I found some old Cosmos seeds that must be at least 6 years old. Do you think they will germinate ? I sowed them about 5 days ago so i am hoping, and i wont have lost anything if they dont i suppose. Any experience of old seeds ?
Worth a try (assuming you've sown indoors) - online sources say 3-4 years longevity for cosmos seeds. Plenty of time to buy & sow more if they don't germinate.
A trowel in the hand is worth a thousand lost under a bush.
The viability of seeds varies a lot from one species to another ironically size seems to be inverse except for some pea & bean seeds. Some large seed such as acorns must be sown fresh, Poppy seed (which is tiny) can last for years even in the ground waiting till soil is disturbed and then germinating. As Bob says there are lists & charts on the internet that will tell you the expected viability time.
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But I wouldn't bother trying with old parsnip or carrot seeds ... I discard them after a season ... they just don't remain viable.
Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.