How to grow Cosmos Seeds

Cosmos grow easily in beds and make great cut flowers. When established, the plants can handle drought, poor soil conditions, and general neglect. They even self-sow. This is a truly low-maintenance plant.

While some pests, like aphids, flea beetles, and thrips, do enjoy cosmos, they're easy to control with a strong spray of water or insecticidal soap. Aster yellow, bacterial wilt, and powdery mildew may also affect cosmos. Space plants accordingly to ensure good airflow to avoid diseases.

Light: For the best flowering, choose a site that gets full sun. Cosmos seeds will grow in partial shade but will have fewer blooms and are less vigorous when planted in shady areas. They thrive under uninterrupted full sun in the hottest conditions. These plants are native to the arid regions of Mexico and Central and South America, so they will thrive in conditions that mimic those found in these regions.

Image: Cosmos Bipinnatus Fizzy Rose Picotee
Cosmos Bipinnatus Fizzy Rose Picotee

Cosmos plants prefer neutral to slightly acidic soil although they will grow in poor soil where many other flowering plants languish. Cosmos perform best in medium moisture, well-drained soils, but they will perform adequately even in dry soils. Avoid soils that are too rich, which can cause the plants to get too tall and flop over. You can prevent this by staking the plants or growing them close enough to other plants that can support them.

Water: Once established, you should not need to water your cosmos plants at all unless there is a prolonged drought. Where water is limited, these are the last plants that require irrigation.

Temperature and Humidity: Hot weather is ideal for cosmos, and they thrive under nearly any humidity level.

Fertilizer: Unless your plants seem to be struggling, there is no need for fertilizer. Cosmos can handle poor soil, and fertilizing can actually have a negative impact. Too much fertilizer can create strong plants with lots of foliage—but few blooms.

Pruning: The only real maintenance cosmos plants need is deadheading which will prolong the flowering season. If you fall behind, simply shear the plants by about one-third, when most of the flowers have faded. This produces a second flush of leaves and flowers.

Propagating Cosmos: These plants readily self-seed, and it is also easy to collect the dried seeds at the end of the season to save for next year. Remember that seeds from hybrid varieties may not "come true" to the parent plant and may produce plants that revert to the species.

White Cosmos Flowers Seeds

Growing From Seeds: Although nursery seedlings are available, cosmos are so easy to grow from seeds that it makes little sense to overspend by buying nursery plants. You can start cosmos seeds indoors, four to six weeks before the last frost, but cosmos sown outdoors directly in the garden will quickly catch up. Cosmos typically germinate within 7 days at 23 degrees C, followed by flowering in about 50–60 days.

Many suppliers recommend precise spacing, such as at 60cm intervals, but at Fontana Seeds we believe you will get a better display if you simply scatter the seeds and let the plants support each other as they grow. You can always thin if you need to, moving the extra plants to another part of the garden.

Wait until all danger of frost has passed before sowing or transplanting outdoors. Cosmos grow very quickly but can be killed by a late frost, so don't rush it.

Do cosmos come back every year?
Cosmos is a moderate reseeder, which means that it drops plenty of seeds to bring it back year after year without becoming an uncontrollable nuisance. For cosmos to reseed itself, you have to leave the faded flowers in place long enough for seeds to form.

How do you start cosmos seeds indoors?
Start cosmos seeds in flats or pots indoors. Planting containers should be at least 7cm deep with drainage holes. Fill the containers almost full with starting medium, scatter the seeds thinly over the surface or place them about 2.5cm apart, then cover the seeds with 0.5cm of medium.

Can Cosmos survive winter?
They are not hardy and if you leave them in your borders over winter there is a real chance they will be killed by a sharp frost, or rot in cold wet soil.

Is Cosmos an annual or perennial?
Cosmos are half-hardy annuals with daisy-like flowers that are easy to grow from seed and are incredibly long flowering, from midsummer until the first frosts.

What to do with Cosmos after flowering?
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. Simply strip off the lower leaves and put them in a vase.

Read the most frequently asked questions about cosmos seeds

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