Why Combining the Roots and Tops of Plants Produces Better Plants.
Grafting is a technique that vegetatively joins two plants into one. Instead of cross-pollinating two plants and producing a hybrid seed, grafted plants use the roots and bottom portion of one plant (rootstock) and attach it to a tender shoot (scion) from the top portion of another plant. This is often done with trees and shrubs, to combine the best characteristics of the two plants.
Most fruit trees today are grafted onto rootstock. Besides imparting specific characteristics to the resulting plant, it is a quick and reliable means of reproducing plants that do not grow true to type from seed. Unfortunately for the backyard gardener, that means we cannot save seed and grow more plants. In fact, many grafted plants are patented.
What Is a Rootstock?
The lower plant portion used in grafting is called the rootstock. This is usually a healthy root system and some portion of the stem. You've probably seen a nubby bump at the base of rose bushes or fruit trees, like the one in the photo. This is where the graft was made; the graft union. Everything below the bump is rootstock.
The characteristics of rootstocks can make it possible to grow plants faster and in less than desirable conditions. One of the most common uses for rootstocks is creating dwarf fruit trees. Most fruit trees are not only too large for the average backyard, they take years to mature to a size that is capable of bearing fruit.
By grafting a favorite fruit tree onto a rootstock that produces dwarf trees, we are able to create a tree as short as only 6 ft. tall. This is an easy height for a gardener to maintain and pick from and it helps the commercial orchards get up and producing sooner.
Besides dwarfing, rootstocks can contribute traits to improve yield, cold or drought hardiness, and even disease resistance.
Many European wine grapes are grown on a North American rootstock that was discovered to have a resistance to phylloxera, an insect that was threatening the vines in the 19th century.
What is a Scion?
The portion above the graft is called the scion. It is a young shoot or bud from a plant with beneficial characteristics like great flavor, color or disease resistance. All of the top growth of a grafted plant, leaves, flowers, fruits, etc., comes from the scion. By combining the rootstock and the scion you can be reasonably assured you will wind up with a reliably hardy and productive plant.
The rootstock and scion do not have to be from the same species, but they should be closely related, for instance grafting a plum tree onto a peach rootstock. It is also possible to graft several scions onto one rootstock, as they do for apple trees that produce multiple varieties on different branches.
Special Growing Considerations for Grafted Plants
You have to be careful when planting grafted plants. If the graft joint is buried underground, the rootstock can sprout its own top growth or the scion can send down its own roots. When that happens, you lose the characteristics selected for when the plant was grafted.
A Granny Smith apple tree may start producing unrecognizable red apples, from rootstock shoots.
There are also times when the rootstock needs some winter protection, as with many grafted roses. That's why it is recommended that cold climate gardeners cover the graft in late fall, but remember to uncover it in the spring, so the rootstock does not sprout.
Want to Try Your Hand at Grafting?
Grafting is an ancient practice, but most of the grafted plants available today are the result of research done within the last century. Current trials continually push the limits of where plants will grow, such as the USDA Zone 4 cold hardy 'Reliance' peach. If you are interested in trying your hand at grafting, it takes dexterity and patience, but it can certainly be done by home gardeners. Standard root stocks are available from many mail order nurseries.
They describe the attributes of each, so you can do your own experimenting. Here's some help to get you started: