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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Hi Suellen,
Cosmos certainly are a great annual to enjoy through the growing season. With that being said, your issue with thick stems, lots of leaves, and few flowers is an interesting occurrence. One reason for it could be too much nitrogen. Cosmos are sensitive to nitrogen and it can cause exactly what happened to some of yours. If you have been fertilizing those cosmos (or maybe it came from fertilizer spread on the lawn) you should cut back on fertilizing—or stop all together. If you need to fertilize, do so with a mix that is low in nitrogen. It is completely up to you if you want to pull them out because of space issues, but you could also wait to see if flowers arrive later in the season.
Most cosmos are annuals, so after all the blooms are spent this fall you can cut them back/pull them from the ground. Cosmos self-seed so if you do not want them to grow in the same area, make sure you remove spent flower before they go to seed. If you want them to remain in that area of your yard or garden, you can allow the flowers to remain on the stalks so that the seeds fall to the ground and then remove. You can also remove the flowers and save the seeds to plant in the spring, either indoors 4 to 6 weeks before your last frost date or directly into the ground once the threat of frost has passed.
An important part of maintaining your cosmos is to deadhead. When you deadhead spent cosmos flowers, it forces the plant to put its energy into creating more flowers instead of making seeds. When the blooms are spent, cut back the stem to above the next set of leaves and soon your plants will produce a new set of buds and before you know it a new set of flowers will be brightening up your garden!
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