How to Grow Cosmos for Floral Design
Cosmos are one of those garden plants that everyone seems to have grown at some point. Whether the usual wild pink variety of bipinnatus , the gold and orange sulphureus, or the fancier varieties like the ‘Double Click’ or ‘Cupcake’ series, all gardeners seem to fall in love with cosmos at one point or another.
A very easy plant to grow, cosmos are one of those garden plants that seemingly need little attention in order to thrive. In fact, they really seem to grow the best with neglect!
That’s not to say there’s not an art to growing them - especially if you’re going to be growing them for floral design.
1. Selecting Your Variety
As I mentioned previously, there are several varieties of cosmos, of which there are three main varieties that we need to concern ourselves with.
The first is the most popular and the best for cutting - that of the Cosmos bipinnatus variety. The usual single and double forms are all great varieties to choose from, ranging from pure white to dusky red to candy-striped to picoteed petals, and they are all productive and easy to grow.
Our favorite by and far is Cosmos ‘Purity’ - a pure white single version. Although not long lasting and quick to brown in our high heat and high levels of sunlight, ‘Purity’ is extremely vigorous and productive - which allows us to harvest bucketfuls of long stems of cosmos from the garden on a weekly basis. Their white flowers are a joy to anyone who sees them, adding a wonderful bit of character and sparkle to any arrangement they are a part of.
Also a favorite are the fluffy double flowered ‘Double Click’ series of cosmos. Shaped more like a scabiosa or a Matsumoto aster, their cheery little faces are packed with petals, allowing for a longer vase life and longer harvest window. Our favorite is the dark rich fuschia ‘Cranberry Double Click’ with the white ‘Snowpuff’ coming a close second. Similarly, the ‘Cupcake’ variety is a big billowy cupcake-liner-shaped bloom that is more like a poppy than a cosmos in shape and a lot of fun to use in arrangements and in the garden.
But wait - there are so many other varieties to enjoy! The variety ‘Happy Ring’ has a gorgeous coloration that reminds me of Japanese anemones with their bright yellow centers and stiff pink petals, while ‘Daydream’ is inversely colored with pale petals and dark pink centers. And there is also ‘Xanthos’ - a dreamy creamy yellow that while on rather short stems (12” or so) is beautiful and unique enough to earn a place in the cutting garden. The deeper burgundy and fuschia shades of ‘Dazzler’ and ‘Rubenza’ are a favorite jewel tone of both Sarah Raven and Arthur Parkinson and are thus also a favorite of us as well.
You can grow the varieties of Cosmos sulphureus such as ‘Bright Lights’ with their cheery orange and gold blooms. They are a favorite in the garden, constantly sprouting a haze of orange-y blossoms - just don’t hope for more than 24 hours with the cut blooms, which make for great for quick photo opportunities, but not much else as far a designing goes.
There’s also the rare and exotic Cosmos atrosanguineus - the latter of which translates to “grim blood”, a befitting moniker for the dark blood-burgundy color of its petals, which actually grows more like a dahlia or ranunculus - from a very small tuber - than a garden variety of cosmos. You’ll have to purchase in either plugs or tubers to grow them, and they’re pretty spendy - but the blooms are pretty spectacular and are in great demand with floral designers.
Cosmos will grow well at any time of the year so long as it’s after the frost free date. Although they perform best with hot and sunny days and cool nights, they’ll perform well in humidity and heat as well.
If you want a headstart on cosmos, you can start them in flats or pots before you last frost date - up to three weeks before, so you have nice big transplants. Contrary to some popular gardening advice, cosmos don’t mind being transplanted - and will grow nice and big and full in no time.
Cosmos have a seed to bloom time of around 75-90 days, meaning you’ll have to give them a couple months to get to blooming stages. We prefer to plant ours early to ensure that we get blooms early on in the season.
As I mentioned, you can definitely start cosmos ahead of time indoors to get a head start on the season. Generally speaking, I’ll press them into the top of the soil and leave them exposed to light in order to trigger germination. Although they’ll also germinate if under the soil, I prefer to be able to see the seeds sprouting.
On the other hand, if you’re an effort-challenged gardener, you may just want to end up direct sowing your cosmos. The seeds are nice and big and are easily pressed into the ground, and germinate very quickly if given enough moisture and heat.
4. Growing Them Out
Cosmos, if given plenty of water and fertilizer will have no problem growing into a very nice and big plant. A huge plant, if I’m to be honest, shooting up to around 7’ tall on average around here.
While a lot of fun if tucked into the back of the border or allowed to reseed freely into a meadow, for cutting into arrangements cosmos is a lot more usable if on shorter stems. Like I’ve mentioned with zinnias and dahlias, it’s good to pinch out the initial growing tip to encourage the plant to send out lots of numerous side shoots - all of which will form nice and long stems that are perfect for the vase.
Make sure that when your cosmos start blooming, you keep them deadheaded. If allowed to go to seed, they’ll stop producing flowers and shift their energy production towards producing seeds - so keep them from doing that by clipping off dead flowers or cutting all the flowering stems to enjoy in arrangements.
While cosmos only have a very short vase life, we usually harvest not just single stems of cosmos, but large branching stems to extend the vase life. When you have multiple buds on a stem, it doesn’t matter if one blossom shatters or wilts - there are three or four more that will continue to blossom after that, giving the illusion of a longer bloom window. We harvest each branching stem when the first bud just starts to open, knowing that the rest of the buds aren’t far behind.
Accordingly, we don’t just harvest one stem - we go towards the base of the plant and chop off an entire branch. Because we’ve pinched our cosmos plants already, they are bushy and full of many lateral sideshoots that have produced abundant stems with abundant flowers. Don’t worry about chopping off branches either - cosmos are one of the most productive and abundant flowers out there, you won’t hurt the plant by chopping off branches on a regular basis.
Each branch goes into a bucket of deep cool water and is allowed to hydrate overnight before being used in arrangements, just to ensure that there aren’t any blooms that might shatter or any stems that will go wlty.
If you’re not already growing cosmos, you should give them a try! Seed is readily available, it grows easily and quickly, and will provide you with a seemingly limitless supply of flowers for bouquets, arrangements and the garden all season long.