Janet wrote:

It's a translocated weedkiller, which means the weed has to be in
active growth to move the chemical from where it makes contact (the
leaves) to where it takes effect (the roots). Applying it in the dark
rainy cold days of November is a waste of money.

Thanks for the warning.
I should have said that the garden in question is in Italy,
and there were a few very sunny days after I applied the roundup.
But I'll read the label as you suggest before applying it next time.