E0048 | Cold Stratification of Seeds

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Welcome back to another episode of the Homegrown Liberty Podcast, this is episode 48, and today I’m teaching you all about how to cold stratify seeds for planting in the spring! This is super timely right now as we are headed into winter and for some of the seeds you might want to start it could take months of cold stratification to get their germination parameters met for sprouting. So if you’re wanting to get a bunch of tree seedlings planted in the spring, then now is the time to start!

Why do we stratify seeds?

Most seeds have built-in dormancy mechanisms that prevent them from germinating before a good season for growth has arrived. These germination prohibiting mechanisms keep the seed dormant through times of drought, or harsh conditions. Some of these systems keep them dormant through fall when temperatures are cool, there’s adequate moisture and otherwise it’s perfect growing conditions. This prevents the seed from germinating shortly before killing frosts arrive in winter. It can also keep them dormant through a warm period in spring and prevent them from sprouting too early. Because these systems exist, we as growers must sometimes artificially or pointedly supply the seeds with all the things they need to successfully and consistently germinate. We have to mimic nature, or at least be aware of the plant’s needs so that we can supply what it needs to grow and thrive!

Stratification is…

The process of breaking seed dormancy and promoting germination. Mostly what we’re talking about today is going to be Cold Moist Stratification, but some seeds need a combination of warm and moist and cold and moist, or sometimes there is need to alternate several times to replicate what happens in nature.

Look up requirements for each species you intend to start from seed.

It’s easily found online or in reference manuals. If you want to go the free route, just type into google’s search feature “stratification requirements for (insert species name)” For example, to find stratification requirements for apple seeds, type in “stratification requirements for Malus domestica” or for peaches, “stratification requirements for Prunus persica” but if you want specific information that is reliable, you must use the binary nomenclature or the scientific name. If you don’t know it, then just type into the google search feature “(latin) or (scientific) name for X” and it’ll probably be the first result. Now that you know the scientific name for the genus and species, you can look up the germination needs for free!

How does it work really?

Well the seed coat may be thick and hard and need to be softened by consistent moisture for a certain amount of time. There could be germination inhibiting hormones present such as abscisic acid. The seed coat might be thick and hard enough to prevent oxygen from getting to the embryo and thereby inhibit respiration, and keep the seed in a dormant state.
The short answer is that cold stratification softens the seed coat, leaches out hormones, allows moisture and other elements into the seed and signals that it’s time to grow.
Some seeds need a certain amount of light, some need a particular temperature range, some need to have their seed coats damaged by fire, or the digestive process of an animal to trigger the growth response. So by using “Cold Stratification”, we can simulate winter conditions and this one method is probably the most commonly needed when it comes to breaking seed dormancy. That’s why I’m talking about it first!

What kinds of seeds is this appropriate for?

Generally, temperate climate tree and shrub seeds will benefit from cold stratification, and a lot of the perennial herbaceous plants will also. This is generally not necessary for annuals because they are designed for quick growth. So specifically if you’re looking at a perennial, especially a tree or shrub, you should check to see if it needs stratification treatment of some kind. Most likely, you’ll find that cold stratification is just what you’re going to do.

How to do it.

I’ll tell you the short and simple condensed version, then give you the details. Simply put, you need to use some moist media at a ratio of about 3 to 1 of media to seed, and keep that medium and seed mixture cold for a period of time. That’s the short easy explanation. But you’ll have questions like, “What kind of medium? How cold? When should I start the process? How wet? What kind of container?” Well it just so happens I have answers to those!

What kind of medium?

You can use:

  • Sand –
    • Pros – Easy to sterilize with boiling water, fast draining, easy to spread later, cheap
    • Cons – Drains fast and might dry out if you’re not careful
  • Vermiculite
    • Pros – Holds moisture very well, easy to sterilize with boiling water, easy to spread later
    • Cons – Water retentive and hard to gauge moisture levels, rot prone
  • Potting Soil
    • Pros – Holds moisture very well, easy to find, cheap
    • Cons – Difficult to sterilize, not easy to spread later, can harbor disease
  • Peat Moss
    • Pros – Cheap, holds moisture very well, easy to find
    • Cons – Difficult to sterilize, harbors disease, quite acidic, goes from moist to dry quickly
  • Perlite
    • Pros – drains super well, fairly easy to find
    • Cons – does not hold moisture very well at all

I suggest some mix of sand and vermiculite or just sand. I’ve used potting mix in the past with success, and sometimes with failure due to not sterilizing properly. So I say if you wanna make sure you have sterile medium to work with, mix up a 50/50 sand and vermiculite mix, put it in a pot of boiling water, boil for a couple minutes, then pour it through a fine mesh strainer, put it back in the pot, put the lid on it and let it cool down to room temp, wash your hands well and use that. You can do the same thing with potting soil and peat moss but it is a mess. Perlite is just not much good for this purpose except for cutting vermiculite down to a lower moisture retention level, but I’d just not worry about it. I have to mention it or else I’ll get a thousand emails asking if you can use it.

