A simpler method ... IMO, anyway ... would be just to use a disposable foam paintbrush and paint the new ivy sprouts with brush killer. That's how I control the wild blackberry that would happily devour my entire yard if I allowed it.
It seems to me that leaving jars of the stuff sitting around risks the chance of some child or animal knocking it over, which would spill it on the ground around your hydrangea. I definitely think stuff like weed killers and brush killers should be carefully applied to the leaves of the plant you want to kill, not broadcast around an area. And you definitely want to keep it off the ground. But, that's all just my opinion.
"Isn't it enough to see that a garden is beautiful without having to believe that there are fairies at the bottom of it too?" - Douglas Adams