Spot Spraying Question

Looking out over my dormant Timothy field, seeing a few milkweed plants spring up. Not so many that I can't walk around and hit them with a hand sprayer and roundup.

Question is - we're in September - when is the best time to hit milkweed with roundup? Will probably encounter a touch of horse nettle too. Round up effective this time of year on it too?

Thanks,
Bill

Spot Spraying Question