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
Soil: 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.
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.
Read the most frequently asked questions about cosmos seeds