What Is Cold Stratification
Cold Stratification is a natural process that plays a crucial role in the germination of seeds. It requires a seed to go through a period of cold temperatures before it is able to germinate. In addition to cold temps most seeds need moisture as well during this period. The long period of cold temps and moisture gradually break down the tough outer shell of the seed so it can germinate when soil temps rise in the spring.
Cold Stratification is an important mechanism for the survival of many plant species, specifically wildflowers and native species. Without it, seeds that drop in the fall may quickly germinate if there is a small stretch of warm weather. If the seed germinates in the fall, the seedling will certainly die during the winter months. Cold Stratification prevents the seeds from germinating in the fall and instead forces seeds to wait until spring for germination.
Which Seeds Need Stratification
Not all seeds need to go through a period of cold stratification. Generally, annuals have a lighter outer shell and do not need cold stratification. Hardy perennials, wildflowers, and native species however, may benefit from a period of cold stratification. Here is a list of some popular seeds that require cold stratification:
- Apple (Malus spp.)
- Cherry (Prunus spp.)
- Peach (Prunus persica)
- Plum (Prunus domestica)
- Dogwood (Cornus spp.)
- Lupine (Lupinus spp.)
- Columbine (Aquilegia spp.)
- Delphinium (Delphinium spp.)
- Sweet William (Dianthus barbatus)
- Hollyhock (Alcea rosea)
- Milkweed (Asclepias spp.)
- Larkspur (Consolida spp.)
- Poppy (Papaver spp.)
- Coneflower (Echinacea spp.)
- Black-eyed Susan (Rudbeckia hirta)
- Butterfly Weed (Asclepias tuberosa)
- Hawthorn (Crataegus spp.)
- Bee Balm (Monarda spp.)
- Goldenrod (Solidago spp.)
- Wild Strawberry (Fragaria vesca)
How to Cold Stratify Seeds In Refrigerator
While Cold Stratification is a naturally occurring process, if you have a packet of seeds that you wish to germinate in the spring, you’ll want to mimic the stratification process so the seeds are ready to go when spring comes along. Luckily, it’s very simple to cold stratify seeds right in your refrigerator:
1. Figure timing. Most seeds need a cold stratification period of 4-5 weeks before being planted outside. You’ll want to time it so that you put the seeds in your refrigerator 4-5 weeks before the threat of frost is gone in your area. When that occurs depends on your region. Check this map to see when the threat of frost is typically gone in your area.
2. Soak the seeds for 2 hours.
3. Drain the water and pour the wet seeds onto a paper towel. You can also use a spoon to fish out the seeds. The paper towel will become wet from the seeds, that’s a good thing.
4. Take a dry paper towel and cover the seeds and the wet paper towel. You want the paper towels to be moist but not soaking wet.
5. Place the paper towel and seeds into a Ziplock bag and seal it shut.
6. Label the bag and place it in the refrigerator for 1 month (some seeds may need longer).
7. Sow seeds directly in the ground or in a pot when the threat of frost is gone in your region.
If your seeds start to germinate in the refrigerator then remove them from the refrigerator immediately and plant the seedling in a pot. Keep indoors until the weather allows you to place the seedling outdoors.
Natural Methods for Cold Stratification
Instead of the refrigerator, you can use one of these sowing methods below to let nature do the work for you:
Simply plant seeds directly in the garden soil in the fall. The seeds will experience the natural temperature fluctuations of the changing seasons, providing the required cold and moisture for stratification. When soil temps are consistently warm enough in the spring, the seeds will germinate.
Sowing Seeds In Milk Jugs
Cut the bottom off a clean milk jug, fill it with potting mix, place seeds in the potting mix and water it gently. Place the jug outdoors, allowing it to experience the winter temperatures. The jug acts as a mini greenhouse, protecting the seeds while still subjecting them to the necessary cold period. This is a great method for giving seeds a little head start in the spring. The green house effect of the milk jug will warm soil temps quicker and allow for quicker germination. For more on this method check out the full step by step tutorial.
Open Pot Method
Simply sow seeds in small plastic pots with potting mix and place the pots in a plastic container with drainage holes. Place an animal barrier over the container and set the container outside all winter. In the spring your seeds will be ready to germinate as soil temps warm. For more on this method check out the full step by step tutorial.
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