I have been growing cosmos almost for as long as I have been gardening. I remember the first batch, which were the ‘Sensation Mixed’ strain, dubbed sensational because they would bloom before the short days of autumn as their ancestors were want to do. But even then there were some individuals that wanted to become trees before they thought about flowering and one white-flowered plant in particular still sticks in my mind, a towering ferny monster 2.4m high – well 8ft since metric was then a foreign language – both impressive and disappointing until just before November frosts when it deigned to open a few flowers.
Cosmos have come of age. They are now the easiest way to fill beds and borders with summer colour and the shorter kinds cheerfully and cheaply fill large patio pots. They are also one of the simplest and most satisfying plants to grow from seed. The seeds are large, they germinate quickly and the seedlings are of rapid growth. With just one caveat however – last year I had trouble with germination and I am not sure why. It was like when your go-to sponge recipe fails. Was the baking powder off? Did someone open the door halfway through cooking? Did those eggs and butter curdle? In the end it was not a disaster because tall cosmos are great brutes when they get established and they filled in but I am curious as to what went wrong.
Cosmos are, as Robinson explains, closely related to dahlias ‘Mexican plants allied to the Dahlia. C. bipinnatus is a handsome annual, 3 feet to 5 feet high having finely-divided, feathery foliage and large Dahlia-like bright red-purple blossoms, with yellow centres. It is best raised as a tender annual by sowing the seeds in February or March in a heated frame, and transplanting in May in good, rich soil with a warm exposure.’
I would suggest that February is too early, especially if raising plants on the windowsill since the seedlings will be too tall and straggly by the time it comes to planting out safely. I always leave them till April. But Robinson was not growing the dwarf and early-flowering kinds we have today so perhaps a longer growing season was necessary.
And today we are truly spoiled for choice. The dwarf ‘Sonata’ and ‘Apollo’ strains are perfect for front of the border and for pots. It is a matter of taste but there are now ‘doubles’ such as the ‘Double Click’ series but I prefer the singles for garden display. I am still concerned about how on earth the ‘Cupcakes’ series is morphologically possible (see older posts). But this breakthrough from Thompson & Morgan is now well established and very popular in pink and white shades. Is it better than the singles? I am not sure but it is a nice thing. I have a soft spot for the picotee colours with dark edges to pale petals.
Although the orange and yellow cosmos (C. sulpureus) have been around for ages, they have never been quite as popular. They are a different species and the flowers are smaller, the leaves less ferny and the plants a bit scrawny. In the last few years yellow, albeit pale yellow, has been added to the C. bipinnatus cosmos and now things have been mixed up well and truly with the pink and apricot ‘Pink Lemonade’ and curious pomegranate ‘Xsenia’. I don’t know if C. sulphureus was involved in the breeding of these – I suspect it was – but these new colours do seem to be more delicate in habit than the standard C. bipinnatus.
It is always worth buying the best, named varieties and I would avoid cheap, un named mixtures which are often disappointing. Whatever else you do, grow your own from seed. Because the seedlings grow so quickly they are often sold in large pots in spring and summer at ridiculous prices – I mean too much! This is a great moneymaker for nurseries but once you have raised your own you will realise how much more you have been paying than you needed.