Most of the usual garden veggies (such as tomatoes, peppers, and lettuce) can be sown directly into warm, moist potting mix in the spring, and they will germinate and begin growing within a couple of weeks. However, many of the wilder plants such as native plants and medicinal herbs require special pre-treatment before they will begin to germinate and grow. One such treatment is called cold stratification. Cold stratification is a fancy-sounding term which only means that the seed must experience winter before it will germinate (sprout).
If your seeds require cold stratification, that is probably stated on the seed packet or in the catalog description. If you suspect your seeds may require a special treatment but the seed packet and description don’t mention it, you can search online for more information about the species.
My introduction into cold stratification was prompted by the Alpine Strawberry. In my second year as a gardener, my enthusiasm was high. I was devouring library books about gardening, dreaming about vegetables, and hoping to try all the delicious-sounding heirloom varieties in the seed catalogue. I desperately wanted to try an alpine strawberry. Alpine strawberry plants were not available from any stores or catalogues I could access, and seed packets were hard to come by. When I finally scored some seeds, they proved exceedingly difficult to grow. They required a cold stratification process, about which I found diverse and conflicting information. I tried all the ways: putting the seed packet in the freezer for two months, putting the seed packet in the refrigerator for two months, planting the seeds in a bag of moist potting soil and placing that in the freezer, planting the seeds in a bag of moist potting soil and placing that in the refrigerator, and planting the seeds outside in fall. Many of these methods were unsuccessful, for various reasons. There are three methods that I’ve found to really work, and the best one depends on the type of seed you’re trying to grow.
Cold Stratification Method #1: Plant The Seeds Outside In Autumn
This method is the least effort for the gardener, and works well in the following scenarios:
– You have a huge quantity of seeds to plant, and you don’t mind if some of them get lost.
– The seeds are very large and resilient (pawpaw seeds, persimmon seeds, nut seeds, etc).
– The species needs many freeze-thaw cycles (packet will usually specify this).
– You have a well-prepared, weed-free, well-protected garden bed in which to plant the seeds.
There are some situations in which planting seeds outside in autumn may not work well. For example, two of my acres are in a river floodplain. We regularly experience winter flooding in which the river swells until it flows over my field and whisks many seeds away downstream. Autumn seeding doesn’t work well there. The seeds will not be where I left them by spring. Another example in which I’ve had poor success autumn-sowing is with tiny seeds. The alpine strawberry, for example, (and the native wild strawberry) have seeds about the size of a grain of sand. These seeds wash away and blow away, and come spring, few are left. Those that remain germinate with extremely tiny, extremely delicate new growth in spring, and it’s very hard to see them, weed around them, and water them without damaging the new growth. I experience much better success in growing tiny seeds with cold stratification indoors in a protected environment. Another case in which I have poor success planting seeds outdoors in autumn is when the seeds are especially delicious. If you’re planting nuts or another very tasty seed, you may need to take precautions to protect your autumn-sown seeds from squirrels, voles, gophers, and other hungry wildlife. Lastly, if you live in a warm climate that doesn’t experience cold winters, outdoor cold stratification will not work for you.
If you’re ready to plant your seeds outdoors in autumn, you can either broadcast-sow your seeds (for large quantities of small seeds), bury each one individually (large seeds such as nuts), or carefully prepare a protected garden bed and seed according to packet directions (for smaller quantities of precious seeds). It’s best to plant them before snow arrives, so they’re in direct contact with the soil.
Cold Stratification Method #2: Refrigerate Planted Seeds
Placing your dry seed packets in the refrigerator or freezer will not cold stratify the seeds. To begin the cold stratification process, you must plant the seeds into a moist medium, and place the moistened seeds into the refrigerator. The freezer is not an ideal place to cold stratify the seeds. In most cases, freezing will not harm the seeds you are trying to stratify, but the cold stratification process may take longer in the freezer than it would in the refrigerator.
Some people suggest planting your seeds in a plastic bag filled with moistened sand, and placing the bag in the refrigerator. I have not personally had good luck with this method. I find that the moist sand is very likely to grow mold. I like to plant my seeds into potting mix. If you have a soil blocker, you can make soil blocks of the smallest size that works for your seed type, sow the seeds into the blocks just as you’ll have them when they germinate, loosely cover (I use wax paper or parchment paper), and place into the refrigerator. When stratification is done, you can simply transfer the soil blocks to your warm germination area. This is my preferred method, but with one caveat: be careful that your seeds don’t get mixed up during this process! Last year, I found that some of my seeds that required very long cold stratification periods got mixed up as the soil blocks started to break down over time, or through watering accidents. This year, I’m being extra careful to separate my cold stratification species into separate containers and label them really well so they don’t get mixed up.
After planting my seeds and placing them into the refrigerator, I take them out every couple of weeks to examine them. If I see any mold beginning to grow, I remove the covering and allow the tray to dry out at room temperature for a day or two. If the potting mix seems too dry, I water it and then place it back into the refrigerator. The soil should be ideal for germination, slightly moist like a wrung-out sponge. Too moist and it may grow mold, too dry and the seeds may not germinate. Continue the cold stratification process for the number of days indicated for the species you are growing. Cold stratification times range from about 30 days to about 120 days. Generally, I find that leaving the seeds in the cold stratification environment for a month or two longer than needed does no harm.
Cold Stratification Method #3: Stratify Your Flats Outdoors
Assuming you live in a climate that does experience winter, you might be able to cold-stratify your seeds outdoors in bags or flats. This is different than method #1, because you’ll be growing your seeds for transplants instead of seeding them in place in the ground. This method works just like Method #2, except instead of placing your planted seeds in the refrigerator, you take them outside. This method works sometimes, but the seeds are in more danger outside than they are in the refrigerator. If you have a cold frame or a cold greenhouse or a screened porch where you can protect them, your chances of success may be higher. I don’t prefer this method because I find it’s hard to regulate the moisture in my potting soil, and the seeds freeze completely during most of my Indiana winters, so they take longer to stratify than refrigerated seeds. However, when my refrigerator fills up, I start taking my seeds outside to stratify on my screened porch. It usually does work reasonably well.
Whichever method you choose, cold stratification is easy for even a novice gardener to achieve. Now that you are armed with this thorough cold stratification guide, you are ready to begin. Go get growing!
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