It’s Time for Cold Seed Stratification
Although humans tend to interfere with plant growth processes, the truth is that seeds by nature have everything they need to germinate, thrive, bloom, and reseed themselves—at least under the right conditions. But mitigating factors, from increasingly extreme weather changes to the spread of invasive species, mean they need a little more help from us than they actually should.
For example, perhaps you buy seeds and set about germinating them indoors and planting the resulting shoots where and when they are most likely to be successful. This is great, but you may be missing an important step for many seeds: cold stratification. Many seeds require a period of severe, wet cold during the winter to germinate in the spring. Tossing the seeds into the ground before winter (hopefully) will allow them to experience these conditions, but you can also simulate this process indoors.
Many types of seeds require cold stratification.
Most of your tall, spiky flowers will need stratification: agacha, delphinium, foxglove, liatris, larkspur and hollyhock, to name a few. But it’s not just the tall varieties: rudbeckia, aster, echinacea, lupine, beesbalm, lavender and the notoriously difficult to germinate milkweed and poppy all need that blast of cold to ensure good germination. As a general rule, before purchasing flower seeds, check the planting instructions to see if they will benefit you.
As a rule, vegetables do not need stratification, but it should always be checked. Asparagus and artichokes (both perennials) benefit from cold, as do a number of herbs, from chamomile to garlic (though I’ve managed to grow both without it).
How to cold stratify seeds at home?
First, make sure your seeds are not too old. Seeds have an expiration date . While the dates listed on the packages are only a guide, they are based on years of gardening experience, so set your expectations accordingly if you are using “expired” seeds. Different seeds require different times for the stratification process: from several weeks to six weeks. Google to find the ideal timing for your specific seeds.
All you have to do is place the seeds between two sheets of damp paper towel, secure it, and let it hang in the back of the refrigerator (where the temperature is likely to be most stable) for the allotted time.
I like to place a sheet of paper towel on my work surface and fold it in half, then fold it in half again, creating a little seed book. You can dampen the paper towel with a spray bottle, but don’t overdo it: damp means damp, not wet. Spread the seeds out, spray them, and then wrap them in a towel. Label the resulting package to remember what seeds are there.
You’ll want to protect your little seed packets, so it’s a good idea to place them in disposable food containers or a plastic bag, but be sure to give them some ventilation. You can poke a few holes in the food container to allow the bags to breathe, or you can leave the plastic bag unsealed.
Seeds should be checked every three days for mold (which is caused by too much water and not enough air flow) or dryness (not enough water, too much air flow). If you find mold, you’ll have to start over. Dryness isn’t that big of a problem, but it will prolong the process.
Common mistakes when stratifying seeds
Layering takes work, and while the internet is filled with shortcuts, I’ve tried them all with incredibly poor results. The most obvious failure was the proposal to freeze the seeds in ice cubes: in two seasons I had almost zero germination. Likewise, simply storing seeds in the refrigerator for six weeks had little effect—you need moisture as well as cold.
What to do after cold stratification
If you time your stratification correctly, you will be taking your seeds out of the refrigerator as you plant, so ideally your seeds will go straight from stratification to your seed starting medium, just like your planting trays. Germination is not necessary at this stage (that’s our next step), but some seeds are too eager.
If the seeds have sprouted, carefully cut a small spot of them on a paper towel and carefully plant it in the original seed medium. If they do not germinate, you can remove the seeds from the paper towel and plant them in the growing medium at the appropriate depth. Sometimes seeds will stick to the paper towel, but this is not a major cause for concern – it decomposes quite easily, so if necessary, just plant the seed on a small area of the towel.
Handle the seeds as usual. Let them warm up on heating mats under grow lights as if they were feeling the melting snow and spring warmth. If everything works, you should notice much higher germination rates than if you skipped the entire process.