Weed killer to get rid of agapanthus
Certified GKH Gardening Expert
I would not recommend applying anything for this case. Killing this plant will require killing other plants in the area as well. If you do decide to use this, then you will want to give the area several months before you plant anything, so the poison has time to get out of the soil.
Unfortunately, the best way to control is to pull them up manually.
Here is a link to more information: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/how-long-does-weed-killer-last-in-the-soil.htm