Q: English ivy is taking over my back yard. It has probably been there for twenty-plus years. What’s the best product to kill it?
A: I’ve used glyphosate (click for sources) and triclopyr (click for sources) very successfully to kill ivy for years. In my experience, you have to be patient. You won’t see damage to the vine for a couple of weeks. I spray every three weeks to keep a three-foot wide ivy-free zone around my fence line.
Regarding the long time between spraying and seeing damage, I think it has to do with the “layers” of ivy leaves. On your first spray, the upper layer is covered but not the lower layer of leaves. When you come back weeks after spraying you might conclude that the herbicide has been ineffective.
My experience proves that herbicides do kill ivy. The key ingredient is the process is patience.
If you have a large, flat area of ivy, mow it first, as low as you dare go with the mower blade, then spray the sprouts when they emerge in a few weeks.