When to stop deadheading?

SYinUSA

I have several plants I would like to leave to self-seed. How do you decide when to stop deadheading for the season and let the seedheads develop? Is it a certain number of weeks before your first frost date? Or should the self-seeding be done earlier in the season so it can grow before the frost?

Dovefromabove

I would make my decision early on and leave the best few seedheads on the best plant (tie a piece of bright coloured knitting wool around the stem in case you absentmindedly forget your plans).  That way there’s plenty of time for the seeds to swell and ripen properly before you harvest them. 

Then you can either sow immediately and get good sturdy young plants before the winter, or dry them properly and sow in the spring.