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How to Grow Cosmos

How to Grow Cosmos

So many of you are chomping at the bit to get going so here we go.....everything you need to know about sowing, growing and giving Cosmos.

First of all - why wouldn't you? It is the floweriest flower you could have in your garden - the more you cut it, the more it flowers, it will fill your posies all summer long until the first frosts and it's one of the simplest ways to achieve flowery success.

If you're a fan of pink - you can't not. (Other colours can be grown).

Here are my 2 little beds of Cosmos last year to inspire you as to what's coming.

And I'll be doing a whole load of new videos this year as we sow our Cosmos-filled Flower Kits together in The Flower Club so check back here for updates.



The biggest mistake many growers make is to rip open the packets enthusiastically too early in the year.
Cosmos are half-hardy, bordering on tender annuals which means they cannot and will not handle frost.
All well and good, you might think, I'll just grow mine on inside my greenhouse or plastic white box. These things grow fast and they grow big and what you'll find is that they will a) be a constant challenge to keep alive as the earlier you sow = more frost days to survive and b) they will take over ALL your under cover space and you'll need to keep potting them on into bigger and bigger pots as they grow and you'll have no room for anything else!
So, basically, chill your boots and wait. There is no rush.
The ideal time for sowing is 6-8 weeks before your last frost date. You can download the Floral Project Planting Calendar here to find your last frost date and your perfect sowing date.
Mine is around 26th March as my last frost here in Surrey is around 20th May.
(If you're reading this thinking 'Oh shit, I already did mine' - no problem - just be aware of a) and b) above and you'll know better next year!)
Sow one seed per module or sprinkle a few into a seed tray/old yoghurt pot with holes in the bottom filled with no-peat compost and cover lightly. I'll do you a video and add it here when I sow mine.
Water gently and place into your greenhouse or kitchen windowsill. Add a plastic lid or clear plastic bag over the top to allow for some humidity to get them going.
Watch them.
Watch them.
Check them.
Watch them.
Check them.
Repeat this every 10 minutes for about a week - it DEFINITELY helps with germination  😃  😃  😃
Make sure the compost doesn't dry out and it stays moist but not damp.
As soon as you see the first little signs of green, take the lids off and get those babies out into the greenhouse, cold frame or plastic crate - nobody wants a leggy Comsos.
This is the point they need LIGHT - more light than your windowsills or conservatory can give them if you want the healthiest plants possible.

Pricking Out

If you've sowed into a seed tray, prick your plants out when they have their first two true leaves. 
If you've sowed your seeds into 40-cell trays, you'll need to pot them into a 15-cell tray or 9cm pots when their root ball fills the cell they are currently in.
Keep some horticultural fleece handy and watch the weather app like you have  weather-based OCD. If you see temperatures dip, get them covered or bring them indoors for the night.


Once they are around 20cm high, you'll need to pinch their main tip out.

This simply means taking the main stem between your two fingers (or using a sharp, clean pair of secateurs or scissors) and removing the stem, leaving the first couple of sets of leaves.

If feels so cruel, but if you don't you'll have one spindly tall plant - we want bushy, stocky fellows.



Keep potting them on if their root balls fill their current pot.

Hardening Off

About 7-10 days before your last frost date is predicted to be, start moving your Cosmos plants out of the greenhouse during the day and back in again at night, gradually building up the time they are exposed to the elements.

This will toughen them up so that when they do go out, their stems will be stronger and tougher meaning they are more likely to win in the inevitable battles against slugs and heavy rain.

Planting Out

After your last frost date has passed, and not a moment before, you can plant them out into the garden. I

In my 1m wide beds, I planted 2 rows with plants spaced 30cm apart and they flourished last year. See video above.


These plants can grow up to a metre tall and if you don't stake them, they will simply snap in the first strong winds.

Here's how I support mine - I place a layer of netting about 30cm up across the beds and then use twine to zig zag around the stakes at a height of about 60cm as they grow. You can see this in the video above.


As with all cut flowers, cut your flowers before the sun reaches them in the morning or after it has left them in the evening to get the longest vase life.

I am lazy so I have never done the former - but if you do, full kudos to you, you can call yourself a proper flower farmer :)

For the best vase life, you'll want to cut them, just as the buds are starting to crack open. Because these things have so many flowers, I pick some at this stage, and then some that are fully open so my posies have instant wow appeal for the people to whom they are given, but also the buds slowly open over time - for me, I love seeing this happen with my flowers at the kitchen table over the course of a week or two.


Once picked, you'll want to condition them, which simply means to strip all the lower foliage off and place them up to their necks in cool water in a cool place for a few hours - I use my stone shed for this and because I cut in the evening, I just leave them there overnight so they are ready for wrapping and giving the next morning.


You can cut them short and create jam jar posies.

You can keep them long and wrap in simple brown paper.

You can mix them with just about any other flower in the garden, or give them all by themselves.

You definitely want to take some 'armfuls of flowers' pictures with these and share them in our Facebook group :)

Then give.

That's it.

If there was one flower you could grow in your garden this summer (and Sweet Peas had been removed from the planet) - these would be the ones.

I hope you decide to join us as we sow, grow and give Cosmos this summer - honestly, you'd be hard-pressed to find anything that will give you as much joy.


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Everything you need to create your own cut flower patch - even if you've never done it before.

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