How do I kill off very old IVY PLANT

YOu need to treat all roots with SBK, a brushwood killer. Fiddly, time consuming and back breaking - dig the area with a fork, with the bottle of SBK handy, sharp secateurs and possibly an electric drill for large roots, and a pile of old pots. Expose roots, no matter how small, drill or make cuts into them and apply SBK to the holes or cuts, preferably without spilling it on the soil. Use neat, wear gloves. After application, cover with a suitably sized upturned flower pot, and push the soil back in place. Then move to the next area, and repeat the process. Do not remove the pots even if you plant the area - they help to exclude soil and keep the SBK in place. If you spill the SBK, don't plant for 6 weeks.

We had a huge old one on the wall of the house. I cut it off at ground level and applied SBK (or similar, it was in the form of crystals) to the cut surface. This killed the roots but they are still in the ground. We planted around and above the ones that we did not remove.

Snoop, if you want to kill the roots so it doesn't grow again, spraying with SBK on the leaves won't work. And no, SBK is not inactivated on contact with soil, it doesn't kill 'through the green' as in the case of something like glyphosate.