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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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I live in the SF bay area (zone 9b) and planted a few cosmos seeds (Sensation blend) at the end of April and then again in early May this year. It's the end of June now - about two months later - and each plant is just 4 - 5 inches tall. The plants are in almost full sun - they get several hours of sun per day. They look healthy and green but aren't growing much. I'm wondering what could be the problem...I was hoping to have at least some flowers by now. Anyone have any ideas on what might be going on?
Cosmos likes at least 6 hours of sunlight each day. It may take about 85 days for the Sensation variety to reach maturity, so it could be that it just needs more time. Be sure that the soil is well-drained. Also be sure that the plants are thinned to the proper spacing and that there are no weeds nearby that are competing for nutrients, space, and light.
I was stunned that my pictoee cosmos only took 4 days to sprout from seed as the package said 7 days minimum. That's fast! I started them indoors at the same time as about four dozen other flower and vegetable species, and they're the only ones to break the surface so far. I used soil-less substrate in paper pots, kept just moist enough, and they are under full spectrum artificial lights for several hours a day.
Hi,
I have started a few Sensation Mix cosmos from seeds. I would like to know what would be an ideal container size for transplanting these.
If you are growing one plant in each pot they don’t need to be too big. A 6-inch pot will do great. Good luck!
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Does it take two to three weeks for the cosmos flowers to budd because I read this and I really need them to for my project!
Thanks
If you are growing cosmos from seeds, it takes just 5 to 7 days to germinate but about 4 to 6 weeks to first bloom.
ok so im doing a science project and i need to know what type of cosmo grows the fastest from seed?
Cosmos will bloom in approximately 50 to 55 days after germination. So not sure the variety is that much of a big deal. If you wish, go to the site of a retail seed seller (Burpee, Johnny Seeds, American Meadows, etc.) and give them a ring. Keep in mind that growth is also affected by weather, soil temperature, and germination speed.
