They are various members of the Bittercress family which previously had small white flowers; another, polite, name for them is 'poppers'.
They are a nuisance annual and it is better to try to get them out before they pop as the seeds will come back to haunt you next year.
It is not possible to get rid of them completely!!!!

It's a common mechanism for many plants, so we'd need a photo, or more detailed description to be sure. Hairy Bitter Cress is a possibility, but rather smaller than you describe.

Some bbc photography was fascinating to watch in slow motion....

hairy bittercress...absolute nightmare !!!!!

With you there, budd

thanks for all your advice.

was going to add a photo but have just googled bittercress. now i know it is hairy bittercress. thanks again for all your replies!