By late September the sunflowers that were still gold in August have gone over: petals dropped, heads bowed, the back turning from green to brown. That is the seed ripening, not the plant failing. Left on the stem much longer it gets stripped by birds or spoiled by the first heavy rain, so this is the point to bring the heads in and finish drying them somewhere controlled.

Save Sunflower Seed from Dried Flower Heads

What you need

3 paper bags large enough to cover a seed head
1 ball garden twine
4 paper seed envelopes for storing dried seed by variety
1 marker pen for labelling variety and date

Step by step

Look at the back of the flower head rather than the face. It's ripe when it has turned from green to yellow-brown and gone papery, the petals at the front have wilted and dropped, and the seeds themselves are plump and striped rather than pale and flat.
5 min
If a head isn't quite ready, slip a paper bag over it and tie the neck loosely around the stem below the head with twine. This keeps birds and squirrels off the ripening seed without stopping air getting to it.
10 min
Once ripe, cut the stem around 30cm below the head with secateurs, leaving enough length to hang it by.
5 min
Hang the head upside down somewhere dry, airy and out of reach of mice — a shed rafter or garage beam works well. Tie a paper bag loosely around it to catch any seed that drops. Leave for two to three weeks.
10 min
Once the back of the head is fully brittle, hold it over a tray and rub the face firmly with a gloved hand or a stiff brush to knock the seeds loose.
15 min
Pick out any shrivelled, pale or mouldy seed and discard it. Spread the rest in a single layer on the tray for another two or three days so any residual damp evaporates before it's sealed away.
10 min
Pack the dried seed into paper envelopes, one per variety, and write the variety and date on the outside. Keep them somewhere cool and dry until sowing time.
5 min

Afterwards

Compost the spent heads and stems once you've stripped what you want — broken up, they rot down quickly. Check stored envelopes again after a week; if any feel damp or smell musty, tip them out and re-dry rather than let the mould spread to the rest. Sown seed germinates best within a year, so use the oldest envelopes first next spring.

Questions

Can I save seed from any sunflower I'm growing?

Only from open-pollinated varieties. F1 hybrids, common in bedding-pack sunflowers, won't come true from seed — the resulting plants are unpredictable, though usually still fine for feeding birds.

Do I need to remove the husks before storing?

No — leave the seed in its husk. It keeps better that way and you can hull it as you use it.

What if it rains before I get to the heads?

A light shower won't ruin a ripe head, but leave a soaked one somewhere warm and airy for a day before bagging it, or it will mould instead of drying.

Can I just feed the seed straight to birds instead of storing it?

Yes — a dried head can be hung whole as a winter bird feeder with no processing needed.