If you had asked me a year ago how to grow cosmos, I would have had absolutely no idea.
In fact, I still wouldn’t claim to know very much.
What I do know is that an old cardboard egg carton, a conservatory windowsill and a plastic bag produced dozens of healthy cosmos seedlings and gave me a dangerous amount of confidence.
The funny thing is that I repeated exactly the same process with a different packet of seeds and achieved almost complete failure.
Gardening has a wonderful way of keeping you humble.
Why Cosmos?
This spring felt like the right time to try growing something from seed.
Our garden is still very much a work in progress. While we continue uncovering the established plants and perennials hidden throughout the garden, I wanted to add something of our own.
The newly created driveway border seemed like the perfect place.
The border itself is not ready yet. In truth, it is currently more weeds than flowers and still waiting for a proper clear-out. But I could already imagine clouds of cosmos softening the space and adding colour while the rest of the planting scheme slowly comes together.
So I bought three packets of seeds and decided to see what happened.
My Very Unscientific Growing Method
As previously established, I am not a gardening expert.
There was no greenhouse.
There were no heated propagators.
There was no carefully researched growing setup.
Instead, I filled an old cardboard egg carton with compost, sowed the seeds and placed the carton on the conservatory windowsill.
Technically it wasn’t a greenhouse, but on a sunny spring day our conservatory comes surprisingly close.
To help the seeds germinate, I placed a clear plastic bag over the entire egg carton to create a warm, humid environment. Once a day I would open the bag, check on the seeds and give them a gentle mist before sealing everything up again.
As soon as the first seedlings appeared, the bag came off.
The cardboard carton had a habit of leaking whenever I watered it, so rather than throwing the bag away, I simply placed it underneath to catch the drips.
Simple, effective and entirely held together by optimism.
Cosmos Purity Seedlings
Thompson & Morgan 'Purity' cosmos seedlings at seven weeks
Three Varieties, Three Completely Different Results
This year I sowed three varieties of cosmos:
All three were sown at the same time. All three were grown using exactly the same method. All three sat on exactly the same windowsill.
The results could not have been more different.
Thompson & Morgan 'Purity'
Thompson & Morgan 'Purity'
The Purity seeds were exceptional.
Every single seed seemed determined to grow. Germination was fast, the seedlings were strong and healthy, and before long I had more plants than I knew what to do with.
If someone had handed me only this packet, I would probably be writing this post convinced I had discovered a hidden talent for gardening.
Suttons 'Apricot Lemonade'
Suttons 'Apricot Lemonade'
The Apricot Lemonade variety was slightly less enthusiastic but still produced nine healthy seedlings.
For a beginner gardener, that felt like a huge success.
They developed steadily and have remained strong throughout the growing process.
Johnsons 'Tetra Versailles Mix'
Johnsons 'Tetra Versailles Mix'
The Tetra Versailles Mix was another story entirely.
Despite receiving exactly the same treatment as the other two varieties, only one seedling has emerged from the entire packet.
Even now, that lone survivor appears to be struggling with the commitment.
Whether I received a disappointing packet, encountered some gardening mystery I do not yet understand or simply used up all my beginner’s luck on the first two varieties, I genuinely couldn’t tell you.
Potting On and Hardening Off
Once the seedlings had developed their first true leaves, I hardened them off over three days.
After that, they were moved into larger pots.
I couldn’t tell you the exact size because they were simply a collection of old pots I already had lying around.
Each pot received three seedlings and has lived outdoors ever since.
The entire process felt far less complicated than I had imagined.
Learning to Pinch Out Cosmos
One of the most useful tips I discovered came from a Sarah Raven reel.
Once the seedlings had developed a couple of sets of true leaves, I pinched out the growing tip to encourage bushier plants and, hopefully, more flowers later in the season.
The Purity seedlings were around two weeks ahead of the Apricot Lemonade plants, so they were pinched out earlier.
I will admit that cutting the tops off perfectly healthy seedlings felt completely wrong at the time.
After spending weeks willing them to grow, deliberately removing part of the plant seemed slightly counterproductive.
Thankfully, they appear to have forgiven me.
Hardening Off Cosmos Seedlings
After developing their first true leaves, the Cosmos 'Purity' seedlings spent several days hardening off outdoors before transplanting.
Potting On Cosmos Seedlings
The next stage of the experiment. The strongest seedlings were carefully separated and transplanted into larger pots after hardening off.
Cosmos Purity After Potting On
Freshly transplanted Cosmos 'Purity' seedlings settling into their new pots. Each pot contained up to three seedlings before their final move into the driveway border.
Where We Are Now
And that brings us to today.
The seedlings are thriving.
The driveway border they are destined for is less so.
At the moment it remains a rather weedy work in progress, waiting patiently for some much-needed attention before planting can begin.
The cosmos are still sitting in their pots, full of promise.
I haven’t seen a single flower yet.
There are no dramatic before-and-after photographs.
No overflowing borders.
No finished garden reveal.
Just a collection of healthy seedlings and the excitement of imagining what they might become.
Hopefully, by the end of summer, these little plants will be filling the driveway border with colour and movement.
For now, I’m simply delighted that most of them germinated at all.
And if the Tetra Versailles Mix decides to produce a second seedling, I will consider that an added bonus.
Reporting from the dust,
xo Lois