Cosmos Cultivation Notes

Cosmos, in its annual form is native to Mexico, and is often known as the Mexican aster. It wasn’t collected until the late 1700s and the flower made its way to England in 1789, thanks to the Marchioness of Bute, wife of the English ambassador to Spain.

They are prized for their abundant, daisy-like flowers and are simple and inexpensive to grow from seeds. Plant them in full sun, providing protection from strong winds for some of the taller cultivars. Cosmos tolerates a wide range of soil types, including poor soil. Plants need even moisture to get started, but mature cosmos can be quite drought tolerant; plants produce more and larger flowers, however, if they are watered regularly. Regular and methodical dead-heading prolongs the flowering season. They will happily self-seed if required.

Cosmos bipinnatus is a tall, bushy annual which grows up to 2.5m, with very finely divided, mid-green leaves, and large flowers up to 8cm across, usually white or pink the wide outer rays surround a central disc of tightly clustered, usually yellow, inner disc florets. They flower in late summer and into autumn, until the first frosts arrive. Many named cultivars and selections are available, including doubles, forms with tubular rays, and picotee colours. They can be grown in containers, or directly in flower beds, with some of the taller cultivars being suited to back of bed positioning. A great plant in terms of attracting pollinators such as bees to the garden.

Cosmos are one of the easiest plants to propagate from seed. Seeds can be sowed at 16°C in the UK from February onwards, although a later sowing is advisable in order to keep the plants frost free until planting them in situ in early May, depending on your location. Alternatively seeds can be sown in situ in late spring in a weed free bed.

Cosmos bipinnatus have relatively few pest and diseases that cannot be kept in check. New seedlings can fall foul of inquisitive birds pulling them up, or slugs, which can be combated with selective mulching products. Similarly if rabbits are an issue, they make take a liking to fresh young leaves, so protective measures may need to be employed. Cosmos can be impacted by grey mould, which can be combated by extra ventilation when growing under cover, removal of infected material, and not overcrowding with your planting schemes. Historically a fungicide has also been used in order to prevent foot rot, although there is evidence some treatments still on the market can impact pollinators.