Stratification

Stratification is a process that fools seeds into thinking they have experienced winter, spring, or both, to help them break dormancy.

Traditionally, stratification referred to the practice of layering seeds with a moist growing medium, such as vermiculite, peat, perlite, sawdust, composted bark, or potting soil. As seeds germinated, they would be removed to a more permanent growing space. There are three types of stratification: warm, cool, and variable.

Learn from your plants
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Many plants have evolved to use cold temperatures as a period of rest and warmth as a trigger to gear up for germination in spring. We still don’t completely understand the magic that happens within a seed. [Part of me hopes we never do!] Simple starches, sugars, and genetic information are somehow transformed into a living, breathing, growing organism. It’s really amazing when you think about it. We can look to those natural processes to get more out of our gardens. There’s no sense bucking millions of years of evolution. Even better, we can put all that evolution to work for us. Keep in mind, however, that some seeds need the extra time for the embryo to fully develop. Pushing them to do too much too soon only weakens them.

Benefits of stratification

​By artificially stratifying seeds, you can decide when they will germinate. This can help you control when a crop might reach harvestable size. It can also help avoid predictable pest or disease infestations. It can also be sued to extend your growing seasons. Since seed dormancy can be quite variable, with some seeds taking over a year or two to germinate, stratification can be used to provide a more favorable environment for new seedlings.

Preparing seeds for stratification

Start with firm, certified disease-free seeds. These seeds will need a little moisture for their period of hibernation, but too much moisture sets the stage for decay. Seeds destined to be stratified need to be soaked for 24 hours. Then shake off excess water and place them in a plastic container with three or four times the seed volume of some type of growing medium. Some people use folded paper towels, which you can certainly use. Professional growers prefer sphagnum moss or vermiculite. Whichever you prefer, add water to the plastic bag and allow the medium some time to absorb all that moisture. Vermiculite absorbs water quickly, while moss may take 8 to 10 hours. Next, squeeze the bag to get rid of most of that water, the same way you would squeeze out a sponge. Gently shake the seed-medium mixture, to distribute the seeds and to incorporate air, before placing the bag in the appropriate temperature-controlled environment.

Be sure to write the seed name and the date stratification was begun on the bag. The next day, pour or squeeze out any excess water. Then, let nature take its course. Periodically check your seeds for signs of rot, desiccation, or germination. Rot should be completely wiped off and the seed allowed to dry out before continuing stratification. Otherwise, the seed can be planted after all signs of decay have been removed. Dried out seeds need more water and closer monitoring. Germinating seeds need, you guessed it - planting!

Cold stratification

Most perennial woody shrubs and trees require cold stratification to germinate. Lettuce, milkweed, delphinium, and violets can also benefit from this process. Cold stratification mimics the conditions of a cold (31 to 41°F), wet winter and is generally used on seeds that naturally ripen in late fall or early winter. Your refrigerator provides the perfect place to cold stratify crops intended for spring planting. You can leave moistened seeds in the refrigerator for 1 to 4 months to get the desired effect. Just make sure they are not sitting in water, or they will rot. Also, do not plant cold stratified seeds in autumn. The double whammy of cold will be more than most seeds can handle.

Warm stratification

Warm stratification is used on seeds from trees and shrubs that naturally ripen in early fall. This is the majority of plants grown in gardens and landscapes. Warm stratification mimics the conditions of spring with warmth (68-86°F) and moisture. The moisture softens the seed hull, entering the seed and providing the water needed by the embryo to complete its development. Moistened seeds can be placed in a plastic container on top of the refrigerator for 60 days, or until 20% germination is seen. When warm stratifying seeds, you will need to keep a look out for mold, which will need to be wiped off regularly. Some growers apply fungicides to seeds being stratified. I do not.

Variable stratification
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In some cases, seeds need a combination of warm and cold stratification to stimulate germination. Starting with warm stratification, moistened seeds are given 60 days to soften the seed hull before being placed in a cold environment.

To determine whether or not stratification is needed by your seeds, find out how it grows in nature.