Hawthorn rootstock, also know as just 'thorn' used for grafting Medlar and other Hawthorn. Used as a nurse rootstock, that is the graft with say Medlar is as low down as possible. Then it is planted out with the graft joint below ground level. In time the scion will root and then the rootstock should die off. At the very worst you should only get very few hawthorn sprouts that will need cutting off every year.