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
This may sound odd, but cosmos like very tough, poor conditions. Low soil fertility. Very little water. If you overcare for them (too much water, too much fertility and too much shade), cosmos becomes tall and spindly and blooms sparsely. They like it hot and dry. Think Texas. When not exposed to "tough-love" growing conditions, this otherwise beautiful flowering plant becomes tall, spindly and sparsely floriferious.
Cosmos love lousy soil, so you've mastered that. It may be that the potting soil in you pot is too rich! Soil that is too nutrient rich produces plants that are weak-stemmed and have few to no flowers. Be sure that it drains really well!
As noted above in response and in guidance, cosmos are not demanding. Your generic potting soil is probably fine; "lousy" was your word re your in-ground cosmos and we meant only that potting soil does not need to be rich/composted/fertilized. Another thought is that you use the in-ground soil in the pot. If indeed the pot is appropriate. You may have identified the problem as a too-small pot; the cosmos may need more leg room, so to speak. Does this help?
We imagine you'll want flower pots that are heat-tolerant and can take full sun. Consider gomphrena, lantana or scaevol, for starters. For annuals in containers, use a medium weight potting mix. But first fill the bottom 1 to 2 inches of the pot with coarse gravel and then a 1⁄2-inch layer of fine gravel or sand.
If you are experiencing particularly wet or dry conditions, this could be the cause of your short-lived flowers. Cosmos are generally drought-tolerant, but if it's been very dry and hot, then your flowers could be getting sunburnt.
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