Do cosmos reseed themselves? Do I take off the spent blooms or leave them to dry and fall off?

Most cosmos reseed themselves but not always reliably or true to the variety. The common cosmos in Michigan is an annual, so the best way to have nice plants that are the color and height you want is to sow new seeds each spring when the ground has warmed to 70 to 80 degrees. Or you can purchase plants from local garden centers. Plant them in full sun in the back of the garden where a fence or other plants can hold them up. They tend to flop in wind or heavy rain, and it is difficult to stake them because of their thin stems. In her book Annuals for Michigan, Nancy Szerlag suggests placing twigs around the young plants for support that will be covered when the plant is older. The plants germinate easily in the garden with only 1/8 inch of soil covering them. They are very drought tolerant and don’t need much, if any, fertilizer for the whole growing season.

To keep the plants blooming throughout the summer it is best to remove spent blooms. It also makes the plant look neater. If they get too tall and blooms are weak you can trim them back to half their height. They will grow back bushier to bloom again later in the summer. You can selectively cut them back, so there are still some blooming while the others grow back. If you want to encourage reseeding (and that means anywhere in the garden with help from the wind and critters), leave the seed heads on in the fall. The finches and other small birds love the seeds, and what they don’t eat may reseed.