How much seed?

Try to stick to about 3 parts seed stratification medium to 1 part seed. So let’s say you have 1 cup of seed, that means you should have about 3 cups of seed stratification medium to keep it cozy and moist. If you have too little medium in there, you will run the risk of the seed imbibing (sucking in) almost all the moisture in the medium. It’s important they don’t dry out, our goal is to get those seeds to suck up all the moisture they need and still stay snug in a moist cold environment.

How cold?

Anywhere from 34F to 40F or 1C to 4.4C. You don’t want the seeds to freeze, but you want to stay below pathogenic fungal growth temp ranges. Normal fridges will fluctuate and have warm spots that will bump up into that fungal growth range and sometimes also dip down to sub-freezing temps as well. So my little trick is to get one of those small lunchbox sized coolers and put the seed inside there along with a little bag of ice, or a bottle of ice. That will keep the temp nice and cold inside the cooler, and also help to buffer any temp swings while hopefully protecting them from freezing. If they freeze it’s not necessarily the end of the world, you may lose some to freezing, but you’d be surprised at how many will make it just fine.

When do I start?

Ahh, here’s an important bit. When to start the process… Well that depends on when the seed needs to be sown. If you have something that requires 2 weeks of cold stratification, you should start stratification 2 weeks before it’s time to sow them. If you have a seed that needs 120 days of warm stratification plus 120 days of cold stratification, better start them 240 days before it’s time to plant. FYI that’s 8 months for cherry seed. But most things are in the 30-90 cold stratification time window, which is 1-3 months. So again, it all depends on the particular seed. I look at my calendar and figure out when it’s going to be most convenient for me to plant the seeds and then count backwards to figure out what date I need to start the process and even go so far as to make reminders on my phone or build a calendar with start dates etc…

How wet?

If you get things too wet, they will rot due to lack of oxygen. The seeds are alive and you’re waking them up from dormancy. One of the triggers is oxygen content, so too much moisture and they’ll suffocate. You’re shooting for a nice middle ground of moist mix here. If you were to get a big handful of it and squeeze as hard as you can, a few drops of water should come out, or if it’s sand and vermiculite, you should be able to put it in a towel and sling it around once and get a little bit of water to sling out. If you leave a huge spray of water slinging that stuff around, that’s fine, keep doing it until there’s a little trickle. Or if you squeeze that potting mix hard and a ton of water comes out, keep squeezing until just a few drops are dripping, then mix it up again and you should be good. The trick is to keep it moist to the touch, but not so much that any moisture pools in the bottom of the container.

What kind of container?

Well there are several ways to go about this, most people will just put the seed mix into a resealable plastic bag and call it good. That’s probably the easiest and simplest solution so that’s what I normally do. It takes up the least amount of space in the fridge. But if you pick up an extra fridge to put in your garage or whatever, having that extra space can open up lots of opportunities for doing things differently. You could plant the seeds in seed flats covered in the seed starting mixture and then they’re ready to go whenever it’s time to get germination kicked off. You just fill the trays, sprinkle the seed, and cover with your seed starting mixture, and tamp it down. Then stack another tray on top, keep doing that till you have as many trays filled as you need. Set them all in the fridge and check on them periodically to make sure they aren’t drying out, and to check for signs of germination. Then when stratification time is up, just pull the tray out, set it somewhere to warm up and your seeds should start sprouting and growing! It removes a step and makes your life a little easier, but it takes up a lot of space. I don’t do it that way because we have better things to do with the fridge space all winter.

Other than plastic bags, you can stratify seed in containers in the ground outside. The way you do this is to dig a hole, and put a hardware cloth box inside the hole, then simply fill some sanitized pots with your seed stratifying mixture, once all the pots are underground where it will be nice and cold, protected from freezing, and safe from rodents, seal it up with more hardware cloth and some weights, and top it off with either a pile of mulch or soil and a tarp. Then you know it will stay moist, cold and safe under there. This way is what I’d suggest for larger seeds like nuts or especially anything you are doing on a large scale.

