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
The botanical name for the orange variety is Cosmos sulphureus. Most cosmos varieties develop seedheads if you leave the faded flowers on the plants. The seedheads may look a little different—spiky—but you'll be able to see and collect the seeds after they have dried.
The seeds are in the center of the flower. As the flower dries they start to separate and you can see them quite clearly about an eighth of an inch long. Let them dry right on the flower but make sure you get them as they start to open up and separate. Put the flowers into a paper bag and shake out the seeds.
There is a wealth of knowledge in the comments by editors and gardeners below. We invite you to peruse it.
Cosmos are aggressive reseeders, so it's likely that you've got some seeds in the pot already. However, if the pot has had cosmos this year, you're better off starting with fresh soil next year. This way, you won't risk the continuation of pests and disease, and next year's plants will have plenty of room and nutrients to help them grow their best.
To save seeds, watch for seedpods to develop on the plant and snip them off just before they a start to look dry. Keep these in a paper bag in a cool, dry place and spread the seeds once again in the spring.
Lack of blooming could be caused by too much water, too little light, or too much fertilizer. We're not personally familiar with the climate in Singapore, but if it's too humid and wet, the cosmos may struggle. They prefer to be grown in well-draining soil in full sun.
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