I would spread out 1/2 cm to 1 cm of sawdust or wood shavings, followed by a cautious portion of pelletised agricultural limestone. Follow that with a few hours of soaking with a bubbler on the end of the hose, unless you get a very heavy rain. The wood absorbs the excess nitrogen, while the limestone neutralizes the resulting acid. If your soil is chalky, you won't need the limestone. Note that the taller varieties of Cosmos often don't bloom until well into fall no matter what you do.

I'm so pleased that you said that about the taller varieties Tug, as I don't have any of the substances you suggest. These are tall varieties so I think I'll just live in hope/

I made the same mistake with my impatiens. I've flushed out the fertilizer with the garden hose. I've only used fish fertilizer which is very mild. They eventually resumed flowering after a week or two. Depends on how much fertilizer you used and what kind. Too much of certain types can damage the roots.

Bathgate, it was tomato food and I think did it twice. When i looked online it said that overwatering can stop them flowering. However, I'm going to water them when the sun goes down as they are drooping. It's been so hot today. I've never used fish fertiliser but I'll look out for it ; sometimes good to have a milder boost.

OK if it's granules, that will be difficult to eradicate. -

Bathgate, it was liquid seaweed not granules. Hope that is easier to dilute than granules. Thankyou so much.

Ooh I fed my white cosmos with tomorite, as all my other plants, and they're fine....mind you they do have entry of (rain) water🙄