two pear varieties grafted onto the same rootstock
No one knows when grafting of plants began. Initially, grafting was thought to be an extension of vegetative or clonal propagation by cuttings. Just as softwood (shoot) or hardwood cuttings, detached from certain plants and placed directly into the ground at the proper time of the year, will strike roots and grow into new plants, it was thought that grafting was a similar process. The only difference was that, in grafting, the detached shoot from one plant would supposedly “take root” in the stem or the trunk of another.
Today, we know that the cambium or actively growing layer of cells just below the bark is responsible for a successful graft. Where the cambium of the recipient stem unites with the cambium of the bud or stem grafted into it, the graft is successful and the two plants now grow as one.
The First Practice of Fruit Tree Grafting
The practice of grafting is best understood against the backdrop of fruit tree domestication, which started in the Middle East with the five Biblical fruits mentioned as indigenous to the Land of Israel. Starting in the fourth millennium B.C., olive, fig, pomegranate and grape were cloned by rooting stem or hardwood cuttings, taken in late summer, fall, or early winter, while date palms were propagated from pups/offshoots/suckers growing at the base of their trunks.
Noah, as recounted in the Book of Genesis, is famous for making sure that pairs of every animal species traveled with him in his ark during the flood. Yet Noah was a horticulturist, too. Soon after Noah left the ark, exactly a year after entering it during the fall season, he planted a grapevine which quickly produced grapes. Since the world had been inundated, plant life on earth had disappeared so the grapevine that grew must have been with Noah in the ark. Since the flood came in the fall, that would have been the perfect time to detach hardwood grapevine cuttings and root them in pots of soil that Noah could have brought on board the ark. Inside the ark, with all those animals, it must have have been as warm as a well-heated nursery or hothouse. There was a skylight in the roof through which light would enter. Well positioned hardwood grape cuttings could have gotten sufficient light and, together with the animal generated heat, grown like weeds in this environment. Who knows? Perhaps a year in the ark would have been time enough for those woody cuttings to produce grapes from which Noah made wine, albeit with unfortunate consequences, soon after leaving the ark.
It took another three thousand years, some time during the first millenium BC, before grafting was put into practice. This procedure was an innovation of people living in the colder climate of northwest Asian, where grafting is thought to have begun. Unlike the Middle Eastern semi-tropical fruit species mentioned above, which could be propagated simply by sticking stem pieces into the earth, the plums, peaches, cherries, apples, and pears of a chillier Asian environment could not be propagated in such a manner.
The first grafts were no doubt a result of seeing two fruit trees self-graft or graft naturally into one another, probably because they were growing so close to each other that two stems touched and then bonded together, as sometimes happens in nature. The branch that grew from that natural graft probably bore fruit that was more plentiful and disease resistant — major reasons for grafting until today — than were produced previously.
Fruit Tree Bud Grafting
Bud grafting of citrus fruit tree
Today, most fruit trees are produced by bud grafting, known in the nursery trade as just plain “budding.” A bud growing on a tree that you wish to clone is grafted into a pencil-sized stem of what is typically a seedling of the same species or of a different, but related, species. The reason for doing this is that the scion will exhibit more vigor when grafted into a rootstock seedling that if it were cloned, for example, as a cutting and grown on its own roots. Today it is possible to grow Santa Rosa plum, Elberta peach, Bing cherry, and Gala apple trees from clonal cuttings but the trees that result are much less healthy than when buds of these trees are grafted onto rootstocks. Grafted trees have increased quantity, and sometimes even better quality, of fruit, disease resistance, pest and nematode (a microscopic, soil-dwelling worm-like organism) resistance, and cold tolerance.
Interspecific grafting among Prunus, or stone fruit, species is common. Almond (Prunus amygdalus) buds may be grafted into peach (Prunus persica), plum (Prunus salicina), or apricot (Prunus armeniaca) seedlings. Intergeneric grafting of plants belonging to the same family is also sometimes feasible. Quince (Cydonia oblonga) buds, for example, may be grated into pear (Pyrus communis) or loquat (Eriobotrya japonica) trees. All three of these trees belong to the rose family (Rosaceae).
Eventually, the bud will grow into what is known, in grafting parlance, as the scion, while the trunk and roots below the budding area compose the rootstock of the developing tree. On a mature fruit tree, the point on the trunk where budding took place is known as the graft union and is typically visible as a swelling or bulge on the trunk. (In the nursery, once the scion bud starts to grow into a shoot, it is trained to grow up vertically while the stem portion of the seedling rootstock above the bud union is cut off.)
All of this is meant as background to a situation described by Shirley Amdisen in an email she recently sent. “I’ve never grown citrus trees,” Amdisen confided, “but when we moved to Thousand Oaks we inherited quite a few trees. One of them is a mystery: it has small bright orange tangerines on the bottom half of the tree, which have a lot of seeds and peel easily, very ripe right now and very sweet. On the top half, there is an entirely different fruit, more like an orange but smaller and yellower and very tart, almost like a lemon. I can’t figure out how two fruit can grow on one tree, divided between top and bottom. What have I got here?”
Although rootstock species impart vigor to the scion, their own fruit is either bland or bitter. What has happened here is that a sucker grew out below the bud union and matured into an undesirable horticultural monstrosity. Whoever was living in your house at the time could have eliminated this outcome by pinching off the young sucker when it was a few inches long. Due to neglect, that sucker turned into its own fruit producing behemoth. What you need to do is remove the rootstock growth. Start cutting it back at the top and proceed down to the point on the trunk where it originates.
Tip of the Week: Every garden should have a mahonia or two. Several are native to California, even though they are generically referred to as Oregon grape. They can take sun or light shade. I have found them to be short-lived, probably because they cannot tolerate any excess water in the root zone and they die either from over-irrigation or because they are planted in soil with imperfect drainage. Many are tolerant of freezing temperatures, down to 0 degrees or colder. Their bluish-purple fruit is highly recommended for making jelly and is a magnet to birds. Joy Krauthammer got me thinking about mahonias after sending me a photo of a Mahonia oiwakensis specimen that she found growing in Northridge. I am partial to the closely related Nevin’s barberry (Mahonia/Berberis nevinii), an endangered species native to the San Fernando Valley and found growing wild, here and there, in Riverside and San Bernadino Counties, too. Only 500 plants are thought to exist. Nevin’s barberry grows into a magnificently symmetrical 10 foot tall by 10 foot wide shrub with a blanket of yellow flowers followed by scads of bird-pleasing red fruit. In addition, its diminutive leaves are spiny and blue-grey. It is an excellent candidate for a living border or fence around your property.