For those who have not used this method here's how I do root cuttings. Pot up the root cuttings into normal potting mix. with the tops a bit above soil level. It is important to get root cuttings the right way up. New roots will grow from the distal end (furthest away from the trunk) and branches will shoot only from the proximal end (closer to the trunk). It is easy to get them around the wrong way because there's often not much difference. Some went in upside down one year when I was not concentrating.
New shoots emerged from the end under ground and then grew up and out of the mix. They even grew roots from the junction of the new shoot and the old root so these Chinese elms are pretty resilient.Sizes above are only a guide. many species will grow from shorter or longer root cuttings. If I find a nicely shaped longer elm root I'll often pop those in as they produce nicely shaped trunks. Elm roots are really flexible so you can also wire them to give really attractive shapes.
One down side of root cuttings is reverse taper. Roots tend to get thinner the further away from the trunk so your new trunk can have reverse taper, especially if you use longer pieces. Select roots that don't taper too much if you want better trunks.
Here are a couple of trees grown from root cuttings. Not all species grow from root cuttings.
Species that do include: any elms, including zelkova, wisteria, apples and crab apples, quinces and flowering quinces. Grant and Barefoot have posted that gingko also grow from root cuttings.
Happy for others to add other species that you have found can be propagated this way.
