Seed Stratification Explained- Part 1
One aspect of growing native plants that can intimidate even the most experienced gardeners is the need to cold stratify. This process is actually quite simple and can be a fun experiment. In this first blog post, I'll explain the WHY behind seed stratification.
Plants that are native to our Northern climate are so beautifully adapted to our seasons. Seed crops on most native flowering plants will ripen in the fall, be shed by the wind or animals, overwinter in a cozy location under the snow and then germinates in the spring. But the challenges there are that the seed needs to survive our cold winter temperatures and also not accidentally germinate on a warm day, in say November. We know that our native seed are cold hardy, so they can survive the cold temperatures fine. I won't get into detail on those mechanisms, perhaps that is a topic for another post.
The second challenge is how do the seed avoid germinating on a warm winter day? Most native seed in our region have evolved a dormancy mechanism that prevents them from germinating until certain conditions have been met. For most, the built-in 'dormancy clock' will not allow the seed to sprout until anywhere between 1 week to 4 months of cold temperatures and often moist conditions have been experienced. This happens naturally for seed that spend the winter in the elements, which is why it is a great idea to plant your seed in the fall. Imitating this process yourself is called cold, moist stratification, or sometimes just stratification.
A gardener may choose to stratify seed themselves for a number of reasons. First, the survival rate of your seed when you stratify yourself will be higher since for those seed overwintering in the garden there is generally a proportion that will get eaten by rodents and birds over the fall and winter. This is why if you are sowing seed outdoors in the fall, it is ideal to wait until as late in the season as possible, ideally right before the snow flies. Some gardeners will even sow their seed in the snow. Stratifying seed yourself also allows you to start your seedlings indoors, so they can be larger when you plant them in the garden come spring. Stratifying seed also allows you to really see the germination rate of your seed and you'll know for sure if the seed was viable. The most common reason gardeners stratify though, is simply that many gardeners like to do most of their garden prep in the spring rather than the fall.
So what happens if I don't stratify my seed? Well first thing, some native varieties don't need to be stratified. They will grow on a warm summer day without any pre-treatment, just like an annual. We have grouped many of those varieties in our Easy to Grow Collection. For those varieties that need stratification however, the truth is, without stratification there is always a percent that will still germinate, it ranges between 10-25% depending on the variety. In many large restoration projects seed is applied in the spring with a cover crop added in for extra protection with the assumption that only a small percent will germinate the first year, the rest will remain dormant until the following spring. Over-seeding is often prescribed in these projects, since during this long dormancy period many seed will get eaten, blown away, rot or be lost to disease.
In my next post How to Stratify- Part 2, I will walk through some simple methods for stratifying your seed.