I had some onions sitting on the counter for a long time and one of them sprouted, so I planted it in a flower plot to see what would happen. At first it grew well, sprouting several long green fronds. In the last week, however, it has declined, the tips of the leaves turning yellow, then withering. Some of the fronds have started to wilt even though I keep the soil watered.
What can I do to keep the onion alive?
When you plant a sprouted onion, the hope is it produces roots at the bottom - if it does, it will keep producing green shoots. It will not, though, produce any onions, as such, but you can use the green shoots as you would chives. You've not said how long ago you planted the onion, but if its only been 2 or 3 weeks, its possible it hasn't produced roots, the top green growth just carried on growing for as long as it could, and now the onion bulb in the soil is simply deteriorating.
It might be worth turning it out of its pot, carefully, to see what's going on with the original onion bulb - if its going rotten, then bin it, if not and its got roots, maybe it needs a bigger pot. More information about using sprouted onions, including growing from them, here https://dengarden.com/gardening/What-to-Do-With-a-Sprouted-Onion
