Apricot cultivars are usually grafted onto plum or peach rootstocks. Some species can be grafted onto citrus rootstocks. Trees grafted on dwarfing rootstocks bear about of fruit per year. Pears are usually grafted onto quince rootstocks, which produce small to medium-sized trees. "Citrus maxima" is usually grafted onto other citrus rootstocks, but may be grown from seed. These can be grafted on to standard citrus rootstocks. On alkaline soils, chestnut trees can be grown by grafting them onto oak rootstocks. They were replanted with Sauvignon Blanc, partly because it grafted better onto the American rootstocks. However, trees grafted on these rootstocks often succumb to black line. Grafting such varieties onto disease-resistant rootstocks offers protection. Layering is the technique most used for propagation of clonal apple rootstocks. Some are herbaceous plants; others have woody rootstocks. There is greater variability than with the vegetatively propagated rootstocks. Grafting is often not a complete cloning method because seedlings are used as rootstocks. The breeding grounds received seedlings free of charge as well as rootstocks and grafts.