I had some sweet potato vines last spring. Try cutting them in half and sticking the pointy end down in the water. That worked for me, but the whole one didn't. It sometimes takes 2 weeks for the sprouts to show up. The vine, by the way, is edible and delicious! So if it gets too long, now you know what to do with the prunings.
Oh yes....I have also regrown celery roots the same was as the green onions...just put the roots in a little dish with some water in a sunny place, and they will sprout. You can then plant out in soil for 'free' celery. I just kept trimming it off and using it for seasoning until the roots petered out from just the water. I assume they would last longer in dirt.
I did the green onions too, and I had only 1 inch above the roots to work with. They sprouted up really quickly and were delicious!
Edited by kappydell, 12 March 2013 - 10:31 PM.