How to: Grow Cosmos

I thought I’d start my ‘How to grow’ series with what I’m considering my most successful flower so far… Cosmos.

Why did I decide to grow from seed?

1. Because it is so much cheaper than buying flowers in bloom from the garden centre. For one pack of cosmos, containing 450 seeds I paid 99p + 95p postage.

2. There’s a larger variety of flowers and colours to choose from.

3. Starting seeds off indoors early means an earlier and longer flowering season.

Cosmos and tomato plants were the first two seeds that I sowed when trying to grow from seed for the very first time. If you read my last post, a gardening amateur, you’ll know that I am not particularly experienced with the garden so I decided to start with cosmos because not only do I love this particular variety of flower but they are considered one of the easiest flowers to grow from seed and so far I haven’t failed. Despite early concerns of leggy seedlings and basically not having a clue what I was doing.

I did some very basic research before sowing the seeds and only needed a few things to get started:

Propagator trays with cell inserts and lid

I bought my first couple of seeds from our local garden centre, Cosmos Rose Bon Bon, Verbena and tomato seeds but I have to say they were quite pricey. I then took to Amazon and found a large variety of seeds and ordered more. The packs had a huge number of seeds and were cheaper than the garden centre. Here is what I ordered:

Cosmos Purity
Cornflower Centaurea Cyanus Pink
Cornflower Classic Fantastic
Sunflower F1 Full Sun
Zinnia Lilliput

The cosmos and cornflowers for my front garden and sunflowers and zinnia’s for my vegetable patch where I’m hoping to start growing some cut flowers too.

On my first attempt I sowed 24 Cosmos seeds and 24 tomato seeds. One tray for each plant. I realised once I started sowing the seeds I should have perhaps done a little more research and then panicked that I’d failed before I’d even finished sowing them. I then did a ton of research and found conflicting information, for example, should I water the seeds before or after they’ve been sown, or not at all, do they require a light mist of water, when should I take the lid off. Once I started, the questions were endless.

My biggest concern with my cosmos seeds is that I should have watered the compost before planting the seed, I read this on a few different sites after planting them!!  I shouldn’t have worried though because it doesn’t seem to have created any problems. I did however spray a fine mist of water the next day to compensate and ensure the compost wasn’t dry.

In light of all my concerns and confusion, here’s how I now sow my seeds and what seems to have worked well for this beginner so far:

Step one: Fill all of the cells with compost, fill just below the top line (so there’s room to place the seed and cover) and firm down with your fingers.


Step two: Water the compost at this stage. I found conflicting information on watering but this seemed to be the most common time to water and from sources that I felt carried more weight.

Step three: Place one seed in the centre of the tray. Some people sow multiple so you have a higher chance of germination of one seed but I was happy with one per cell. If you do plant more than one seed, once they’ve germinated and started growing you prick the smallest seedling out to leave one per cell. A little trick when pricking out seedlings, you could cut the seedling just above soil level so not to disturb any roots of the strong seedling.


Step four: Sprinkle a small layer of vermiculite over the top of the seed. Vermiculite increases water and nutrient retention and aerates the soil, resulting in healthier, more robust plants. I had enough for my first lot of seeds but not the second, including the sunflowers so I used a fine layer of compost for these and so far so good. You may want to experiment with both.

Step five: Label with the flower name and date. Fit the lid and leave in the sunniest window you can find. If they don’t have enough sunlight once they’ve germinated they will become leggy as they try to grow towards the sun.


I planted the Cosmos Rose Bon Bon variety first (6th April) and they germinated within three days. Much quicker than anticipated! The first leaf you’ll see is called the Cotyledons and they are the first leaves to emerge from the soil when a plant germinates. Because they are part of the seeds embryo, they also are known as seed leaves and provide nutrients to the seedlings until its true leaves unfurl, the seeds true leaves will be the very next ones to grow.


As the seedings started growing (20/24 of the seeds germinated) I was concerned with how tall they had become and was worried I was growing leggy seedlings. I did so much research on leggy seedlings, watched so many videos and reached out to Kirsty @my_little_allotment (who I love following) for advice. Determined that while they looked particularly tall and long I would keep going. I concluded that they didn’t look weak, and while some were bendy, most were stable and they weren’t growing sideways towards the sun but upwards to the sky so took these as positive signs. We had grown quite attached to these little seedlings. Checking their growth more than once a day, checking they weren’t dry, had enough sun. And I wasn’t ready to give up on them just yet!

11th April (5 days)

The next milestone was planting them on into a bigger pot so they had more room to grow. But when? Advice was once they have two sets of true leaves, I actually planted on earlier than this, after they had one set, as I wasn’t convinced they were growing well in the small seed trays and honestly, this is the best decision I made. They have grown so big and strong since putting them in a slightly larger pot.

28th April (3 weeks)

Once they’ve been potted on and during the month of May, once the weather becomes warmer, they’re ready to harden off outside in an open greenhouse before moving directly outside. Since we were going on holiday on 4th May I decided to wait until we were home to start this step as I wouldn’t be able to move them inside and out as required to start this process. I am now over half way over hardening them off and so excited to get planting them out in the next week or two. They’re looking strong but I think ready for more room to grow again but rather than disturbing the root system I’ll wait to plant them out.

21st May (6 weeks)

I’ll get a blog put together on how to harden off plants because thats another story. And if you want to grow your own flowers from seed you can now sow directly outside now! In fact, I think I’m going to try growing direct too for even more flowers!

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