All the ones we grow here in zone 4 are annual. It could be determined by the zone you live in.

You didn't tell us anything about the ones you grow, so it's kind of hard to tell you what you have. There are only a few perennial kinds one being Cosmos atrosanguineus (the Chocolate Cosmos) and it wouldn't be hardy in your zone anyway.

OK, thanks! Mine are variations of pinks and whites and are 3-4' tall.

Yours sounds like the annual variety but they self slow like crazy, at least here in central New Mexico. I originally planted them from seed 4 yrs ago and now have them every year. I let them go to seed every year in case birds eat the seeds so I have a ton of them.

Yes they will self sow like crazy and I love that. I hadn't grown them in years, planted some a few years ago and every spring since I have tons of seedlings. It was fun rediscovering something so common yet so wonderful. Mine are still blooming and the finches love the seeds.

Thanks for the info on the self-seeding.

So, my husband wants to know whether he should pull them, or cut them back. If they are annuals, he should pull them, but wait as long as possible in the hopes they will self-seed?

Sorry to be so dense...I'm definitely a beginner gardener!

Not to worry... Google the images for cosmos seed head.

If you find the spikey seed heads, like those pictured, left over from the long ago spent flowers, you can pick them off and scatter the hard, brown, dart shaped seeds where you want them. You can also just cut the seed head off and drop the whole thing where you want. Or, save the seeds and plant in the spring. Then pull up the old plant since it's done for the season.

They do self seed here in cold zone 4 - not as reliably as in warmer spots, but I usually get a few that crop up in unexpected places.

By now, your cosmos plants should have scattered plenty of seed for next year. When you pull up the dead plants this fall, you'll scatter even more seed in the process. I doubt you really have to worry much about finding the seed heads and scattering the seeds yourself.

Thanks for all the great information!