Q.When I try to root clippings in water they will start to root well and then after about a centimeter, the roots turn black on the ends.
Certified GKH Gardening Expert
If you're trying to root a plant with a woody stem, this may not be the best way to go. Softer, more pliable stems will root in water much more easily. Plus, be sure to change the water every 2 or 3 days. Here's an article for you: https://www.gardeningknowhow.com/garden-how-to/propagation/cuttings/plants-that-root-in-water.htm
If the plant you're trying to propagate has a woodier stem, here's an article on how best to do that: https://www.gardeningknowhow.com/garden-how-to/projects/rooting-plant-cuttings.htm