hemisphere: June – July

Obviously, seeds could be sowed directly into the ground or pots during winter, but weather conditions can be unpredictable. Also, some climates may not have a cold or wet enough winter to trigger germination, in other cases, the winter may be so wet that the seeds rot.

Do all seeds need stratification?

No, seeds from warm climates don’t need stratification, however, some seeds require scarification, which is a process in which the seeds are buffed to weaken the outer shell, allowing water in to trigger the natural sprouting process.

Seed stratification process

Stratification

There are several to cold stratify seeds, we will cover the most simple stratification method here.

What you will need:

  • Seeds
  • Paper towel
  • Sandwich bag
  • Marker pen
  • Water
  • Refrigerator (set to 1 – 4°C)

Method:

  1. Mark the sandwich bag with the name of the seeds and the date.
  2. Carefully moisten the paper towel, and squeeze out excess water, it should be damp but not saturated.
  3. Lay the seeds in a thin layer on the paper towel, and fold the paper towel over the seeds to help keep the seeds moist
  4. Put in the refrigerator for 30 – 90 days (until outside temperatures warm up)

If any seeds sprout during stratification, pot them up and keep them in a warm location inside if it is cold outside.

Which seeds require stratification?

Anise hyssop Agastache foeniculum
Arnica Arnica montana
Bellflower Campanula spp.
Black eyed Susan Rudbeckia hirta
Borage Borago officinalis
Butterfly bush Buddleja spp.
Catmint, catnip Nepeta spp.
Chinese lantern Physalis alkekengi
Clematis Clematis spp.
Delphinium Delphinium spp.
Echinacea Echinacea spp.
False sunflower Heliopsis helianthoides
Foxglove Digitalis spp.
Fuchsia Fuchsia spp.
Geum Geum spp.
Hellebore Helleborus spp.
Hibiscus Hibiscus spp.
Hollyhock Alcea spp.
Lady’s mantle Alchemilla spp.
Lavender Lavandula spp.
Lilac Syringa spp.
Lupine Lupinus spp.
Marshmallow plant Althaea officinalis
Masterwort Astrantia spp.
Milkweed Asclepias spp.
Monkshood Aconitum spp.
Pansy Viola tricolor var. hortensis
Peony Paeonia spp.
Peruvian lily Alstroemeria spp.
Poppy Papaver spp.
Primrose Primula vulgaris
Sweet cicely Myrrhis odorata
Violet Viola spp.
Wild garlic Allium ursinum
Yarrow Achillea millefolium

 

Is cold stratification really necessary?

Some seeds may germinate even without stratification, but not all. I have sown a number of cottage plant seeds without stratification. My foxgloves from last year self sowed and I have a large collection of baby foxgloves in the garden. However, if you have purchased cold climate seeds that should be stratified, it is recommended. It is simple to perform and increases the germination rate.