Image Credit:
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Botanical Name
Cosmos bipinnatus
Grown as
Toxicity
Sun Exposure
Water Needs
Height
Spread
Soil pH
Resilience
Plant Type
Planting
When to Plant Cosmos
- Direct-sow seeds outdoors once the danger of frost has passed.
- Alternatively, sow seeds indoors 4 to 6 weeks before your last spring frost date in trays or pots with a good seed-starting mixture. Move them into 5-inch pots as soon as they’re 3 or 4 inches tall.
- Young plants (transplants) can be planted outdoors once the danger of frost has passed.
Where to Plant Cosmos
The soil should be well-draining, but other than that, cosmos don’t need any special soil preparation. In fact, they like soil that is not too rich, as rich soil will encourage foliage at the expense of blooms. Cosmos are also tolerant of most soil pH levels but grow best in neutral to alkaline soils (pH of 7.0-7.5).
Cosmos can tolerate warm, dry weather very well. They are even drought-tolerant.
How to Plant Cosmos
- Just sow seeds lightly—no more than 1/4-inch deep.
- Thin to 12–18 inches apart when seedlings are a few inches tall. Depending on the variety, cosmos can grow anywhere from 18 to 60 inches tall, so plan accordingly.
- If you are growing cosmos from seeds, be mindful that it takes about 7 weeks to first bloom. After that, your flowers should continue to bloom until the first fall frost.
- If you let the spiky-brown seed heads blow away during the fall, cosmos are likely to self-sow throughout your garden.
Growing
Watering
Water regularly until plants are established or if it is unusually dry. According to the North Carolina Extension, “Be cautious not to overwater your plants. That and over-fertilizing can lead to the plant producing fewer flowers.”
Cosmos can tolerate dry soil, even in a hot, arid, sunbaked spot.
Deadheading
To prolong flowering, you should deadhead the plants (prune off dead/faded flowers). This speeds up flower development and aids branching.
Supporting Cosmos
Because some of these plants can grow really tall, staking may be necessary. Offer them protection from strong winds, encourage balanced branching by pinching out (aka pruning) central shoots or stem tips, or stake them. Growing them against a fence can also provide the necessary support.
Seasonal Care
Cosmos beds may become weedy due to the fact that they self-seed, so remember to remove flowers before they go to seed or to thin seedlings in the spring.
Harvesting
Harvesting Seeds and Cut Flowers
- To harvest more seeds, remember to leave a few flowers on the plant because they will self-seed.
- You can cut the flowers off anytime after blooming, but it’s best to pick some right when the petals have opened.
- If you cut the blossoms when they first open, they’ll last more than a week in water. Strip off the lower leaves and put them in a vase.
Types
- ‘Picotee’ has pretty white flowers with a crimson border around the edge of the flower (some are also flecked in crimson).
- ‘Sea Shells’ have white, pink, or red tube-shaped petals.
- ‘Purity’ is a beautiful, classic, white flower that looks spectacular growing against the dark-maroon ‘Niger’ hollyhock or purple basil.
- ‘Cosmos Sonata Series’ looks especially good with Bells of Ireland.
- ‘Chocolate’ cosmos adds distinction to a white campanula, dianthus, or ‘Moonshine’ achillea.
- Tall cosmos are best for the back of the garden and filling in mixed borders among perennials such as foxgloves, phlox, delphiniums, and daisies, as well as annuals such as poppies, cleome, and asters.
- Dwarf cosmos, such as the Ladybird series, are great companions for zinnias, Johnny jump-ups, pansies, alchemilla, or dark-blue morning glory.
More Like This
I planted a combo of seeds purchased from a wildflower farm in Texas in March. The seeds came up but it appears I have two different types of Cosmos. The smaller / shorter plants ~ 2 feet tall are blooming. I also have another version that have very thick stalkes ~ 1" dbh and approximately 4' tall. None of the larger plants have bloomed. They receive full sun and the plants look beautiful but no flowers. ?? Help ??
Cosmos are great for Texas gardens. For more bloom, hold back water and any fertilizer. Give them all-out, blazing sunlight and no shade. They like to struggle and are very drought-tolerant.
You may be babying the cosmos. They thrive on neglect--heat, lack of water, poor soil, etc. Don't feed them a high-nitrogen food or they won't bloom. Withhold water to the extent that you can. Keep giving them sunlight.
when I withhold water they get droopy and seem to shrivel pretty bad. Is that ok? How much sunlight do they need?
Cosmos like full sun (at least 6 hours of sun daily). If the plants shrivel give them some water.
I recently purchased a small Cosmos plant that had a small bud on it, however, I live in a flat so I can't put it outside. What should I do?
Cosmos make great container plants so it should be all set as an indoor plant.
However, make sure the plant receives lots of indoor light!
Thank you for your interest in the Old Farmer's Almanac and our Web site.
I am wondering if they are candy for the deer. Hopefully they are deer resistant. Please someone. thank you
We like to say that there's no such thing as a truly deer-resistant plant, however, deer will usually pick other flowers over cosmos due to the stiff stems and less appetizing leaves. Cosmos is NOT candy. See our top deer-resistant plants here: http://www.almanac.com/content/deer-resistant-plants
I started my cosmos from seed in March, which now I'm thinking was a little too early for these guys. When I went to plant them in pots they had long lanky stems. As of right now they kind of look like ground cover because they can't stand up. Should they lie of the ground like that or should I prop them up? Thanks!
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