There isn't one answer, Kate. Some seeds like to be sown fresh, while others need stratification (a period of cold) and others need to wait until the spring.

Poppies and Coreopsis - both spring. Alcea - sorry, not sure.

I can recommend a super book called 'Seeds' by Jekka McVicar if you want really excellent advice on growing plants from seeds.

My theory is that if the seeds have been left alone in the garden they would have fallen to the ground and been 'sown' now. I sow all seeds as soon as I get them; sown into pots and left outdoors until they germinate. Any that germinate quickly at this time of year will be taken into the unheated greenhouse when the winter gets really bad.

I always do 50/50 - keep half and spread the other half where you want the seeds to grow. Either way you will gain something. Self germinated seedlings tend to be stronger in the long term than ones germinated in spring, even if slower to grow.

Thanks for the great info everyone :)