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
I've started to withhold watering until the very last minute. I have done no fertilizing. The first blooms are still going strong but new blooms are still struggling and I am constantly deadheading. I believe the crowding is the issue. There are about 4 plants in each grouping about 12 inches apart from each other so it is pretty crowded in there. I will completely seperate them if needed
My Beautiful Cosmo Plant is dying. I put in my flower bed with fertilizer it was growing pretty nice unti now its really dead the steams are turning brown and their dying
what can I do?
Be sure that you are not overwatering. Cosmos prefer to dry out in between waterings.
Also, check to make sure that they aren't crowded and that there are not aphids destroying your flower.
I let my little girl plant the seeds, while i was doing other things in the garden and told her to only put 2 in each hole and she put 4 in each. The are four big healthy plants coming out on each section. The majority of the flowers are just beautiful but some are coming out dry and dead now. I'm going to cut back on watering and see if it helps. Thanks
Well I just read in my garden magazine that they do not like fertilizer. In fact it says they grow best in crummy soil as long as it's well drained!
That is correct! 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. Do not baby them at all. They want no lovin'.
My cosmos seem to be fairly healthy but a few plants have leaves that have rolled up. What's wrong?
Check underneath the leaves for bugs; if you see aphids, spray them with insecticidal soap. Also be careful not to overwater cosmos. Water sparingly and at the soil line (not from overhead); they can take the sun and heat. Let them dry out completely between waterings and make sure they have enough air circulation and aren't crowded.
our cosmos are growing nice and tall. We are fertilizing them but they haven't bloomed yet. Keep the soil moist and gettting good sun. Any suggestions?
Do not fertilize cosmos. They need "tough-love" -- very little if any fertility. And water sparingly. They want blazing sun and like it HOT and DRY!
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