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A rootstock is a stump, which already has an established, healthy root system, used for grafting a cutting from another plant. The tree part being grafted onto the rootstock is usually called the scion. The scion is the plant which has the properties desired by the propagator, and the rootstock is the working part which interacts with the soil to nourish the new plant. After a few years, the tissues of the two parts will have grown together, producing a single tree although genetically it will always remain two different plants.

The use of rootstocks is most commonly associated with fruiting plants and trees but is, in fact, the only way to mass propagate many types of plants that do not breed true from seed or are particularly disease susceptible when grown on their own roots.

Although grafting has been practised for many hundreds (if not thousands) of years, most orchard rootstocks in current use were developed in the twentieth century. The reason for the multitude of different rootstocks for the same species is because they all impart specific properties to the scion; vigour, disease or drought resistance, fruit size and precocity are just some of the properties affected by rootstock choice.

The rootstock can be a different species from the scion, but they must be closely related. A grafting can also be done in stages, with a closely related scion being grafted to the rootstock, and a much less closely related scion being grafted to the first scion. Also, with a serial grafting of several scions one may produce a tree that bears a number of different fruit cultivars. The same rootstock takes up and distributes water and minerals to the whole system.

Grapevines for commercial planting are grafted onto rootstocks; back garden viticulturists will probably find that the plants available to them are not. Grape growers prefer not to take the risk of purchasing or growing a whole plant, when it is safer to establish a number of healthy rootstocks and then graft vines onto them as they desire. This provides an extra measure of control over the growth of the plant, since the quality and characteristics of the resulting fruit are so important.

It can be hard to match a plant to the soil in a certain field or orchard. Growers want a rootstock which is compatible with the soil; the fruiting characteristics of the scion can be considered later, once the rootstock has proved successful. Rootstocks are studied extensively and sold with a complete guide to their ideal soil and climate. Growers determine the pH, mineral content, nematode population, salinity, water availability, pathogen load, and sandiness of their particular soil, and select a rootstock which is matched to it. Genetic testing is growing more common, and new cultivars of rootstock are always being developed.