What is seed stratification?
Seed stratification is an important concept for many gardeners to understand. Not all seeds are ready right away to germinate when removed from the plant. Some seeds will only germinate if certain very specific conditions have been met. Many seeds wait to germinate until conditions are just right. By delaying the timing of germination, they improve their success rate.
Most commonly, many seeds will not germinate until a certain amount of time has passed. And some, especially those from stone fruits, will not germinate until they have been subjected to low temperatures for a period of time, such as a winter season. This ensures that the seed doesn’t sprout at the wrong time, only to be killed off by freezing temperatures.
It is not necessary for the seed to actually spend the cold months in the ground. It is only important that the seed ‘senses’ that it did. You can actually mimic this process by placing the seeds in the refrigerator. This allows you to increase the success rate due to a more controlled environment.
Moisture in the Cold Environment
Some seeds require dry cold storage and some need moist cold storage. The dry storage is usually for the seed to continue to develop their immature parts. This is often true of seeds from trees and shrubs.
Seeds that need moisture are those which need the moisture to simulate the natural condition expected from the winter earth. This may be to soften a hard outer shell, or it may be to activate out of dormancy.
For cold stratification, you need to make sure the seed remains slightly moist but not wet throughout the cold period. Depending upon the seed, you can either use a moist paper towel or coffee filter placed inside a plastic baggie inside the refrigerator, such as I did here with my apricot seeds, or you may need to place the seeds in a small container with moist sand or vermiculite.
I’ll be honest, I don’t love that second option, because the idea of vermiculite in my refrigerator isn’t very appealing. We are lucky to have a commercial fridge in our barn though, which we usually only use a few times a year for parties and for keeping the Turkey prior to Thanksgiving. So if you have a basement or garage fridge, that might be your area for seed stratification.
Some seeds may sprout during stratification, especially those that need the moisture to activate or stimulate them. Others may not. Once the seed spends the time in this simulated cold environment, it can be planted in soil at normal temperatures.
What Seeds Require Stratification?
There are a few rules of thumb you can go by, however keep in mind that with all of these “rules”, there are exceptions.
First of all, seeds that evolved in hot climates generally don’t need any stratification. This includes tomatoes, peppers, basil, and many squashes and gourds. Since they evolved in climates without winter, they can just keep on growing year round.
Cold-hardy fruit trees that have a required number of chill hours before they bloom always perform much better with moist stratification. This includes stone fruit such as peaches and nectarines, cherries, apricots, and plums. It also includes blueberries, pears, and apples.
Fruit trees which do not need chill hours do not need any stratification. This includes avocados and citrus. You’ll note that the fruit trees which do not require stratification are also those which do not perform well in cold climates.
Temperate climate trees and bushes need stratification to germinate. This includes many of the cold climate or winter bushes we think of when we think of winter; the mighty oak, walnut trees, many evergreen pine trees, Canadian maple and Christmas holly.
Cold hardy herbs also benefit from stratification. Generally they do fine with dry stratification, unlike their fruit tree cousins. Cold hardy herbs include those which withstand mild freezing temperature, including rosemary, oregano, nepeta (catmint), anise and St John’s Wort.
Lastly, many native perennial flowers require stratification to germinate. The North American Sunflower (helianthus), Prairie Coneflower (Ratibitha), Rudbeckia (Black-eyed Susan), Echinacea (Coneflower) and other beloved plants you may be growing in your garden now.
Stratification Guide
The below table includes some common seeds you can save, as well as the medium and required amount of cold time before you will start to see germination.
| Plant | Temperature | Moist or Dry | How Long |
| Apple Seeds | 40 Degrees F | Dry | 2-3 months |
| Apricot Seeds | 40 Degrees F | Moist | 4-6 weeks |
| Asparagus Seeds | 45 Degrees F | Moist | 2-4 weeks |
| Black Walnut | 40 Degrees F | Dry | 2-4 Months |
| Blueberry | 35 Degrees F | Dry | 3 months |
| Cherry Pits | 40 Degrees F | Moist | 2-3 months |
| Echinacea | 45 Degrees F | Moist | 4 weeks |
| Nectarine | 40 Degrees F | Moist | 3-4 Months |
| Peach Pits | 40 Degrees F | Moist | 2-3 months |
| Pear | 40 Degrees F | Moist | 3 months |
| Plum | 40 Degrees F | Moist | 90 days |
| St Johns Wort | 35 Degrees F | Dry | 1 month |
