Getting rid of roots from Geranium Ibericum

I can't think of a way except for spraying the whole plant with Round up or glyphospate, which will work its way back to the roots and kill the whole plant.

It will need a strong dose as some Geraniums are partially resistant to Roundup.

Glyphosate would be best as it breaks down to harmless substances when it comes into contact with the soil. It may take a year or two to get all the bits.