How to Cold Stratify Chill Seeds

Many perennial seeds, as well as hardy tree and shrub seeds, require special treatment such as cold stratification or chilling before planting to assure germination will occur. (Not sure if your flower is annual or perennial? See Annual or Perennial.) The most common perennial, tree and shrub seed treatment is chilling or cold stratification. Cold stratification mimics the natural seasonal changes of seed ripening during summer or fall, then the cold temperatures of winter followed by seed germination in the spring. Cold stratification can be accomplished by placing the seeds in damp medium and storing them in a cool place to chill for the recommended time, usually between 3 and 8 weeks for perennial flowers although some seeds need an even longer period of cold treatment. When you cold treat or stratify seeds, you must plan ahead and allow sufficient time for this process prior to planting in the spring. (For additional information on other types of seed pretreatment such as scarification, see Special Seed Germination Tips.)

Natural Cold Stratification Outdoors is Risky

In nature, seeds are stratified naturally outside. Certainly, one way to provide a cold period is to plant the seeds outdoors in the fall and allow them to chill naturally during the winter. You can plant them in a nursery bed or directly in the garden or in containers left outside. If you use containers, it is best to protect them from the worst weather and from animals by storing the pots in an unheated cold frame.

Unfortunately, Murphy’s Law and the lack of control over outdoor growing conditions often result in reduced germination. Common causes of failure would include extreme temperature fluctuations, excessively wet or dry conditions, and damage by birds, rodents or other animals, just as a few examples.

Since it is so risky to stratify seeds outside, many gardeners prefer to cold stratify their valuable seeds under more controlled conditions. Luckily, the typical household refrigerator does the job quite well.

How to Cold Stratify Seeds in the Refrigerator

The household refrigerator maintains just about the right temperature for cold stratifying seeds. Place your seeds in a small amount of barely damp medium such as vermiculite, sphagnum peat moss, seed starter mix or clean coarse sand and wrap securely so it stays moist. It should be damp like a wrung out sponge, not sopping wet or saturated.

Be sure to label and date your chilling seeds and also mark your calendar to remind yourself to bring them out of the fridge and plant them. Check the seeds periodically to make sure the soil is still damp and to remove and plant any that may begin to germinate early.

How to Cold Stratify Small Seeds

I usually plant tiny seeds in damp medium in a seed pan, enclose the pan in a plastic bag, and place it in the refrigerator. This makes handling them much easier after the chilling period is over: simply move the container directly from refrigerator to a location with the appropriate temperature for germination. (For more detail, see How to Plant Seeds Indoors.)

How to Cold Stratify Large Seeds

Larger, easy to handle seeds such as Hemerocallis (daylily) for example, can be placed in a sealed plastic bag with a small amount of dampened medium and placed in the refrigerator. Check them often and remove individual seeds as they begin to grow. Plant the seeds immediately; do not let them dry out between chilling and planting.

Once the majority begin germinating in the refrigerator or after the recommended time, plant the seeds.

Have Fun Starting Your Perennial Flowers From Seed

It is such a rewarding experience to propagate or grow your own perennials (and hardy trees and shrubs, too) from seed. I hope these directions on cold treatment or cold stratification help you succeed with your seed starting. For additional information on other types of seed pretreatment such as scarification, see Special Seed Germination Tips.

Happy Gardening!

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