How Does Grafting Work? Joining Two Plants Into One

Apples, pears, cherries and many other fruit trees are highly heterozygous, meaning their offspring vary enormously, so a seed from a particular apple grows into a tree bearing fruit that is genetically new and usually poor. Every named variety is therefore a single original tree, propagated by cutting, and every tree of that variety is a clone of that original, which means the apples in a shop descend directly from one seedling that somebody noticed centuries or decades ago. Some plants can simply be rooted from a cutting, and many fruit trees root badly or not at all, which is where grafting comes in: instead of persuading the cutting to make roots, it is joined to a root system that already exists. That also allows the root system to be chosen independently, which turns out to be at least as important as the variety on top, because the rootstock controls the size, the speed of fruiting, the soil and disease tolerance and the tree's lifespan.

The biology of a successful graft depends on one thin layer of tissue:

• •The cambium, a layer of dividing cells between the bark and the wood, is the only part able to generate new tissue, so the cambium of the scion and the cambium of the rootstock must be brought into contact
• •The cut surfaces produce callus, an undifferentiated mass of cells, which fills the gap between them
• •Cells in the callus then differentiate into new vascular tissue, connecting the water-conducting xylem and the sugar-conducting phloem of the two plants
• •The union must be held firmly and kept from drying, which is what grafting tape, wax and sealants do
• •Timing matters, with most grafting done when the rootstock is becoming active in spring while the scion is still dormant, which is why scion wood is cut in winter and kept cold
• •Compatibility is required, generally within the same species or closely related genera, and incompatible combinations may unite and then fail years later, which is a genuine commercial problem

Several methods suit different sizes and seasons. Whip and tongue grafting cuts matching slanting faces with interlocking tongues, which gives a large area of cambium contact and a mechanically secure join, and is the standard for nursery work on material of similar diameter. Cleft grafting splits a larger stock and inserts wedge-shaped scions at the edges, used for topworking, meaning changing the variety of an established tree. Bark grafting slips scions under lifted bark on a large stock in spring. Budding inserts a single bud rather than a shoot, with T-budding and chip budding used extensively in commercial fruit and rose production because it is fast, economical of scion wood and can be done in late summer. Approach grafting joins two plants while both remain on their own roots and separates them only after the union forms, which is the most reliable method for difficult subjects. Bridge grafting repairs a tree whose bark has been ringed by rodents by installing living bypasses across the damage.

The most consequential development in modern fruit growing came from rootstock breeding rather than from varieties. Work at East Malling in England from 1912 onward classified and bred apple rootstocks by the size of tree they produce, and dwarfing rootstocks transformed orchards from widely spaced standard trees requiring ladders to intensive plantings of small trees on trellises. The effects are substantial: a dwarfing rootstock produces a tree a fraction of full size, brings it into fruiting within two or three years rather than seven or more, allows far more trees per hectare, and makes pruning, spraying and picking possible from the ground, at the cost of requiring permanent support and irrigation because the root system is small. Rootstocks also supply disease and pest resistance, which is what saved European viticulture: the phylloxera aphid devastated vineyards from the 1860s, and the solution adopted was grafting European vines onto resistant American rootstocks, which is why almost every wine grape grown today sits on American roots.

Grafting joins a chosen variety to a chosen root system because fruit trees do not come true from seed, so every tree of a named apple variety is a clone of one original seedling. The union forms when the cambium layers of both parts are brought into contact and generate new connecting tissue, which is why alignment, firm binding and protection from drying decide success. Rootstocks control tree size, time to first fruit and disease resistance, and grafting onto American roots is what saved European vineyards from phylloxera.