probably later summer nearer august or september.
the next stuff is only a guess but it seems to ring true!
i think it depends on the plant though, but as a guide it's before the fluff has gone!
whilst it is in full bloom,
i think mabey the "nut" is the protien the plant uses to grow the seeds, so basically you have to pick the flower just before or after it's polenated certainly still whilst there is a flower to be seen!
earlier before the flower and the nut will basically be "flower petals" folded up,
during flower the plant will be gathering it's nut.
when the fluff is poping out all the nutrition has already been used and the plant is dying, the nut will be dry and worthless.
please any plant expert correct me wherever it needs it!