Image Credit:
Pixabay
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
Cosmos need a lot of sunlight to grow well. You may need more light and warmth for your seedlings to become stronger. Use a growing light placed about 6 inches above the soil if the seedlings don't receive enough sunlight.
Great article, thanks for posting!
I have very limited space, so I have to grow everything in containers. How many plants per container would you advise? And what size of container? Thanks!
You can use small to large containers. Plant 3 to 4 flowers in a small container and up to 8-10 in a bigger container. Try different varieties as cosmos come in different sizes. Make sure to remove spent flowers for a bushier growth.
After the first hard freeze, do I mow down the cosmos or do I pull them out by the roots?
Cosmos are annuals, so digging them up will be your best solution.
Thank you for your reply. My cosmos have been returning every year for years so they must be reseeding themselves - is digging them up still the best solution? I have dug them up every year, but guess I am getting lazy and looking for an easier way if there is one.
I started from seed indoors. Planted outside 8 weeks ago. Cosmos plants are thriving. But no buds at all. Non-amended soil. 6 hours sun. Spotty watering. HELP!!
Not sure what variety of cosmos you have. Give them another week or so. The plants are still pretty short. With less daylight the plants don't grow as fast as in the summer.
i grew Cosmos from seed and they are just THRIVING! I have dozens of healthy beautiful neon orange flowers, but now some of the flowers are blooming already dry and deformed that I have to deadhead them right when they open. The leaves on one plant are turning brown from the inside out. I didn't realize I had been watering them too much(once daily, keeping the soil moist, I'll cut back) I have also been misting which I will stop doing(but it was working great). I haven't used any fertilizer, just the miracle grow I first put them in. They get plenty of blazing hot Utah dry sun. I'm in the middle of the city in a condo complex so I really don't see insects as the culprit. Am I just over watering?
Yes, stop watering and misting! The general rules with cosmos are: 1) do not overwater cosmos; water so sparingly that you wait until the leaves are wilting before you water again, 2) do not overcrowd cosmos which need lots of air circulation (and you sound as if you have dozens in a small space?), 3) withhold fertilizer as they love to struggle in poor soil, 4) look for mildew-resistant varieties, 5) hope for low air humidity, 6) use well-drained soil, and 7) do not water from overhead; water at soil level. All of above keeps cosmos from getting a plant disease such as powdery mildew or a wilt. You may wish to take a sample to your local county cooperative extension or garden center to get a diagnosis.
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