I only realize it's time to harvest seed when the goldfinches go after the heads ;)

When the heads turn brown and dry that is when it's time to harvest the seed. The trick is to get them before they open and spill the seeds. I usually get them when I am dead heading, some of them are ripe and I'll collect the seeds from them. I dead head most of them to keep them blooming, but there are always plenty of seeds. I grow Coreopsis Early Sunrise they will start blooming any day now and I can collect seed all summer long.

Kaydee, I looked at mine this morning before leaving home - I left the plant stick in it for identification and it is the Early Sunrise variety also. If left alone, will they reseed on their own and spread?

I love this whole seed gathering thing, I just have a lot to learn

Coreopsis seeds are fun to gather. Let the head go brown. It will open up like a cup and the seeds will spill out into your hand - you have to move fast before they all take off. The part of the debris that are seeds are the disks that look a bit like a turtle shell - just a little cup with a darker spot in the middle.

Coreopsis like plains coreopsis will have smaller "shells" more like a half moon sliver. Once you find your first batch, you will understand the process and have a ball harvesting.

Personally, I like to winter sow my coreopsis for more control over the sprouts. I'm not sure mine really re-seed on their own that much.

I am in zone 6b NYC and mine do reseed. In the past couple of weeks I have been pulling out the volunteers. I don't get that many because I dead head quite regularly and I collect the rest. The few I miss come up the next year. You really have to dead head this plant so it will continue to flower through out the summer.

Excellent point. When the first flush of blooms fades, I whack the whole thing with my clippers and wait for the second flush.

I don't know why mine don't re-seed that much. Perhaps because I have them in drought conditions and packed in a cottage garden setting.