Let’s Think About Seeds: The Cold Treatment

Sometimes it is best to turn our minds from current events, and return to the beauty of old, continuous events, like sleeping trees, crusty snow, slow and fast approaching spring, quiet seeds sitting on the dining room table… Yes, let’s think about seeds.

Some native flower seeds.
Some native flower seeds, in all their many forms. The fossil-like ones are Western Spiderwort (Tradescantia occidentalis).

An important trait for seeds (and for gardeners to know about) is that, as they reach maturity, many seeds enter a protective dormancy. These dormant seeds are delaying germination until they receive the proper seasonal cues that indicate it is the right time to germinate and grow.  There are many ways that seeds stay dormant, from impermeable seed coats to hormone signaling.  But luckily for gardeners, most seeds can be woken up with the same type of treatment: The Cold Treatment.  

The cold treatment sounds mean, but really it is just a way to mimic winter (nothing mean about pretending to be winter).  During the cold treatment (also called cold, moist stratification), seeds absorb moisture, chemicals in the seed break down, and the moisture and cold provide the environmental signals needed for germination to begin. Depending on the species, cold treatments typically last from 10 days to 3 months. And that is why, nearing mid-January, I begin to think about seeds, and getting my 60 day cold treatment seeds ready to go into the fridge.

Beautiful New England aster seed poof ball (Symphyotrichum novae-angliae)

The Cold Treatment That’s Right For You

So how does one do a cold treatment?  Well, there are several easy methods, but when you look out into the vast expanse of cold treatment information on the internet, there are two main techniques you will see recommended:  Moist vermiculite/sterile sand vs. Moist paper towel/coffee filter. 

In one, you mix seeds with moist vermiculite (or sterile sand), and in the other, you fold seeds into something like a moist paper towel (or coffee filter).  Which technique is best for you depends on how you will eventually plant your seeds.  If you plan to grow seeds in individual cells or pots, by all means go with the paper towel!  It will allow you to see the seeds after the treatment (that’s mostly impossible in the vermiculite method) and transfer them individually to their little spots.  On the other hand, if you think your children will likely find some mortal peril, while you are dutifully picking seeds off of a paper towel, and you need to quickly scatter your seeds on an open tray or directly into the garden, by all means, go with the vermiculite!  

These open trays were all sprinkled with seeds mixed with vermiculite. For home gardening, this open tray method works well for me. Teasing roots apart has never been a problem in the spring (fall can be another matter though!)

How One Gardener Does A Cold Treatment

Now, in no way at all am I saying that my method is The Best Way to do a cold treatment or start plants.  It is really just how I can manage to get it done between cleaning up broken snow globes and making sandwiches.  And for me, it is mostly Good Enough (which, in some cases, is hard to distinguish from perfection).   

Snack-size plastic baggies
A permanent marker
A medium sized bowl
Vermiculite or sterile sand

1. I label my little baggies with the species name, the number of seeds in the bag, and if they are surface sown (because I won’t remember that in a month). 

Dried butterflyweed (Asclepias tuberosa)seed pods in winter.

2. Usually, I fill about ¼ to half of the bowl with vermiculite, and slowly add and mix in water, until I can squeeze the vermiculite and it stays together in my hand, but no water comes out.  In other words the vermiculite is moist, but not sopping wet.  However, I recently read someone else’s technique where they want just a few drops of water to come out when they squeeze, and that is what I am going to try this year. 

It seems there is a little wiggle room here on the moisture, so don’t let the pressure of making this moisture decision deter you from undertaking cold treatments -you can do this!  Seeds in the wild experience a variety of moisture levels as they overwinter naturally, so from a biological perspective, you want enough moisture that your seeds will fully imbibe (technical term for absorb) water to initiate the changes in their physiology, and on the other hand, you don’t want them to be surrounded by so much moisture that they are deprived of oxygen.

3. I grab a little handful, maybe 2-4 tablespoons, depending on how many seeds I’m starting, of the moist vermiculite and put it in a bag and then I add my seeds in and mix it all together, and zip up the bag.  Usually I start something like 100-400 seeds of something.  I try to go by hundreds because I’m thinking ahead to when I will scatter this mix onto open flats a.k.a. germination trays.  For me, thus far, I find about 100 seeds per tray works pretty well.  But I do adjust this based on past experience with certain species -ones where almost every seed germinates I would do less than 100 per tray, and those where it’s been more sparse, I would do more.  

4. I throw the small bags into a larger bag that says if it’s a 30 or 60 day treatment.etc., and toss it all in the back of the fridge for 10 days to 3 months, depending on the species requirements.  

5. Every couple weeks, I check them and make sure nothing’s too dry or molding, or sprouting early.  If too dry, I add water.  If there are seeds molding, I remove the offenders. If they are sprouting early, I plant them.

5. At about the right time, I get them out, fill up a tray with moist potting soil, and then sprinkle the vermiculite/seed mix, onto the top of the tray, then cover them with  a couple more sprinkled handfuls of potting soil (unless they’re surface sown), and just like that, the seeds are cold treated and planted. Hooray!

A Joe Pye weed seed about to take flight.

Below, I will put links to a number of very good additional resources on how to complete cold treatments.  And somewhere out there, there may even be The Best Way to do a cold treatment. Please let me know if you find it!!

I’d also like thank Ann Casper, a grower at Prairie Moon Nursery, for explaining the pros and cons of vermiculite/”goop” vs. the coffee filter.

Cold Treatment Resources:

Germination Codes and Seed-Starting Basics for Native Plants, Prairie Moon Nursery *Excellent pictures and instructions for both methods!

Native Seed Propagation Methods, Missouri Botanical Garden
*Interestingly recommends first 24 hours in extra moist conditions, then adding extra vermiculite to reduce moisture for the rest of the cold treatment.

Native Plants: Learn to Grow Your Own Webinar

Other Resources:

Dormancy and Germination: Making Every Seed Count In Restoration. O.A. Kildisheva, K.W. Dixon, F.A.O. Silveira, T. Chapman, A. Di Sacco, A. Mondoni, S.R. Turner, A.T. Cross, Restoration Ecology Vol. 28, No. S3, pp. S256–S265, August 2020

4 replies on “Let’s Think About Seeds: The Cold Treatment”

I used sand instead of vermiculite, and instead of 3 months, I did 7+ months. And instead of a permanent marker, I used a marker that wipes off. Are my seeds still good


Hmm! Hi, Aaron! My guess is that your seeds would be ok… but I am not sure about that. Maybe a low effort way of dealing with uncertain seeds would be to plant them directly out in the garden (shallow rows), instead of trays?


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