How to cold stratify seeds

Seeds from different species have different requirements before they will germinate, some are simple, others are rather complex.  All seeds need moisture to germinate.  All seeds have a preferred temperature range within which they will germinate, for some this temperature range is rather narrow while for others this range is very wide.

Some seeds require cold stratification in order to germinate, some will even need to be cold stratified several times.  Some need the seed coat damaged in some way, in nature this is usually achieved when it passes through the digestive system of an animal.  Some seeds need light in order to germinate, others need dark, and most don’t really care.  Some seeds, particularly tiny seeds such as orchids, require complex mycorrhizal interactions in order to germinate.  Some plants, particularly parasitic plants, need various chemical stimuli in order to germinate.

I have heard people say how they incorrectly cold stratify seeds and then have dreadful results, so thought I would write a post on how I cold stratify seeds.  This may not be the best method, but it has worked for me and has returned excellent results and allowed me to grow some things from seed that otherwise I wouldn't be able.

Strawberry and raspberry seeds germinate better after cold stratification

Some seeds only germinate after winter, so to get them to germinate you need to trick them into thinking they have experienced winter.  You can plant seeds and leave them out in the frost over winter to experience this naturally, or you can cold stratify them in the fridge.

Nature can cold stratify for you

The biggest mistake I hear people make is they just pop dry seeds in the fridge for a while and think they have stratified them, or they have stored seeds in the freezer and they assume that this has stratified them.  I have seen this mentioned in reputable garden magazines as well as on permaculture websites!  You can’t just store seeds in the fridge or freezer and expect anything to happen.  It doesn’t work that way.

The first thing I do is get my seeds and soak them in water.   Some people soak in weak solution of fungicides, I grow things organically so can't comment if this is helpful in any way, I just use rain water.  I soak them until I am convinced that they have all soaked up water, this may be an hour or it may be overnight.  Some people use warm water, others cold water, others room temperature water, I am not convinced it makes any difference.

Apricot seeds germinate better after cold wet stratification

The next mistake I hear people make is not labeling anything, then they forget how long the seeds have been in the fridge.  Stratifying seeds for too little time does nothing, leaving them for too long isn't great for them either.  I label a small plastic zip lock bag, I will forget what is in there and how long it has been there if it is not labelled.  I often include the earliest date that I can plant the seeds. A few extra weeks generally won't hurt them.

The next common mistake people make is using too much water.  For most small seeds I use paper towel, for larger seeds you could use sphagnum moss or something that will hold water but also allow oxygen to reach the seeds.  

I get some paper towel and make it damp.  You want it damp, not dripping as this will normally grow fungi or bacteria that will eventually kill the seeds.   I put my nicely soaked seeds on the damp paper towel and put that in a plastic zip lock bag.   I normally squeeze out most of the air from the bag, I am not sure if that makes any difference but it helps take up less room in the fridge.   Even after I have squeezed the bag there is normally always a little air remaining which is more than enough for the seeds.

Thornless raspberries, so few varieties in Australia, to breed better ones the seeds appreciate being cold stratified

I put these labelled zip lock bags in the fridge somewhere safe, the butter container in the door tends to work well.  I don’t want to lose the seeds or have them crushed by something being put on top of them.  For most seeds you do not want to use the freezer for cold stratification.  Storing dry seeds in the freezer is usually fine as the moisture content is low, putting soaked seeds in the freezer will kill some species as the water expands when it freezes and ruptures the cells.

I then leave the seeds in the fridge for anywhere between 3 and 6 weeks depending on the type, some tree seed needs even longer.  During this time I check on the seeds every now and again to watch out for early germination or issues with mould.

Pomegranate seedlings - Many temperate fruit trees require cold stratification

If they germinate early you can plant them out and are good to go.  If they start being overtaken by something you can rinse them, throw away the paper towel, and put them on new damp paper towel.

Unlike when you are storing seeds in the freezer for long periods of time (where you are trying to prevent any temperature swings) you can allow some changes in temperature while cold stratifying.  During cold stratification there is no need to try and keep the seeds cold while I inspect them as long as I don’t have them out for very long.  In nature they would experience brief warm spells, any condensation that forms on the seeds is not a problem as they are on damp paper anyway.

At the end of their time in the fridge I plant them and treat them as I would any seed.  Quite often germination can be quite fast as the embryo has already started doing whatever it needs to do in order to germinate.  My strawberry x raspberry hybrids germinated in three days after cold stratification.

I sell perennial vegetables, heirloom vegetable seeds, culinary herbs on my for sale page.  At this stage I don't sell any that require cold stratification as so few people understand how to cold stratify, but I am starting to consider if I should sell alpine strawberry seeds, or red fleshed apple seeds, or a few other things.