Cosmos are one of the best annual flowers you can grow from seed. They are quick to germinate, and once established they will give you months of romantic, colorful blooms and plenty of height in a border, too.
I've grown cosmos as a cut flower every year for nearly a decade, and always from seed. Despite my years of experience, each spring no matter how much light I give my seed trays, I always end up with a few leggy cosmos seedlings on my hands. The ones that just get too tall and eager before they put on any true leaves.
In the past, I would have discarded the leggy ones, but never again. It is so easy to fix leggy seedlings, and once you know how to do it with cosmos you'll never lose a single plant again.
How to save leggy cosmos seedlings in 3 simple steps
Cosmos seeds are very quick to germinate. In my experience, if you place them on a windowsill indoors in a propagating bag, they will sprout in around five days.
This is all very exciting when starting to grow cosmos, but what can happen next is that the seedlings grow with gusto, searching for the light. They need a lot of light and a lot of water.
The perfect seedling would be about an inch and a half high before it starts to produce true leaves (these are the leaves that form after the first two when a seed germinates), and have a light green stem.
Badly behaved, exuberant cosmos seedlings might have a long red stem, and be 2-3 inches tall before putting on extra growth, making them weak and floppy. If this has happened to any of your cosmos seedings, here is what to do:
- Carefully remove your seedling from its seed tray using a dibber, making sure to keep all the roots in tact. This wooden dibber from Amazon is very similar to the one I use, and is ideal for the job.
- Place fresh potting soil (and horticultural grit if you have any) into a small (10cm) plant pot, and make a fairly deep hole in the center using a pencil or your dibber.
- The key step is to place your seedling into this hole and cover with potting soil right up to its first set of leaves. Then firm in gently and water. It will grow on beautifully, and once it has established with healthy growth of about 4-5 inches, you're ready to transfer it to a bed or a cutting patch.
How to prevent leggy cosmos seedlings in future
As soon as your seedlings begin to germinate, move them outside. They can be kept in a greenhouse or a covered frame, but they need to have constant daylight to avoid them going leggy.
Resist the temptation to keep them on a windowsill, even if it is a super bright and sunny spot.
Shop for your cosmos seed sowing kit
I like using these biodegradable seed pots for all my seedlings, as you can plant them out directly into the soil without having to disturb the roots - that is, unless you are repotting those leggy cosmos seedlings.
Cosmos seeds are about 1cm long with little pointed edges. They are a bit easier to handle than some tiny seeds, such as poppy seeds. This apricot variety is one I look for every year as I love the peach and lemon pastel shades.
Coco coir pellets are a super easy and organic way to sow seeds. Simply place the dry pellets into a tray, add water until they expand, then put your seeds on top and push them into the pellet.
Growing annual flowers from seed is something I do every year, and I still learn from my mistakes as I go. To help you, we have an expert guide on the key seed sowing mistakes to avoid, so with the above tips and this in hand you will be ready for flower growing success.
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