The scion is the part of the grafted plant that will produce the plant’s shoots. It will, in the future, give rise to all of the plant’s leaves, stems, flowers, and fruits.
The scion is typically the top part of the grafted plant. If it is inserted lower down on the plant during the grafting process, everything above the scion is usually cut off in the spring. This forces all of the nutrients and water from the rootstock into the growing scion.
The rootstock is the lower portion of the grafted plant, the part that produces its roots. “Understock” and “stock” are synonyms for “rootstock.”
Usually, the top of the rootstock is about at the soil line, but not always! When plants are “top worked,” they might be grafted at a point many feet above the ground. In this case, everything below the graft, including perhaps an old tree’s trunk and branches, is the rootstock. Topworking is how weeping standard trees are produced: by putting a weeping scion on a non-weeping rootstock.
Why Scions Are Chosen
A scion is chosen for the good characteristics it can give to these plant parts. There are many possibilities, such as beautiful flowers, a special form such as weeping, or delicious fruit. In apple growing, for example, the type of apple produced is controlled by the scion. If a grower grafts twigs of a ‘Jonathan’ variety tree onto a rootstock of a different variety, it still produces ‘Jonathan’ apples.
In-home grafting, scions that are woody twigs or larger will probably be easiest to work with, but scions can be many things. A single bud can be a scion, producing all future growth over time. Working with bud scions is generally called “budding,” but budding is just a particular type of grafting.
Why Rootstocks Are Chosen
Since the scion produces the parts of the plant that we see and the parts we eat and enjoy, it’s a little harder to imagine why a rootstock might be special. Sometimes, having a particular variety of rootstock is not important: you graft the scion you want onto the root system of a compatible plant you happen to have. As long as a good union can form, the new scion will benefit from all the water and nutrients that that already-established rootstock can take up.
As a home gardener, you won't be picking specific rootstocks unless you go out of your way, but the stocks of the plants you buy will be silently at work for you. And when you see a sucker rise from the ground and have different wood or flowers from the rest of your tree, you'll know why: it's the sucker of a rootstock.
But the right rootstock can be more than just roots. The right variety of rootstock can be adapted to a type of soil that the scion alone would fail in, such as clay-soil adapted apple rootstocks. Some pear rootstocks make the plant bear fruit and ripen earlier (“precociousness”). Some grape rootstocks are chosen for resistance to parasitic soil nematodes.
In a counter-intuitive example of rootstock usefulness, many apple rootstocks are chosen because they grow slowly and keep the whole plant small. This ensures that the apples are borne low to the ground, at easier picking reach than in old-fashioned orchards where full-size trees were grown and required ladders.
Rootstocks are produced by research and sold to trade growers, not the public, so they tend to have less sexy names. One popular pear rootstock that dwarfs the tree and resists common diseases is called OHxF 87!