How Do You Join Two Plants? One Root System, Somebody Else's Branches

Grafting joins a piece of one plant, the scion, onto the root system and lower stem of another, the rootstock, so that the two grow together as a single plant while retaining their separate genetic identities. The join works because a thin layer of dividing cells lies just beneath the bark of both, and if those layers are brought into firm contact they produce new tissue that bridges the gap and reconnects the vessels carrying water and sugars. The scion determines what the fruit or flower will be and the rootstock determines the root system, the vigour, the eventual size and the tolerance of soil conditions and diseases. Success requires close botanical relationship, with most grafts working within a species or between closely related species and failing beyond that.

Several reasons make grafting not merely convenient but unavoidable:

• •Most fruit varieties do not come true from seed, since seedlings differ from the parent
• •A named variety is therefore a single original plant propagated vegetatively ever since
• •Rootstocks control tree size, which determines orchard spacing, pruning and picking
• •Rootstocks supply disease and pest resistance the fruiting variety lacks
• •Rootstocks tolerate soil conditions including wetness, drought and lime that the scion cannot
• •Grafted trees bear fruit years earlier than seedlings would

The most consequential application saved European wine production. An aphid-like insect attacking grapevine roots arrived from North America in the 1860s and spread through European vineyards, destroying a very large share of them over three decades and causing enormous economic damage and displacement. American vine species had evolved alongside the insect and tolerated it, while European vines had not. After chemical and cultural remedies failed, the solution adopted was to graft European fruiting varieties onto American rootstocks, which resists the insect while leaving the fruit unchanged, and essentially all European vineyards were replanted this way. The practice continues, so nearly every bottle of wine from such regions comes from a grafted plant, and the episode is a standard illustration of the danger of moving organisms between continents.

Grafting supports several practices that sound implausible and are routine. A single tree can carry several varieties grafted onto different branches, which is done to supply pollination partners, to fit several varieties into a small garden and occasionally as a display piece carrying dozens of varieties. Top-working replaces the entire canopy of a mature tree with a new variety, which brings a producing tree of the new variety in a few years rather than a decade. Bridge grafting repairs a tree whose bark has been stripped around the trunk by rodents, by inserting shoots that span the damage and restore the connection. Inarching adds a new root system to a failing tree. And grafting is used in research to determine whether a signal travels from root to shoot, since joining a mutant to a normal plant shows which part carries the effect.

Several techniques exist and each suits particular material. Whip and tongue grafting joins pieces of similar diameter with matching diagonal cuts and an interlocking tongue, which maximises contact. Cleft grafting inserts a wedge-shaped scion into a split in a larger stock and is used to change the variety on an established tree. Budding inserts a single bud under the bark of the stock and is standard for roses and for producing fruit trees in quantity. Approach grafting joins two plants while both remain on their own roots until the union takes. The limits are real, since incompatibility between species produces unions that fail years later, and some plants graft badly or not at all. Virus transmission through grafting is a recognised hazard, which is why certified material matters.

A scion supplying the fruit is joined to a rootstock supplying the roots, and the union forms because dividing cells beneath the bark of each bridge the gap. Fruit varieties do not come true from seed, so a named variety is one original plant propagated ever since. Grafting European vines onto resistant American roots saved the wine industry after an insect destroyed the vineyards.