Cold stratification – part 3, the practicals

Cold stratification – part 1, dormancy
Cold stratification – part 2, FAQ

Let’s dive into the methods used for providing cold stratification as a seeds germination treatment, drawing from my personal experience. They often overlap with the sowing methods (also mentioned for seedlings winter protection). It makes sense because all are interrelated .

Natural cold stratification = sowing in the ground (I also call it sowing ‘in situ’)

This is the easiest method of providing cold (moist) stratification for seeds. We actually reproduce what’s happening in nature!
This type of sowing/stratification can be done until the ground freezes, so probably early November for our region would be safe to give as a limit. In case of  ‘in situ’ summer sowings (Corydalis, Asarum, Hepatica…) you will ensure first a natural warm/moist and then a cold stratification.
Incorporating potting mix in the top layer of sowing will provide a better substrate for the future seedlings.

Cons: selecting a proper location is the key to this method; it should be good for the germination and further growth of the plants. In this case we usually do not aim to transplant the young seedlings.
The seedlings are prone to be easily damaged in first stages and so their mortality may be higher than in pots. Sowing seeds in larger quantity it is necessary in order to establish young plants.

Despite the cons, many wildflowers are self-seeding with success in nature, and even in the garden. Lobelia siphilitica, Asarum canadense, Corydalis, Hydrophyllum, Geum rivale, Iris versicolor, Lathyrus vernus, Primula mistassinica to mention just a few, are regularly self-seeding in my garden.
In case of hemiparasitic species it is definitely worth trying. Although I had Castilleja species germinating well in pots, a flowering plant was obtained one year only after sowing seeds directly in the garden.

Another nature inspired method involves Providing cold stratification in moss (outdoors).
This was shown for Gaultheria procumbens, and it involves sowing the seeds in moss, in the garden or on a piece of moss placed in a pot outdoors. The moss will provide protection and constant humidity for the small seeds and seedlings.

For those more creative, a piece of decaying stump or a boulder are good opportunities to try and establish moss and then sow suitable species.

This method can be tried for Gaultheria, Coptis trifolia, Trientalis borealis, Maianthemum canadensis, Cornus canadensis, shade growing Saxifraga, Rhododendrons and probably many others. In nature, we always see young Hepatica americana and Gaultheria procumbens seedlings germinated in moss patches.

Cold stratification in pots or larger containers buried in the ground

In this case we sow in pots or in larger containers and then place them in the ground. We can start with the first summer sowings and continue until late fall before the ground freezes. You can use an ’empty’ area between the garden plants, or organize a special place in the garden ‘out of sight’.
Sowing in a larger container works very well for species with the same germination requirements that need to be grown in the same place for 2-3 years.
Before the winter, add a layer of dry leaves/mulch for extreme cold protection; it will also help compensate for the lack of snow (if).
Avoid low areas where a lot of water might stagnate in early spring after snow melt!

Cons: the location to keep the pots for the winter may not also be the best for the germination and growth of the seedlings; you may need to move them in the spring in another location. For this reason, many times I keep the pots buried for the winter and then move them as soon as the spring arrives. It adds as extra work to be done.
Summer sowings need constant monitoring/watering and being ‘buried’ in the ground, makes it easy to forget about them.
The seedlings are also more prone to damage in early stages.

Cold (moist) stratification above ground

For those who prefer the ‘cleaner’ or classic method, sow in pots/containers and provide the cold stratification above ground.
We need something to keep the pots together, hence the box method, easy to bury in the snow during the winter.

Any other container type, small or large, plastic trays, work well to keep the pots in place. Over the years I have also used plastic (nursery-type) trays with a mesh on top, but that came with the challenge of fixing the mesh. Another option is to wrap the trays in a plastic sheet, which works pretty well.

As previously mentioned, make your cold storage pile in a shaded area to make sure the snow remains compact and doesn’t melt during our rare sunny days. Be creative in using your garden space: underneath a patio table, a bench, beneath eavestroughs or a large evergreen tree…This way the pots will also be protected from occasional heavy rains, an important fact to keep in mind in the spring when the emerging seedlings are very fragile.
Cons: snowless winters can be problematic for protecting the pots from extreme cold.
The pots will need to be handled/moved around multiple times. 

If you want to grow a lot of plants/species, a classic frame or a simple wood framing are very valuable in our climate. They do not need to be fancy and have multiple advantages.

Place your pots inside tight together and towards late fall add some insulation along the edges: plastic sheets, bubble foil insulation…); on top better to add a thin fleece and a plastic foil.  A lid is beneficial and it can be easily improvised, by making a light frame with plastic in between.  This is what I used when I had my very simple, home-made frame and it worked well.

The advantage of a frame is that you can easily add bubble sheets and tarps on top of the frame to compensate for the lack of snow. You will still find the pots frozen when first opening the frame, but apparently the temperature remains acceptable. Various seeds always germinated well and all young seedlings survived.
A frame also provides a good place for keeping all your pots organized together: sowing and seedlings during the growing season (a shading cloth/anti-critters mesh to be used during the season).

Cons: a limiting factor in this case is having a proper, large enough space with good exposition for installing the frame. Avoid a south facing location because during the summer it will get too hot for small size seedlings and the pots will dry faster.
For people with mobility issues, working with a ground frame may pose problems; there is a lot of bending and crouching involved to oversee the pots.

Cold (moist) stratification provided in the fridge

In this purpose we place the seeds between moist paper towels enclosed in plastic bags, in moist media (vermiculite, perlite…) or keep whole pots in a dedicated fridge/shelf. This method was advertised a lot as a space saving method and it looks ‘attractive’. I employed it for a while, discovering its many disadvantages in the process.

Cons: the seeds need to be inspected frequent, especially in moist towels. Even if you pay a lot of attention, you may find germinated seeds with the roots entangled in the moist towel, or even worse with etiolated shoots. 

Most important, most seeds do not germinate uniformly and they would need potting up when/as it happens; damage to the rootlets when potting up is also a factor.
Poor germination results are also possible due to the constant fridge temperature, and also because of the molds sometimes forming on the seeds (when using moist towels).

For these reasons I now consider this as a last method for cold stratification by those living in a cold climate. In any case, stratification in moist media is preferable than using moist towels, with the exception of very fine seeds.

I recommend the fridge stratification only if it is too late to provide the stratification outdoors, to keep seeds moist at cold for a very short period, or when a reliable method that works well has been developed, like for example: How to easily germinate Hydrastis canadensis.

***Special attention is needed for the seeds that were sown early in the season like: Hepatica, Sanguinaria, Corydalis, Jeffersonia… in fact the whole category of the ‘moist-packed’ seeds which have been discussed ad nauseam in this blog.

But let’s not forget that other species also require first a lightly warm cycle followed by a cold stratification in order to germinate well: most Paeonia species, Helleborus, Actaea, Aralia, Hydrophyllum, Viburnum…

Keep in mind that in this case, by fall there are radicles/tiny rhizomes growing in the pots!!! (the lightly warm/moist cycle), even if we do not see any growth above the potting mix.

A cold stratification it is then needed for the shoots to be induced into growth. For this reason I sometimes called them ‘double stage germinators’ (easy to remember what’s happening). 

The best methods for cold stratification for these species are: pots or containers buried in the ground or a cold frame.

 

Next in Part 4 –  spring arrives: what to expect and how to handle the start of germination following all the shown methods.