A rootstock is part of a plant, often an underground part, from which new above-ground growth can be produced. It could also be described as a stem with a well developed root system, to which a bud from another plant is grafted. It can refer to a rhizome or underground stem. In grafting, it refers to a plant, sometimes just a stump, which already has an established, healthy root system, onto which a cutting or a bud from another plant is grafted. In some cases, such as vines of grapes and other berries, cuttings may be used for rootstocks, the roots being established in nursery conditions before planting them out. The plant part grafted onto the rootstock is usually called the scion. The scion is the plant that has the properties that propagator desires above ground, including the photosynthetic activity and the fruit or decorative properties. The rootstock is selected for its interaction with the soil, providing the roots and the stem to support the new plant, obtaining the necessary soil water and minerals, and resisting the relevant pests and diseases. After a few weeks, the tissues of the two parts will have grown together, eventually forming a single plant. After some years, it may be difficult to detect the site of the graft although the product always contains the components of two genetically different plants.
The use of rootstocks is most commonly associated with fruiting plants and trees, and is useful for mass propagating many other types of plants that do not breed true from seed, or are particularly susceptible to disease when grown on their own roots.
Although grafting has been practiced for many hundreds of years, even in Roman times, most orchard rootstocks in current use were developed in the 20th century.
A variety of rootstocks may be used for a single species or cultivar of scion because different rootstocks impart different properties, such as vigour, fruit size and precocity. Rootstocks also may be selected for traits such as resistance to drought, root pests, and diseases.