Hi Leo:
As an alternative, why not follow USEFUL instructions??'

These instructions of potting, refrigerating, whatever, are new to me.
Just sow upon receipt by pressing the seeds in individual pots in agood
commerical compost and leave themn alone in a preferably frost free place
with not much direct sun. By the summer the tuber will be dormant. If you
have room you can plant in the ground your next autumn before it sprouts
and there it will stay for the next 40 or more years, provided you can
give it "Cape bulb" conditions. But, it is a large plant, it comes from a
frost free region, it can take slight frosts with no harm but in an
exposed chilly spot the plant will grow slowly and will not flower in the
second/third season as would be normal.

Finally, they come from a year round rainfall region and the dormant
tubers receive some rain in summer. This also implies that it is best to
sow the seed without delay although they will of course germinate with
the onset of cool weather in autumn. The tubers, so much Dahlia like are
extremely fragile hence at first it is safer to sow the seed
individually.