Sweet Peas vines! I have so many of them on a flower hill, we look at the hill and don't know which invasive plant will win; including the sweet pea there must be seven other ones. Cut them anytime you want. If I had the time I would cut them before they had the chance to drop their seed. But if you want to share seed or keep sowing others for yourself you will need to allow them to mature. Cutting them down in the fall will be just a matter of looks.

Diane