I dug up a tree a year ago and the roots seem to still be spreading under ground. Every once in a while I will see it sprout above ground. I keep pulling it out but it comes back. How do I kill the root forever without hurting the grass?
Something like Brush Be Gone, which is selective for dicots over monocots, should prove useful. That said, some tree types have roots that can live underground without foliage for many years. Killing the thing may well be a multi-year project.
Put epsom salt on the roots. Multiple applications. Another option depending on there size is poke a few drill holes in the roots and poor in some high nitrogen fertilizer. The decomposers in the soil feed of the nitrogen and process the wood much faster then usual.