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Planting, Growing, and Caring for Cosmos
Planting
The soil should be well-draining, but other than that, cosmos don鈥檛 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鈥檙e 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鈥攏o more than 1/4-inch deep.
- Thin to 12鈥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 , 鈥淏e cautious not to overwater your plants. That and over-fertilizing can lead to the plant producing fewer flowers.鈥
- 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.
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鈥檚 best to pick some right when the petals have opened.
- If you cut the blossoms when they first open, they鈥檒l last more than a week in water. Strip off the lower leaves and put them in a vase.
Catherine Boeckmann
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Thank you that does help. I am just impatient for them to flower. They do look lovely and healthy though.
I have a Cosmo that I planted at the end of May, not from seed though. It continues to grow in height, but there still is no sign of flowers getting ready to bloom. Why?
Cosmos usually take a long time to grow the stem and leaves before the buds show. You should see some flowers soon. Remember to dead head the faded flowers to have more buds and flowers through the end of summer.
I have cosmos in containers on the deck & while the plants are growing well, the original hot pink coloris all washed out. They are a pinkish gray or dull looking pink. I bought these as sedlings at a nursery in a 4-pak. What blooms they had at the time were very vibrant. Is it not enough sun? Most of the sun is indirect, with the strongest & most direct sun after 2 p.m.
The flowers on my cosmos are getting smaller. What am I doing wrong? Thank you!
Cosmost likes full sun. It's daylight sensitive, so if it's not getting enough sun, it may be failing in health. Are you deadheading? Removing spent blooms encourages the plant to put its energy into new flowers, not seeds in old ones.
I'm getting ready to plant beginners that are currently in Dixie cups. I'm in eastern Nebraska. The area I will be planting in is mostly shade. Will they grow right or do they need mostly sun.
I live in Maryland. Last year I put half my seeds in full sun and half in mostly shade. I got tremendous results from the full sun. The mostly shade grew a lot slower, were much smaller, and had a lot fewer flowers. - Good luck!
is it advisable to cut the cosmos in half when still growing to promote stronger thicker stems
We would not recommend it. Cosmos do not ask for much: they thrive in fairly poor soil, just make sure the soil drains well (and the plants are not in puddles). Too-rich soil may result in thin-stemmed, weak plants; cutting them will only hasten their demise. Stake tall stems again heavy rain or wind, when forecast. Or plant the flowers closer together than seed packages or plant labels advise so they can support each other. Pinching off flowers encourages blooms. Pinching stem tips can reduce height and encourage branches to grow but it is not necessary.
