One critical thing is to consider the plants in question. Grafts
usually work only between very closely related species. Examples:
Plum, peach, nectarine, apricot, and cherry are all _Prunus_, which is
why you can make a tree with a different peach or plum variety on
every branch. Pear, while in the same family (Rosaceae--rose family),
is in a different subfamily and would probably not be compatible with
Prunus. Mountain ash (which someone mentions downthread) is in the
same subfamily as pear, so it's not surprising that a pear graft on it
would flourish.

Monique Reed
botanist, Texas A&M

Jerry Small wrote:

Just curious if any home gardener has had success in grafting say a
hybrid peach, plum, pear twig to a volunteer tree, ie, one that grew
from seed. If so, what are the critical points in the process that
determine success or failure?