I have a large planter where healthy shrubs and vines have been growing. Now my wife wants to replace them with something more “flowery”. I weed-whacked and pulled out a bunch but the roots seem to be in all the soil and I need to figure out how to kill off the existing remnants so when we plant our new flowers they will not have to compete with the old foliage. How can I do this?
To be honest, if it is a planter, I would completely replace the soil. Throw away the soil that is in the planter (which would also remove the roots) and start fresh. The reason being is that plants in planters can quickly drain the soil in the planter of nutrients, the soil breaks down over time and is not as healthy and there could also be diseases and pests in the soil from the last plant. It is just good practice to replace soil when you plant a new plant. If it is a large planter, you can buy potting soil by the bale - which is much cheaper than the bag. Many garden centers will sell it by the bale.
If replacing the soil is not an option, then you can trim a bit off what is left and paint the fresh cuts with Round Up. The roots will suck in the Round Up through the cuts and that will kill the roots. It will still take quite a bit of time for the roots to decompose though. But new plantings should be fine while the roots decompose.