Most cosmos are annuals, so after all the blooms are spent this fall you can cut them back/pull them from the ground. Cosmos self-seed so if you do not want them to grow in the same area, make sure you remove spent flower before they go to seed. If you want them to remain in that area of your yard or garden, you can allow the flowers to remain on the stalks so that the seeds fall to the ground and then remove. You can also remove the flowers and save the seeds to plant in the spring, either indoors 4 to 6 weeks before your last frost date or directly into the ground once the threat of frost has passed.
Most cosmos are annuals, so after all the blooms are spent this fall you can cut them back/pull them from the ground. Cosmos self-seed so if you do not want them to grow in the same area, make sure you remove spent flower before they go to seed. If you want them to remain in that area of your yard or garden, you can allow the flowers to remain on the stalks so that the seeds fall to the ground and then remove. You can also remove the flowers and save the seeds to plant in the spring, either indoors 4 to 6 weeks before your last frost date or directly into the ground once the threat of frost has passed.