In message , Ian Wilson
writes

The "ask a scientist" page was interesting. A study of the length of
time seeds are viable would be interesting, but is going to be too long
for my disertation. My main study will be next year & will be based on
axil pools. For now though I may spend some time this autumn trying to
get a clear correlation between flower head size and seed
numbers/viability. At some point flowering heads seem to become too
small to produce fully mature looking seeds. If I can get a good
correlation it might even reduce my counting work next autumn!! I'm
expecting to need at least 10 or 12 plants in each of 3 groups. If
plants really do have 3,000 seeds each I might be looking at 90,000
rather hard to extract seeds to count next autumn - so any sort of
short cut I can devise would be wonderful!

Once you have an average weight for seeds you can clean the seeds from a
plant and weigh them. (The problem is non-random variation is seed
weight, i.e. between plants and between flower heads.)
--
Stewart Robert Hinsley