Frost-tender. Start indoors 5 to 7 weeks before your last frost; plant out 1 to 2 weeks after your last frost; sow outdoors up to 2 weeks after your last frost. First blooms 60 to 90 days from sowing.
Put in your ZIP code to see when to start, plant and harvest cosmos where you live, from your own frost dates.
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From seed to harvest
SowSeed goes inWeek 0
SproutFirst shoots showWeek 1–3
Thin or plant outGive each plant roomWeek 3–5
BudsFlower buds showWeek 7–10
BloomIn full flowerWeek 9–13
Keep cuttingCut often for moreWeek 9–14
How to plant and grow
Sow in warm soil
Sow outdoors after all frost danger has passed and soil reaches 65 °F. Scatter seed and rake it in about 1/8 inch deep. Deeper seed sprouts poorly.
Keep seed moist
Keep the surface moist for the first week or two. Seedlings appear in about 7 to 21 days, faster in warm soil.
Space the plants
Thin or set plants about 1 to 2 feet apart. Closer spacing lets plants hold each other up and makes a fuller screen.
Skip the fertilizer
Grow in average or poor, well-drained soil. Rich soil and extra fertilizer give floppy stems and fewer flowers.
Water lightly
Once seedlings are up, water deeply about once a week. Too much water reduces blooming. Established plants handle heat and dry spells.
Deadhead to rebloom
Remove spent flowers to keep blooms coming. Plants cut back rebloom in about a month. Leave a few heads only if you want self-sown seedlings.
Common problems
Photo: “Aphids on Kale leaf” by Sanjay Acharya, CC BY-SA 4.0, via Wikimedia Commons (cropped). The cropped version is shared under the same licence.
AphidsPest
What you'll see: Clusters of small, soft-bodied green, black or gray insects on new shoots and under leaves. Leaves curl, and a sticky film may coat them.
Prevent
Check leaf undersides each week; a few aphids do little harm.
Pull nearby weeds, such as wild mustards, that feed aphids.
Grow flowers for lady beetles, lacewings and hover flies, and avoid broad sprays that kill them.
Organic options
Knock aphids off with a strong spray of water.
Insecticidal soap or horticultural oil on leaf undersides, repeated as the label allows.
Neem oil, or OMRI Listed pyrethrins (without piperonyl butoxide).
Conventional options
Malathion or permethrin, if the crop is on the label. These also kill the insects that eat aphids.
Bee caution: Don't spray open flowers. Spray in the late evening, when bees aren't flying.
Photo: “Powdery mildew 9” by Dmitry Brant, CC BY-SA 4.0, via Wikimedia Commons (cropped). The cropped version is shared under the same licence.
Powdery mildewDisease
What you'll see: White, powdery patches on the tops of leaves and on stems. Leaves yellow and dry, and fruit may ripen poorly.
Prevent
Grow resistant varieties.
Give plants sun and space for air flow.
Remove badly infected leaves, and clear debris after harvest.
Organic options
Sulfur or potassium bicarbonate, from the first spots. Sulfur can burn some vine crops in heat, and never use it near an oil spray.
Neem oil. Look for OMRI Listed products.
Conventional options
Myclobutanil or propiconazole, if the crop is on the label.
Bee caution: Don't spray open flowers. Spray in the late evening, when bees aren't flying.
Avoid broad insecticides; they kill mite predators and can start an outbreak.
Organic options
Hose leaf undersides hard, often.
Insecticidal soap or horticultural oil on leaf undersides, repeated as the label allows.
Release predatory mites once pest mites are present.
Conventional options
Same as organic. Broad insecticides often make mites worse.
Bee caution: Don't spray open flowers. Spray in the late evening, when bees aren't flying.
Read and follow the product label: it is the law. It sets the rate, the crops it is for, and the days between spraying and picking. Products registered vary by state; for organic growing, look for products allowed under the USDA National Organic Program (OMRI Listed). We name active ingredients, not brands or rates.
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Sow
Sprout
Thin or plant out
Buds
Bloom
Keep cutting
