By Amsterdam
Shropshire,
United Kingdom 
Cosmos not flowering but growing very tall. I have grown cosmos from seed and have had lots of lush growth but hardly any flowers this year. They are 6ft tall and look healthy. I cut the tips off 6 weeks ago in the hope to encourage flowers and stop them growing even higher. How has anyone else got on with Cosmos this year? These are photographed whilst in shade but get over 6 hours of sun every day.
28 Sep, 2020
Answers
Half of mine are flowering, the others are doing the same as Amsterdam's. Same soil, same situation.
28 Sep, 2020
I sowed a whole packet of cosmos seed and only one came up! That has been a magnificent specimen, however, and has bloomed continually throughout the summer. It's just in ordinary compost in full sun.
1 Oct, 2020
Yes, it’s been a mixed bag this year for cosmos.....
3 Oct, 2020
Oops! The flowers form on the tips! So you have probably set them back.
Two other things help Cosmos to bloom:
They need at least 6 hours of direct sun a day, so you might want to check the sun exposure where you plant them.
They also like poor, sandy soil, so you may want to cut down on the compost, and even mix in some sand before planting, if you have exceptionally black, rich soil.
28 Sep, 2020