Many crop plants (and cultivated garden and house plants) can reproduce asexually.

Many plants can be increased by artificial methods:

Those crops which are grown from asexual reproductive structures have generally been found to be highly heterozygous, which probably reflects the superior vigour of these commercial varieties. Potatoes are the supreme example. Each variety of potato consists of a huge number of clones, which explains the uniformity of crop from each variety. Genetically each of these varieties is very heterozygous and new varieties can be produced by crossing the existing types. However, many potato varieties are reluctant to flower. Many need to be stem grafted onto tomato plants (which are closely related) and they will then produce flowers. These flowers are hand pollinated to give the required crosses between varieties. The potato fruit is a small green tomato-like object. The seeds produced can then be grown and subjected to selection.

Grafting. There are many varieties of plant which have a useful characteristic, eg large flowers in a garden rose, or fruit yield in an apple tree. But they often have less desirable traits such as weak stem growth or poor root development. These undesirable traits can be avoided by grafting the variety upon a rootstock which is known to be viable, resistant and strongly growing. It is possible to graft a number of varieties onto the same rootstock and thereby gain the benefits of a varied crop even in a limited growing space.

Grafting can be performed in a number of ways, the most common being a) bud grafts, where buds from the scion are grafted onto the rootstock, and b) whip or tongue grafting in which a stem from the scion is grafted onto the rootstock. Bud grafting is most successful in the spring and early summer, while stem grafts are most successful if carried out over the winter period.