Many think that seeds are something you only sow in the spring. And for most of the annual vegetables, this is also the right time to sow. However, when you start working with perennial plants in a cold climate, the question of stratification will soon arise.
There is a whole range of plants that needs a cold period before they will sprout. Mainly this is plants that grow naturally in the colder parts of the world, where the winter is a natural part of the years’ cycle. Without this period of cold, these seeds just will not sprout. There is an intrinsic knowing in the genetic code of the seeds that they should not sprout until they have experienced a period of cold. This is part of ensuring the survival of the plant. If the seeds spout late in the summer or early fall, they will not have the time to grow strong enough to survive the winter. Only if they sprout after the winter, they will have a long enough period of warm weather to grow strong before the next winter arrives.
Some seeds are fine to plant directly outside in the autumn. This year I have for instance collected seeds of wild clover and plantain. These I have spread on areas in the garden where I would like to have these plants. Clover I want to be a ground cover between some of my raised beds, and the plantain is a medicinal plant what I would like to have access to in my garden. It grows wild along the roads and walking paths here, but I am slightly worried about picking leaves from those plants as there are lot of dogs in this area.
Then there are other seeds that are more delicate, that you would want to give a period of cold exposure in your fridge instead. Then plant the seeds inside in appropriate containers to grow strong before you plant the small plants outside. This way you give these plants a better chance of surviving than if they would have to compete with weeds and fight of hungry slugs’ form day one.
I tried cold stratification for the first time last season. I then had seeds of beach cabbage (crambe maritima) and hablitzia (hablitzia tamnoides). The beach cabbage has large pea sized seeds, and these came up fine. I transplanted some of these plants outside in the garden, and some I have on my balcony in pots. The hablitzia has tiny seeds and I found them difficult to germinate. I think I drowned some of the seeds, and only one plant seems to make it. I am overwintering this on my balcony, and hoping it will come back next spring. The beach cabbage is a perennial cabbage you can use as any other cabbage. The hablitzia is a climbing perennial plant with leaves that you can use in the same way as spinach.
This year I have seeds of the cold hardy kiwi (actinidia arguta) which also needs a period of cold stratification. These seeds needs to be thirteen weeks in the fridge, and I am just about to put them in now end of October or beginning of November. Then they should be ready to plant around the end of February next year. The cold hardy kiwi is also a climbing wine that produce small fruits about the size of goose berries, tasting slightly like kiwi. These plants I will keep on my baloney the first winter for extra protection, before planting them out in the garden the second season.
There are different technics to use when you stratify the seeds. Some seeds are fine to put in a small glass with water. I have done this several times with apple pips. After a few weeks they actually germinate and sprout in the water. Then they are ready to be planted in pots with soil. With small seeds like the halblitzia or the cold hardy kiwi, it’s easiest to place the seeds in a plastic bag or a small container with some soil, in the fridge. It’s important to ensure the soil is moist, but not soaked. Keep checking the soil to avoid any mould growing. When the seeds have been long enough in the cold, you just spread the soil with the seeds on top of a planting tray or any other container with soil for the seeds to germinate and set roots.