Sulphur Cosmos

Year-round

Sulphur cosmos (Cosmos sulphureus) is native to Mexico and Central America, not to Ohio, so in Columbus it is grown as a garden annual rather than a native planting. It is a fast growing, heat loving plant that fills out into an upright clump covered in gold, orange, or yellow daisy like flowers from summer until the first fall frost kills it. It draws butterflies and other pollinators to the yard, and it is not on the ASPCA toxic plant list for dogs, cats, or horses.

• Give it a full sun spot with at least six hours of direct light a day.
• Plant in average to poor, well drained soil and skip the fertilizer. Rich soil makes it flop over instead of bloom.
• Deadhead spent flowers regularly to keep new blooms coming all season.
• It self seeds freely, so expect volunteer plants to pop up nearby next year.
• Not on the ASPCA toxic plant list for dogs, cats, or horses.

Fall

Sulphur cosmos keeps blooming right up to Columbus's first fall frost, which usually arrives in the second or third week of October. Once a hard frost hits, the frost tender foliage blackens and dies quickly, ending the plant for the season since it cannot survive an Ohio winter. Many Columbus gardeners leave some seed heads standing into October so birds can feed on them before doing a final fall cleanup.

• Expect blooms to continue until Columbus's average first frost, roughly mid to late October.
• After frost blackens the foliage, pull or compost the dead plant. It will not come back in spring.
• Leave a few seed heads standing into October if you want to feed birds like goldfinches, then clear the rest during fall cleanup.
• Let some seed drop if you want volunteer cosmos next year, or remove all the seed heads first if you would rather not.

A blackened, frost killed cosmos plant reads as unkempt from the street, so plan to pull it during fall cleanup rather than leaving it standing through winter.

Winter

Sulphur cosmos is a frost tender annual and will not survive a Columbus winter, so there is no live plant to tend once cold weather settles in. If you let some flowers go to seed in fall, those seeds can overwinter in the soil and sprout on their own next spring. Otherwise the bed is simply open and ready for you to plan next year's planting.

• The plant is finished for the season. There is nothing to water, prune, or protect over winter.
• Seed that dropped in fall can overwinter in the soil and self sow again once the ground warms next spring.
• If you do not want volunteer cosmos, clear any remaining seed heads and mulch the bed before winter.
• Store any seed you collected in fall in a paper envelope indoors if you want to control where you replant.

Clearing the spent bed before winter keeps it from looking neglected through the cold months on a street-facing yard.

Spring

In Columbus, wait to plant sulphur cosmos outdoors until after the average last spring frost, which typically falls between late April and early May. Direct sow once the soil has warmed, or get a head start by sowing indoors four to six weeks before that date and hardening seedlings off before they go in the ground. Pick the sunniest bed you have and hold off on rich soil or fertilizer at planting time.

• Direct sow or transplant outside only after Columbus's average last frost, roughly late April into early May.
• Cover seed lightly with about 1/8 to 1/4 inch of soil and keep it evenly moist. It germinates in 7 to 21 days.
• Thin or space seedlings 12 to 18 inches apart once they have a few true leaves.
• Choose lean, well drained soil over compost heavy beds. Cosmos blooms best when it is not overfed.

Summer

Sulphur cosmos hits its stride once Columbus summer heat and humidity arrive, blooming steadily from June into fall. Once plants are established they handle dry stretches without regular watering, though taller varieties can flop in a summer storm and may need light staking. Keeping up with deadheading through the hottest months is what keeps the plant producing new flowers instead of going to seed early.

• Deadhead spent blooms every few days to keep the plant flowering through the heat.
• Stake taller varieties, over about 3 feet, before a big summer storm. The stems are hairy but not especially sturdy.
• Water only during extended dry spells. Established plants tolerate Columbus's humid summer heat well on their own.
• If a plant gets leggy and stops blooming well, cut it back to 12 to 18 inches and it will rebloom.

Regular summer deadheading and staking keep sulphur cosmos upright and tidy instead of sprawling, which matters most on beds visible from the street.