Hi,
You should leave them on the plants until they turn brown. Picking earlier lowers the germination rate greatly.
They may vary in size. That they are hard and not easy to crush is more important.
Remy

Thank you!

My nasturtiums have been blooming all summer and I am unable to find any seed pods on them. Does anyone know why this is. I have looked underneath and there is nothing there.

I do have a groundhog that eats a few leaves. Would he have eaten all the seed pods?

The nasturtium I planted last season re-seeded by themselves. I got some interesting color combinations this year! One plant has flourescent orange flowers, one is blood red, and I have several different yellows and some bi-colored ones, too. Beautiful!

Last summer the few I planted got some seeds but there must be something in my garden that likes to make a snack of them. They disappeared while still green.
Just unlucky I guess.