Sulphur Cosmos Unveiled: The Ultimate Growing and Care Handbook

The Cosmos sulphureus, also known as Yellow Cosmos, Golden Cosmos, Sulphur Cosmos, or Yellow Blossom, is an annual herbaceous plant from the Asteraceae family, indigenous to Mexico and growing naturally in areas below 1600 meters in elevation. The species name ‘sulphureus’ literally means “of sulphur”.

This plant thrives in full sun to partial shade, has moderate cold tolerance, and is a natural hybrid of the Cosmos bipinnatus and other species within the same genus. It is now widely cultivated as an ornamental flower and as material for cut flowers.

The Sulphur Cosmos has a somewhat untidy growth habit and is therefore suitable for mass planting, unlike the Cosmos bipinnatus, which can be used for structured flower beds.

I. Characteristics

The Sulphur Cosmos is a highly branched plant, reaching heights of about 30 to 100 centimeters, typically around 60 centimeters tall, with potted varieties maintained at a dwarfed height of 20 centimeters.

It features opposite, bipinnate leaves that are deeply divided, with lance-shaped segments ending in a fine point. Compared to the Cosmos bipinnatus, its leaves are broader and its margins are more coarsely toothed.

The achene, a type of dry fruit, is 1.8 to 2.5 centimeters in length, brownish in color, hard, rough, and hairy, with a slender beak at the tip. In the heat of summer, the flowering period may start earlier.

Its long blooming season spans from June to November, typically from June to August for spring sown plants, and from September to October for those sown in summer.

The flowers, 3 to 5 centimeters in diameter, are yellow, golden, or orange, with improved varieties also producing red blooms; the central disc flowers are yellow or brownish-red.

The flower forms include both single and double petals, with the double varieties predominantly sold in the horticultural market.

The Sulphur Cosmos is not prone to pest or disease problems, but aphids may grow near the new shoots in early summer.

In poor soil, it requires only moderate watering to thrive and is a hardy, low-maintenance plant. It can be cultivated in less sunny conditions, although leaf deformities should be watched for in such environments.

II. History

At the end of the 18th century, the Sulphur Cosmos was first planted in the Royal Botanic Garden in Madrid, Spain, and was introduced to Europe by the garden’s director and the plant’s namesake, Antonio José Cavanilles.

According to historical documents, the Sulphur Cosmos was introduced to Japan in the early Taisho period and has become one of the main ornamental flower varieties there.

The plant is also commonly found in Korea, growing abundantly along streets. In the 1950s, Dr. Yu Chang-Chun, an agronomist and botanist, suggested its introduction to Korea, where it has since been extensively cultivated.

In 1996, the Sulphur Cosmos was declared an invasive species by the Southeast Exotic Pest Plant Council (SE-EPPC) in the United States.

III. Cultivated Varieties

Some of the main cultivated varieties of Sulphur Cosmos include:

• ‘Klondyke Mix’: Flowers in shades of yellow, orange, and red.
• ‘Ladybird Dwarf Red’, ‘Ladybird Dwarf Gold’, ‘Ladybird Dwarf Orange’, and ‘Ladybird Dwarf Lemon’: All dwarf varieties.
• ‘Strawberry Chocolate’: A hybrid between Sulphur Cosmos and Chocolate Cosmos (Cosmos atrosanguineus), cultivated in Japan.

IV. Growth and Care

Germination takes 7 to 21 days, with the optimum temperature around 24°C, and flowering occurs 50 to 60 days after germination. It is tolerant of poor, sandy soil, originally thriving in the alkaline soils of Mexico, hence suitable soil pH ranges from 6.0 to 8.5.

Bright, sunny weather favors blooming, but it can also withstand partial shade. Once sprouted, the plant is drought-tolerant and not easily susceptible to pests or diseases, demonstrating its vigor in some pest-prone areas of the United States.

V. Symbolism

The flower language of the Sulphur Cosmos speaks to a wild beauty. This flower was featured as a central symbol in the 1997 Japanese movie “Remembering The Cosmos Flower,” starring Akane Oda.