After-ripening
There are so many interesting seeds-related subjects that sometimes it is hard to choose what to write, or the time is too short.
An email arrived yesterday saying:
“I thought I’d better drop you a line before I forget. Have been now been giving most of my north american seeds at least a week of moist and warm temps before chilling. Today, after less than a week of 65 to 70 degrees f I have 25 yellow asclepias germinated and now in cells. I have had such luck with Asclepias tuberosa that I have collected and germinated so I am not shocked but especially since these are the yellow form, I did not expect such germination.
Most impressive, Thank you, Regards Catherine”
My answer was, besides thank you and so on… that the very good germination at room temperature may be due to the ‘older’ seed lot from 2020. (Last year there were no fruits on the yellow Asclepias). Maybe others are also familiar with the knowledge that ‘older’ Asclepias seeds germinate better than fresh ones, which could be explained by the after-ripening (post-maturation) requirement.
I don’t know for sure of this is the reason; usually I recommend to sow Asclepias species in late fall/early winter because there are some which require a cold/moist stratification for good germination (A. exaltata for example). If they don’t, it doesn’t matter, they will germinate in late spring when it gets warm.
In any case, it is a good time to discuss a bit about the AR.
First, what is this after-ripening? Some define after-ripening (AR) as “a time and environment regulated process occurring in the dry seed, which determines the germination potential of seeds”.
Very simple, I would say that after-ripening is a process through which dry seeds are ‘preparing’ themselves to get out of dormancy and germinate, if and when the right conditions arrive. The AR involves the loss of water content of the seeds up to certain point which naturally happens in nature after the seeds ripen and fall on the ground/remain in the dry fruits.
Not all dry seeds species require an after-ripening period. It is known for sure that many many grasses require AR, but otherwise we do not have a comprehensive list of species that require AR. I only know about Asclepias, Polygala, and various Brassicaceae species.
AR is a complex process during which various physiological and molecular mechanisms are taking place within the dry seeds. If for some reason AR is not complete, the seeds will not germinate even if they are placed in proper condition (water, light/dark…); instead they will remain dormant (eventually they will germinate, but give them time…).
It would be impossible to approximate the exact duration for AR I guess, considering it cannot be the same for all species. Usually seeds collected in the summer and kept dry at room temperature should have enough time for after-ripening until late fall/early winter.
It has been shown that increasing the temperature at which seeds are after-ripened generally increases the rate of dormancy loss, but on the other hand, at high temperatures, the seed viability loss accelerate. So, it is a give and take if we want to manipulate AR or store the seeds long-term.
Conclusion:
Just like many seeds require to be sown right away (those recalcitrant ones), others require a ‘quiet time’ by themselves :) in a dry state, to be ready for the germination.
For gardeners and small operations, after collecting the seeds it is best to let them dry in paper bags in the house until late fall (in contrast to putting them in the fridge right away!). Exception: fleshy fruits/seeds which need to be cleaned first.
And yes, the after ripening requirement could be another reason that sometimes we stare at an ‘empty’ sowing pot wondering what went wrong.
Not one boring minute when dealing with plants and seeds :)
*I am always happy to receive good/interesting germination updates about the seeds you received. If you have bad news, it is fine as well. Maybe we can figure what went wrong together.
