One of my favourites. Because they come in all sorts of colours and have a good long (and often late) flowering period. Mine are still in flower in September.
There's bound to be an approved method of collecting seeds and putting in a lot of hard work and effort to rearing them and potting them on etc etc etc but IME they're dead easy to do.
They self seed and spread everywhere if you leave them to their own devices. That will be why you've got them growing in the cracks of paving and it is how I've got so many in my garden.
I just collect the seeds once they're ready and sow them where I want them to grow. Do that now and they'll be coming through next year.
I use canes or twigs to give them support.