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Cosmos are lovely big plants that look like daisies on steroids, not surprising as they are in the same family - Asteraceae. Don't ask me how it's pronounced! I've grown these a few times over the years to add a bit of colour to my mainly veg producing garden and they are really easy to grow from seed. They are native to large parts of the American continent, where they are mainly a wild perennial meadow plant, although I treat them as an annual as they don't seem to like a British winter.
Now is the time (late winter / early spring) to start planting the seeds indoors or under frost protection. I plant them into pots on a window sill, one seed per little pot buried about half a centimetre deep in moist compost. They take about a fortnight to sprout, then when it gets to late April / early May they can be planted outside, about 15 centimetres apart. I use sticks and stakes as supports as they can grow over a metre high. When they flower, they have big elongated petals growing around a circle in the middle, similar in appearance to dahlias, but more like a daisy. The colours vary and popular types of cosmos (which are easily available from seed producers) include Double Allsorts Mixed and Sensation Mixed.
Cosmos prefer a sunny position, but the soil doesn't have to be mega rich - their natural habitat is quite poor soil so all in all they are relatively easy to look after. If they are grown in a rich soil then they will produce lots of foliage but not so many flowers.
They make an excellent cut flower, and have attractive foliage in the form of wispy fronds (not dissimilar to fennel leaves) that looks ok in the vase if left on the stalk. Dead heading will make the plant produce more flowers, so get picking! Also, the seeds can be saved for the next year, so long as the ones you've grown weren't F1 Hybrids.
A very easy to grow plant that doesn't require much in the way of special treatment; try growing some and you'll enjoy their big daisyesque petals swaying in a breeze.