- If you want a head start, you can plant cosmos indoors 4 to 6 weeks before the last spring frost in trays or pots with a good seed-starting mixture. Seedlings grow fast, so move them into 5-inch pots as soon as they're 3 or 4 inches tall.
- Plant seeds in moist, well-drained soil about ¼-inch deep and 12–18 inches apart after the danger of frost has passed. You can also plant transplants instead of seeds. They also like soil that is not too rich.
- They can tolerate warm, dry weather.
- Depending on the type of flower, cosmos can grow anywhere between 18–60 inches tall.
- If you are growing cosmos from seeds, be mindful that it takes about 7 weeks to first bloom. After that, though, your flowers should continue to bloom until the next frost.
- In order to prolong flowering, you should deadhead the plants (remove the dead/faded flowers).
- Because some of these plants can grow really tall, staking may be necessary.
- Water regularly, but make sure you don't over-water the plants. Over-watering and over-fertilization can lead to plants with fewer flowers.
- Cosmos beds may become weedy due to the fact that they self-seed, so remember to check them.
- 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 on good stems when they first open, they'll last more than a week in water.