Cosmos is a genus of around 25 species of annual and perennial plants in the family Asteraceae from Mexico. Two species frequently used as annual bedding plants are Cosmos bipinnatus (pink, white) and Cosmos sulphureus (orange and yellow). The word 'cosmos' derives from the Greek 'kosmos' for order or adornment. See Revision of the genus Cosmos for historical details.
Cosmos atrosanguineus is commonly called chocolate cosmos; previous names include Bidens atrosanguineus and black bidens. At the end of the 20th. century, gardening pundits firmly placed it in an interesting band of plants described as extinct in the wild and surviving in cultivation as a single non self-fertile clone. Other examples are the saffron crocus (Crocus sativus) and tiger lily (Lilium lancifolium). The situation was not so simple and by 2010 there were reports of Cosmos atrosanguineus plants producing viable seed and the appearance of named cultivars, see plant patent 12587072.
This species was originally found in Zimapán, Hildago, Mexico. It is said to have been introduced to cultivation in 1835 at Ipswich by William Thompson (born 1823, and later to found Thompson and Morgan) who received seeds from Mexico and passed the plant on to Joseph Hooker at Kew. Hooker then described it in the botanical literature. Popular through the early part of the 20th century, by 1970 the only widely known population was at Kew, from which the single non self fertile clone was propagated. Seemingly whilst the pundits spoke, fertile forms were quietly growing in gardens in New Zealand, and this is where the new cultivars and seed have appeared from.
The Latin term atro means dark and sanguineus means blood-red. The roots are similar to Dahlia tubers and cultivation is similar to Dahlias, with the tubers being put somewhere frost free for Winter (see this posting for crucial differences). It gets its common name from producing a fragrant chemical constituent of vanilla, often chocolate has vanilla added to it.
The centre of the flower is composed of a number of florets, each of which has its own petals. As is typical for Asteraceae there are five stamens with anthers that are fused into a tube, pollen is released inside this and pushed out by the developing style. The florets open one by one, overall this gives the effect of pollen being produced first and then later all the florets being in a pollen receptive state.
First photograph by Jacob Uluwehi Knecht, rest by David Pilling. The fourth annotated photograph is of the centre of the flower. The next photograph shows (arrowed) styles breaking through the anthers. The last photo is of a style with developed stigmatic tissue. Given the history above, these photographs are of the same plant; flowers are on stems over three feet high.
Photograph 1 is of roots along with a one pound coin which has a diameter of 22.5 mm, the stem is towards the top right. Photos 2 and 3 are of developing seed. Photo 4 is of seed received from 2013/2014 SRGC seed exchange; visible on the end of the seed are the remnants of the two styles (left hand end of top seed, right hand end of bottom seed). Photo 5 shows shoots in spring.
Photograph 1 shows the contents of a packet of seed, the numbers by each seed are the weights in milligrams (see Cleaning Seeds). Seed was placed in a zip lock plastic bag with moist kitchen towel and kept at room temperature (around 65 °F), germination started within a couple of days. Photos 2 and 3 show the seed from photo 1 which weighed 11 mg. The seed which weighed 3 mg was the only one to not germinate; no surprise; yet it was one picked for the seed photo above; showing it is not easy to tell good seed by eye. Photo 5 is one plant grown from the seeds; by the end of July it was full size and flowering. Photo 6 compares a flower of the usual clone (right) with the flower of a seed grown plant; this particular situation (of an old flower and a new one) exaggerates the differences. It seems that pollen from the seed grown plants produces seeds easily.
Cosmos atrosanguineus 'Chocamocha' is a new cultivar, widely available in 2012. It is shorter than the usual one at around 40 cm, and according to vendors has redder and more strongly scented flowers. The first photos trace the development of one flower over three days; Cosmos flowers do become lighter with time. In the first photo the green bracts at the back of the flower are visible; the petals around the outside are provided by special ray florets. The last two photos compare 'Chocamocha' (left) with the common form.
Cosmos diversifolius is a lilac flowered species related to Cosmos atrosanguineus.