Hi Morvan,
back to your original question:
Yes, a great solution would be to sow a separate bed of old seed. I would probably sow side by side beds of increasing ages if you have them, and see how the germination declines if at all. Keep in mind the survival strategy of many wild populations is to have a soil-borne seed store in case any particular germination event ends in catastrophe. However the potential for any given crop to have long seed life will vary considerably. I imagine biennials probably have a bit less reliance on soil seed banks since there are likely to be both 1 and 2 year old plants in the population, and a missed year can be replaced by the subsequent generation still growing. Plants with wide dispersal mechanisms have less reliance, since the population can be replaced by new, distant arrivals next season. Desert plants where good seasons are few and far between adopt different strategies.
By way of illustration, the seed dormancy (and presumably seed lifespan) of Themeda triandra (kangaroo grass, an indigenous grain crop in Au) varies widely. Where i live it has a 3 month dormancy, so seed doesn’t germinate in summer storms, but waits for the reliable soil moisture of the autumn break. In Canberra where they get hard frosts, the dormancy is 12 months, since seedlings will be killed in winter frosts. In central australian deserts, there is no dormancy, since the plants need to exploit any rainfall event, even tho the chance of survival may be slim. While dormancy and seed lifespan are different, i can’t help but think they are related. I’m not sure of its strategy in the African savannahs where it is known as elephant grass.
I’m in my 15th year of Landrace Gardening. In that time, I could have grown 3 generations of 5 year old parsnip seeds. That would get me to the 3rd generation that so often seems magical to me.
I love planting experimental seed at the start of a row, and then the rest of the seed further along. For example, I could plant those 10,000 5 year old parsnip seeds, and see if anything grows.
My general impression, is that most seeds stay viable longer than intimated by the mega seed companies.
I figured as much. It would make sense if expiration dates on seeds are similar to expiration dates on food – way, way too early, because a) then the company can guarantee peak quality, and b) the company makes more money if customers feel the need to buy things more frequently.
Since a) is beneficial for their reputation, and b) is beneficial for their bottom line (even if they may prefer no waste, which to be clear is the case for many companies!), I suspect expiration dates are going to have a tendency to be estimated shorter and shorter with each passing decade, regardless of reality.