Nurse grafting
Nurse grafting is a method of plant propagation that is used for hard-to-root plant material. If a desirable selection cannot be grown from seed (because a seed-grown plant will be genetically different from the parent), it must be propagated asexually (cloned) in order to be genetically identical to the parent. Nurse grafting allows a scion to develop its own roots, instead of being grafted to a rootstock. Nurse seed grafting. A large-seeded woody species, e.g. the chestnut, retains the cotyledons inside the seed coat below ground while the radicle grows downward and the  shoot appears aboveground. To make a nurse seed graft, a germinating seed is needed. A knife is used to cut an opening between the petioles of the cotyledons. The scion, taken from dormant wood of the previous season's growth, is cut to a wedge shape at the end and inserted into the cut between the cotyledons, so that the cambium surfaces of each can join. The grafted plant is then set in a rooting medium with the union about 1.5 inch below the surface. This graft allows the scion to live on the seed's roots long enough to  form adventitious roots of its own. This technique is used for camellias, avocados, and chestnuts. Nurse root grafting. In this technique, a scion is grafted to a piece of root to keep it alive long enough for it to form its own roots. The graft union is planted below the surface of the growing medium, as with the nurse seed method. Once the scion has formed roots of its own, the rootstock can be removed, or it will die off, as will happen in situations when the scion and rootstock are not closely related. This method works well with apple  cultivars, cherries, plums, nectarines and pears. It is also useful for propagating a rare isolated plant that may be unique and should not be moved from the wild. Such a plant can be propagated by a small amount of plant material that will not harm the parent plant by its removal. Nurse root grafting is the best method for propagating tree peonies. Unlike herbaceous peonies with fleshy roots, which are propagated by division, tree peonies have woody stems and extensive root systems that are impractical for division. Their woody stems have few stored reserves and stem cuttings fail before roots  are produced. The problem of keeping the scion alive long enough to produce roots (generally a year) is solved by grafting a tree peony shoot on an herbaceous peony root section; the section of root sustains the scion with its ample stored energy. (In contrast, Itoh peony hybrids, which are crosses between herbaceous and tree peonies, are best propagated by stem cuttings, which root easily.) The basic process for nurse-root grafting of peonies is as follows: