Photo Credit
Pixabay
Botanical Name
Cosmos bipinnatus
Plant Type
Sun Exposure
Soil pH
Special Features
Subhead
Easy-to-grow annuals that thrive in poor soil and bloom from summer to fall.
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Planting
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.
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.
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
- To prolong flowering, you should deadhead the plants (prune off dead/faded flowers). This speeds up flower development and aids branching.
- 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.
- 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.
- 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.
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.
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.
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The cosmos stems are lanky because they didn't get enough light as seedlings. They are warm-weather annuals and you need lots of indoor light. At this point, can you direct seed? I'm not sure where you live but perhaps the seedlings will germinate at the right time and you'll have sturdy stems. Seed in FULL sun!
I am striving for a self efficient garden and have planted seeds all around my yard and fence to bring in the Praying Mantis. I'm a bit impatient too as bugs have taken over my plants and yard for two years with spraying being non effective so I bought some egg sacks too.
Praying Mantis travel at night too so leaving lights on outside for a bit helps to bring them in.
I am dead heading the dead and dying blooms. PROBLEM: The plants are blooming profusely, however, the blooms are drying out before they open even half way. I have them on a misting system and planted them with Kellogs soil amend. The soil was mostly decomposed granite and juniper tree and oak leaves. Why are the blooms drying out prematurely. They are getting filtered sun all day. Any suggestions?
Plant lice and midges can give you the sense of over or under watering. Lice and midges plant eggs into flower buds so that the babies can be protected as they eat. This causes premature blooming with the look of burnt edges, deformation and lack of color. Prematurity and burnt edges along with a healthy plant are sure signs of lice and midges.
Step 1. Use a water and dish soap spray or a rose spray for midges and lice on the plant, blooms and buds. Let it dry.
Step 2. Immediately after drying, cut all buds and blooms off and place into a plastic baggie or such. Zip or tie the bag and toss in trash.
Step 3. Spray again to kill any that have fallen.
Step 4. Spray all of your flowering plants in cases any lice or midges escaped and ran to them.
This can be done with all of your flowering plants that aren't for human consumption.
I just measured by tallest cosmos plant. From the ground to the tip of the highest flower, it measures nine feet...not joking . I planted it as part of a plum tree guild
I also have cosmos 9ft tall. I took pictures to prove it! It is October and they are just blooming now. How can I submit the photos?
Thoughts: Stop misting. Water very sparingly, just when foliage begins to wilt. Don't fertilize; they need very little if any soil fertility. And give them all-out sun, no shade. Avoid overcaring and practice "tough love" when it comes to cosmos who like to struggle in hot, dry weather. Also, when cosmos grow larger, encourage re-bloom simply by cutting the plants back to 12 - 18 inches high.
A couple possibilities come to mind - that perhaps they don't like the misting (or that the blooms don't tolerate it well, anyways), or that maybe they're not getting quite as much sun as they'd prefer (depending on how filtered the light is)?
I'm growing cosmos for the first time this year myself, so I'm afraid that I speak not from experience but just based on what I've read and heard.
Not sure where you're located or exactly what environment you have the plants in, but one other thought is to wonder if temperatures might have something to do with it - if it got a bit too cold, I could see that affecting any existing blooms.
I live in MD where Fall is almost here. my bushes are in 12 inch pots and are at least 3 feet high. Ideally I would like to remove them from the pot, split them I think and bury them along the side of my house. How do I do this first of all and will they germinate so that new cosmos grow and spread each year? How deep do I bury them and do I cut them down?. Last, do I cut them down and will they survive a cold winter?
Cosmos are annuals; they are one-season (one growing year) plants; transplanting them won’t necessarily guarantee a repeat performance next year, in a pot or a plot. If you move the plants from the pot to the soil now, and the plants have flowers (which contain seeds) on them—even spent flowers—some of the seeds of those flowers might drop on the ground, survive the winter ,and self-seed as new plants next spring. Pssst: The Almanac forecast predicts a cold winter in your area this winter, and we know nothing about the site conditions of the area in which you want to plant, so it’s impossible to say for certain whether self-seeding will occur.
How about this: try it; you’ve got nothing to lose. Plant this year’s potted cosmos where you want them to grow next season. In spring, buy a packet of cosmos seeds, just in case. That way, you’ll be sure to get the plants where you want them. Maybe even twice as many.
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