Cosmos are frost-tender annuals. Sow seed outdoors after your last spring frost, once the soil is at least 65°F (NC State; UF/IFAS says germination is best at 70 to 75°F), which GardenPal estimates at about 1 to 3 weeks after the last frost. For earlier flowers, start seed indoors about 4 to 6 weeks before planting out (University of Missouri Extension) and set plants out once frost danger has passed.
Cosmos grow fast, flower from early summer until frost, and are one of the easiest flowers to grow from seed. Illinois Extension calls them very successful when sown directly in the garden, so most gardeners skip the indoor step. Two species are common: Cosmos bipinnatus, the tall pink, white and crimson cut flower (3 to 6 feet per UF/IFAS), and Cosmos sulphureus, the shorter orange and yellow type (1 to 3 feet), which UF/IFAS says handles heat better.
Planting dates by last frost
The table counts from your average last spring frost. The indoor column follows University of Missouri Extension, which gives cosmos a crop time of 4 to 6 weeks from seed to a plant ready to set out. Transplants and seed go out after the danger of frost has passed (UF/IFAS, NC State). The direct-sowing column waits for soil of at least 65°F (NC State); the 1-to-3-week offset is GardenPal's estimate, so a soil thermometer beats the calendar.
| Average last spring frost | Start seed indoors | Plant transplants out | Sow seed outdoors (soil 65°F+) |
|---|---|---|---|
| Mar 1 | Jan 18 – Feb 1 | Mar 1 – Mar 15 | Mar 8 – Mar 22 |
| Mar 15 | Feb 1 – Feb 15 | Mar 15 – Mar 29 | Mar 22 – Apr 5 |
| Apr 1 | Feb 18 – Mar 4 | Apr 1 – Apr 15 | Apr 8 – Apr 22 |
| Apr 15 | Mar 4 – Mar 18 | Apr 15 – Apr 29 | Apr 22 – May 6 |
| May 1 | Mar 20 – Apr 3 | May 1 – May 15 | May 8 – May 22 |
| May 15 | Apr 3 – Apr 17 | May 15 – May 29 | May 22 – Jun 5 |
| Jun 1 | Apr 20 – May 4 | Jun 1 – Jun 15 | Jun 8 – Jun 22 |
How to sow cosmos seed
Cosmos seed goes in shallow. UF/IFAS says to sow it about ⅛ inch deep or simply rake it into the surface, and NC State warns that sowing too deep slows germination. Choose full sun: UF/IFAS and Illinois Extension both list full sun, and NC State says at least half a day. Indoors, University of Missouri Extension grows cosmos seedlings at about 65°F; harden them off for a week or two before planting out.
Summer sowings and a late-season flush
UF/IFAS says cosmos bloom about 2 to 3 months after planting. That means you can sow again in early summer for fresh plants in late summer and fall. GardenPal's rule of thumb is to make the last sowing about 12 weeks before your first fall frost, so the plants have time to bloom for a few weeks:
| Average first fall frost | Last sowing for fall flowers (GardenPal estimate) |
|---|---|
| Sep 15 | Jun 23 |
| Oct 1 | Jul 9 |
| Oct 15 | Jul 23 |
| Nov 1 | Aug 9 |
| Nov 15 | Aug 23 |
For orange cosmos, NC State has a trick: once the plants are covered in dry seed heads, cut them back to 12 to 18 inches and they flower again in about a month.
Care: lean soil, no fertilizer
| Step | What extension sources say |
|---|---|
| Soil | Well-drained, low-fertility soil (Illinois Extension); cosmos tolerate a range of soils (UF/IFAS) |
| Feeding | Don't fertilize; rich soil gives leaves instead of flowers (UF/IFAS). Illinois Extension also links over-fertilizing and overwatering to fewer flowers |
| Height and support | Tall types may need staking (UF/IFAS, NC State), or plant them close together so they hold each other up (Illinois Extension) |
| Pinching | Pinch young plants for bushier, branched growth (UF/IFAS) |
| Deadheading | Remove spent flowers to keep blooms coming (NC State, Illinois Extension) |
| Reseeding | Cosmos reseed if seed heads are left (UF/IFAS, NC State); NC State warns orange cosmos can become weedy |
Pests and diseases
Cosmos are usually trouble-free. UF/IFAS lists bacterial wilt, canker, powdery mildew, leaf spots and aphids as possible problems, and NC State adds stem canker and gray mold. Good spacing and watering at the base help keep leaves dry.
Cosmos in the vegetable garden
The open, daisy-like flowers are easy for bees and small beneficial insects to use, which makes cosmos a good edge plant for a vegetable bed. They go in at about the same time as zinnias and marigolds; see when to plant zinnias, when to plant marigolds and flowers that attract beneficial insects. Tall cosmos can shade low crops, so put them on the north side of the bed. For an earlier, cool-season flower, see when to plant sweet peas.
Frequently asked questions
When should I plant cosmos seeds outside?
After your last spring frost, once the soil is at least 65°F (NC State). UF/IFAS says germination is best when the soil is 70 to 75°F.
When should I start cosmos indoors?
About 4 to 6 weeks before you plan to set plants out; University of Missouri Extension gives cosmos a 4-to-6-week crop time. Most gardeners direct sow, which Illinois Extension calls very successful.
How deep do I plant cosmos seeds?
Very shallow: about ⅛ inch, or just rake the seed into the surface (UF/IFAS). NC State says planting too deep slows germination.
How long do cosmos take to flower?
About 2 to 3 months after planting (UF/IFAS).
Should I fertilize cosmos?
No. UF/IFAS says fertilizer can stop them flowering, and Illinois Extension says over-fertilizing and overwatering mean fewer blooms.
Can I plant cosmos in summer?
Yes. Because they bloom 2 to 3 months after planting (UF/IFAS), GardenPal suggests making the last sowing about 12 weeks before your first fall frost.
Do cosmos come back every year?
They are annuals, but they reseed if you leave some seed heads (UF/IFAS, NC State).
Check which vegetables you're planting near your cosmos border, and spot any poor pairings before you plant.
Open itSources
- Virginia Cooperative Extension, "Companion Planting in Gardening" (SPES-620P)
- University of Maryland Extension, "Vegetable Succession and Companion Planting"
- Oregon State University Extension, "Companion Planting"
- Penn State Extension, "Herbs Make Good Plant Partners and Companions"
- University of Minnesota Extension, "Planting the vegetable garden"
- USDA Plant Hardiness Zone Map
- NOAA NCEI U.S. Climate Normals (frost/freeze data)
- UF/IFAS Gardening Solutions, "Cosmos"
- NC State Extension Gardener Plant Toolbox, "Cosmos bipinnatus"
- NC State Extension Gardener Plant Toolbox, "Cosmos sulphureus"
- University of Illinois Extension, "Cosmos" (flowers)
- University of Missouri Extension, "Starting Plants Indoors From Seed" (G6570)
Pairings are summarized in our own words from extension-service guidance. Research support for many companion effects is limited, so treat them as good habits, not guarantees.