Generally speaking, wild flowers naturally drop their seeds once they are ripe so cutting the plants back at this time would allow the seeds to fall onto the earth at the right time for next year.
Apophthegm - a big word for a small thought. If you live in Derbyshire, as I do.
It depends whether you want more of them for next year. If you don't, you can cut them down whenever you feel like it.
If you do, then it is best to wait for them to have set seed and let it ripen fully. When this is depends to some extent on the weather, but most likely September/October. Then if you wish you can collect some to sow yourself and cut the plants back. Shake some seedheads over the wildflower area, or let the stems lie for a little while before collecting them up. Rake well to expose a.bit of soil and distribute the seed. Ideally you would then send in your flock of sheep, so that their feet could push the seed into the ground, but in the absence of sheep you will have to walk up and down yourself to try to do the same thing.
Then it is just a matter of waiting for spring to see what will come up.
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If you live in Derbyshire, as I do.