Label the seeds

ALWAYS, ALWAYS label your bags or pots or trays! Put more information than you think you need on the item. Here are some things I always include.

  • Common name
  • Latin or Scientific name
  • Date that I started cold stratifying the seed
  • Range of days for cold stratification (eg. 30-60 days)
  • Date to plant the seeds (date to sow)
  • Source of seed (the place I acquired the seed)
  • Any special treatment done to the seed before stratification (hot water soak, acid, soaking in water)

So a typical bag would look something like this:

Malus pumila  from XYZ seed co.

Antonovka

Soaked 24 hrs

Stratify: 60 days

Started: Feb 2nd  Sow: April 2nd

That’s generally how I label my bags or pots of stratifying seed. This way I know at a glance what the thing is, where I got it from, what I did to it, and when I need to do something else with it.

Tips and Tricks

  • If your seed needs cold stratification, don’t stratify then plant in the fall. That’s a bad idea because it will immediately start sprouting and not have enough time to get ready for winter. Stratify with your planting time in mind so they line up correctly.
  • Some seeds need soaked first. Seed like mulberry does best by soaking for 24 hrs, then draining the water off, soaking again for 24 hrs, and repeating the process once more before mixing with sand or vermiculite for the cold stratification process. So always look up the specific species you are working with for particular techniques you may need to use.
  • Split up your batches of seed. Sometimes one bag will get moldy and another will be fine.

What about disease prevention?

Most of what you’ll need to be concerned with are fungal issues. So the quick, short answer is prevention. Sterilize the media, and wash the seeds with a 50/50 mixture of the regular 3% hydrogen peroxide and water, and let them soak for a little while. If you’re soaking overnight, mix 1 oz of 3% H2O2 and 16 oz of water for a 24 hour soak. That will help eliminate most pathogens and put some extra oxygen into the water for the seeds.

Other than that, the only real ways I suggest to cut down or eliminate fungal diseases, is keep the media cold, below 40F and start with sterile media of whatever type you prefer. If you do those things, you shouldn’t have fungal issues. We could get into all the different types of fungal organisms, we could talk about fungicides, but we can leave that dry material for some other day if you guys are really that interested in something that in depth. The real point is if you just use smart preventative measures, you shouldn’t have fungal issues, so you shouldn’t need to use anything as dangerous as a fungicide. And lemme tell ya, that stuff is super toxic and not to be messed around with.

Recap!

Cold stratification is the process of providing a cold moist environment for the seed to keep it dormant while breaking down the germination prohibiting factors present in the seed. We do this to prepare the seed for successful germination. This happens naturally with winter weather, but we can do it artificially with more control, and keep valuable seed safe and healthy, protected from rodents and insects while it prepares to jump out of the soil in the spring! You’ll want to try and shoot for a 3 to 1 ratio of mix to seed. Keep it moist but not wet. Keep it below 40 degrees fahrenheit and use sterilized media to store the seed in. Do those two things and most disease problems will be prevented completely. But if you need or want some extra disease prevention, use 1 oz 3% hydrogen peroxide to a pint of water to soak the seed in overnight or for 24 hours. And above all else, label your bags or pots of seed! You may think you’ll remember what they were, but one day after a few months in the fridge you won’t remember and you’ll be kicking yourself for not writing it down!

Alright, now I want you guys to know something. I do this all completely for free because I want you to have success! So if you have questions, please go join the facebook group https://www.facebook.com/groups/homegrownliberty/ or ask your question in the comment section below. I’m sure I missed something, or need to clarify. With that said, this is not the end all be all source for how to do this. I’m no expert! I’m just a guy learning about this stuff and sharing with you what I’ve learned. There are probably lots of ways to do this better, more efficiently, or whatever. Experiment, try new things, keep a childlike wonder and love of learning and you’ll never be bored. There is so much to learn and do here on this earth.

Until next week, I hope you have a wonderful day, God Bless. And as always “Go Do Good Things”

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One Response to “E0048 | Cold Stratification of Seeds”

  1. You can use a Julian calendar to help find your start date. Its a calendar that list all dates by the day of the year.

    If you need to plant on march 3rd. And you need 180 of cold. March third is the 62nd day of the year. 62 -180 is -118. 365 -118 is the 247th day of the year, or September 4th.

    I hop that’s clear.

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