Q.potatoes
what herbicide can i use to kill weeds in my garden after potatoes are up

A herbicide will kill any plant it touches, including your potatoes.
Removing weeds safely is best done by hand weeding.
These are some natural methods of weed control, but you still need to use caution as you can kill your plants using these also.
https://www.gardeningknowhow.com/special/organic/using-salt-to-kill-weeds.htm
https://www.gardeningknowhow.com/special/organic/using-sugar-to-kill-weeds.htm
https://www.gardeningknowhow.com/special/organic/homemade-pet-friendly-weed-killer.htm