The annual types of Cosmos are invaluable plants for filling gaps in the herbaceous border left by early-flowering perennials. They have feathery foliage that adds a delicate feel and large flowers in colours ranging from hot shades of orange and red to cooler whites and pinks. Chocoholics will also like this genus for its perennial Cosmos that smells like hot chocolate and has a beautiful maroon-red flower.
There are 26 species of Cosmos within the daisy family Asteraceae, but only a few are used in cultivation. The most commonly known species is C. bipinnatus, an annual type from which the well-known series Sonata and Sensation were bred, with their white, pink and carmine flowers.
The straight species can reach 2m tall in the wild, but cultivars are smaller. The Sensation series is among the tallest at up to 120cm. C. sulphureus 'Ladybird Scarlet' is one of the dwarf varieties - it only grows to 25cm and has bright red flowers. There are also semi-double flowered varieties such as C. bipinnatus 'Bright Lights', which has attractive flowers in shades of orange and is early flowering. A variety with an unusual fluted petals is C. bipinnatus 'Sea Shells'.
There are also perennial species in cultivation, which are similar to small dahlias. The chocolate Cosmos, C. atrosanguineus, is one such. It has velvety petals of maroony-red that smell like chocolate. The cultivar 'Chocomocha' has been bred from this. It is slightly smaller and has more red in the petal colour.
Most Cosmos are frost-tender, so the bedding types bred from C. bipinnatus are produced as half-hardy annuals, raised in a greenhouse and hardened off. The tubers of C. atrosanguineus can be lifted to protect it like a dahlia, but where frosts are brief they can be over-wintered outside if covered with a thick mulch. Mature tubers are more tolerant of the cold.
There is also a hardy perennial species C. peucedanifolius, which is quite new in cultivation. It has large pink flowers that look like those of a bedding Cosmos. In its first year it looks quite straggly, but it will bush out in subsequent years. Cosmos has been crossed successfully with its close relation Dahlia. C. x Dahlia 'Mexican Black' has black flowers with a delicate chocolate scent.
Cosmos are generally easy plants to grow. They like full sun and well-drained but moisture-retentive soil. Fertile soils will lead to excessive foliar growth at the expense of flowering. Annuals should be sown under glass in autumn and grown on at 10 degsC or sown in spring six to eight weeks before being planted out after the danger of frost has passed. Once planted they need watering in dry spells and deadheading to prolong flowering.
WHAT THE SPECIALISTS SAY
- Lyn Southon, owner, Southon Plants, Surrey
"There is a very nice new one called C. bipinnatus 'Seashells' with fluted petals. It comes as a mix of colours from pink to white. It grows to about 1m tall, so is great for gaps in borders.
"We also grow the popular annual C. Sonata, which is a mix of carmine, white and pink and grows up to 60cm tall. Of the perennials types, we grow C. Astrosanguineus 'Chocomocha'. It's similar in cultivation to a dahlia. It doesn't want to be too well fed as it can get soft and snap. I recommend bringing them in over winter.
"Customers are definitely keen on the scent of Chocolate Cosmos, whereas with the annual types it's the foliage and bright flowers that they go for. "
- Derry Watkins, owner, Special Plants, Wiltshire
"We grow the typical annual varieties, which are a range of heights. One of my favourites is C. 'Bright Lights', which produces gorgeous, hot-coloured flowers.
"An interesting Cosmos is the perennial C. peucedanifolius. It's a hardy tuberous species with pink flowers, very similar to a pink annual Cosmos. It gets up to around two foot (60cm) high and its stems trail over. In the first year it can look straggly, but in the next few years it will bush out to around three feet (90cm) across.
"Cosmos like full sun and good drainage. If given that they will flower and flower and flower - that's why people like them."
- Jim Loder, nursery assistant, Primrose Cottage Nursery, Manchester
"We grow popular varieties in the C. bipinnatus 'Sonata' series and C. bipinnatus 'Purity', an older variety with large single snowy-white flowers, reaching around 90cm. A less commonly grown variety we do is C. 'Antiquity', which has rich burgundy flowers, turning to antique bronze/salmon with age. They are just 50cm tall.
"We do stacks of the bedding types, but only around 20 or 30 of the chocolate Cosmos a year, though there is interest in them and we can sell them at a higher price.
"The thing to watch when growing Cosmos is not to let them get pot-bound as they are fairly vigorous plants. We pot them into oneor 1.5-litre pots, and they'll fill them in a couple of weeks. They should also be deadheaded regularly."
Sally Stones, hardy plant section, Squire's Garden Centre, Twickenham, Greater London
"They need a nice, light airy place, so they don't get stretched, and should be kept well watered. We sell them in bedding packs, in one-litre pots and also in five-litre terraplastic decorative pots, displayed on an end bed.
"Cosmos haven't got a long shelf life but they have a high turnover. While they are under your care, keep them regularly deadheaded.
"It's quite hard to get hold of the chocolate Cosmos as nurseries don't seem to grow them in great numbers, but when we get them in customers are very interested. We have them in the patio plants section, where they need to be in flower as that leads to impulse buys."
SPECIES AND CULTIVARS
- C. atrosanguineus, or chocolate Cosmos, is a tender, tuberous perennial with brownish-red blooms that resemble a single Dahlia and smell like vanilla and hot chocolate. Height: 75cm. Spread: 45cm.
- C. atrosanguineus Chocamocha = 'Thomocha' (PBR) has redder flowers and more of a chocolatey fragrance than the straight species. It is more compact and has lance-shaped foliage. Height: 45cm. Spread: 30cm.
- C. bipinnatus 'Bright Lights' is a mix of single and double deep-orange and tangerine flowers. Good as a cut flower. Height: 75cm.
- C. bipinnatus 'Purity' is a tall white-flowered form. It has mid-green feathery foliage and large, saucer-shaped flowers between June and October. Height: 120cm. Spread: 60cm.
- C. bipinnatus Sensation series are tall annuals with flowers ranging from white to carmine. Good for cutting. Height: 90-120cm.
- C. bipinnatus Sonata series are a compact range of annual Cosmos, producing large flowers that come in carmine, pink and white. They have finely-cut feathery foliage. They are also ideal for cutting because they have long stems and last well in water. Height: 60cm. Spread: 40cm.
- C. bipinnatus 'Sweet Dreams' are dwarf half-hardy annuals with pale pinky-white petals with a darker pink base and yellow centre. Height: 90cm.
- C. 'Collarette Mixed' is a mix of half-hardy annuals that have a collarette centre of smaller petals. Colours include burgundy, white and pink. Great for cutting. Height: 90cm.
- C. peucedanifolius is a hardy, tuberous perennial type. It has large pink flowers like annual garden Cosmos. Looks a little straggly in the first year. Height: 75cm.
- C. peucedanifolius 'Flamingo' is a long blooming variety with large unscented lavender flowers, held upright above the plants, which have a sprawling habit.
- C. sulphureus 'Ladybird Mixed' is a mix of semi-double flowers in shades of scarlet, tangerine and yellow. They are half hardy annuals.
- C. sulphureus 'Cosmic Red' is a variety that produces compact plants with intense scarlet-red flowers. Height: 30cm.
- C. x Dahlia 'Mexican Black' is a cross between a dahlia and C. atrosanguineus. The blooms are black with a delicate chocolatey scent and held well above the dark-green, pinnate foliage. Height: 75 cm.