Perennial plants can be propagated either by sexual or vegetative means. Sexual reproduction begins when a male germ cell (pollen) from one flower fertilises a female germ cell (ovule, incipient seed) of the same species, initiating the development of a fruit containing seeds. Each seed, when germinated, can grow to become a new specimen tree. However, the new tree inherits characteristics of both its parents, and it will not grow "true" to the variety of either parent from which it came. That is, it will be a fresh individual with an unpredictable combination of characteristics of its own. Although this is desirable in terms of producing novel combinations from the richness of the gene pool of the two parent plants (such sexual recombination is the source of new cultivars), only rarely will the resulting new fruit tree be directly useful or attractive to the tastes of humankind. Most new plants will have characteristics that lie somewhere between those of the two parents.
Therefore, from the orchard grower or gardener's point of view, it is preferable to propagate fruit cultivars vegetatively in order to ensure reliability. This involves taking a cutting (or scion) of wood from a desirable parent tree which is then grown on to produce a new plant or "clone" of the original. In effect this means that the original Bramley apple tree, for example, was a successful variety grown from a pip, but that every Bramley since then has been propagated by taking cuttings of living matter from that tree, or one of its descendants.
The simplest method of propagating a tree vegetatively is rooting or taking cuttings. A cutting (usually a piece of stem of the parent plant) is cut off and stuck into soil. Artificial rooting hormones are sometimes used to improve chances of success. If the cutting does not die from rot-inducing fungi or desiccation first, roots grow from the buried portion of the cutting to become a new complete plant. However, although this works well for some plants (such as figs and olives), for most fruit tree cultivars this method has much too low a success rate to be commercially viable. Root cuttings (pieces of root cut off and induced to grow a new trunk) are also not used to propagate fruit trees, although this method is successful with some herbaceous plants.
A refinement on rooting is layering. This is rooting a piece of a wood that is still attached to its parent and continues to receive nourishment from it. The new plant is severed only after it has successfully grown roots. Layering is the technique most used for propagation of clonal apple rootstocks.
The most common method of propagating fruit trees, suitable for nearly all species, is grafting onto rootstocks. This in essence involves physically joining part of a shoot of a hybrid cultivar onto the roots of a different but closely related species or cultivar, so that the two parts grow together as one plant. The process of joining the two varieties must ensure maximum contact between the cambium (the layer just below the bark) of each, so that they grow together successfully. Grafting is a preferred method because it not only propagates a new plant of the desired hybrid cultivar, it usually also confers extra advantages as a result of the characteristics of the rootstocks (or stocks), which are selected for characteristics such as their vigour of growth, hardiness and soil tolerance, as well as compatibility with the desired variety that will form the aerial part of the plant (called the scion). For example, grape rootstocks descended from North American grapes allow European grapes to be grown in areas infested with Phylloxera, a soil-dwelling insect that attacks and kills European grapes when grown on their own roots. Two of the most common grafting techniques are "whip and tongue", carried out in spring as the sap rises, and "budding", which is performed around the end of summer.
One reason for grafting onto rootstocks is that this enables the grower to determine the tree's eventual size.
Another desirable characteristic of rootstocks is environmental adaptability. This may be tolerance to wet/dry soil conditions, acidity/alkalinity of soil or even hot/cold air temperature.
Apple tree size classes number from one to ten in increasing height and breadth. A "1" is a dwarf which can be productive and as short as 3 feet (0.91 m) with proper pruning. A "10" is the standard sized tree with no dwarfing and will grow to 20 feet (6.1 m) tall and wide or more, dependent upon the variety chosen. In general the class range is (1) 10-20% of full size, (2) 20-30%, (3) 30-40% and so forth to size 10 which is 100% of full size.
The Malling series and clones have been standard rootstocks for apples for many years and remain the "workhorses" for the commercial industry in the United States and the UK. However, since most of them are susceptible to disease some Malling rootstocks are being replaced by new breeds, including the Cornell-Geneva series, which has resistance to the major problems preventing quality production of apples utilizing organic control systems. One of the newest rootstocks of the "CG" series, only released commercially in 2004, is CG5202(G.202) which adds resistance to the woolly apple aphid, and when combined with highly resistant cultivars such as "Liberty" it is showing great potential. Rootstocks based on Siberian Crab apple are being used in colder areas for more cold tolerance.
The ability of new rootstocks to modify or augment characteristics of fruit trees is limited and may disappoint in the long term. It takes ten years to get a full picture of the effects of any one rootstock, so a rootstock that appears promising in the first five years of a trial may fail in the last five years. The Mark rootstock was such a stock and has now fallen mostly into disfavor. Another, the G.30, has proved to be an excellent stock for production but it was only after a number of years of trials that it was found to be somewhat incompatible with the cultivar "Gala", so that it is now recommended to be staked and wired.
An industry consortium undertakes trials of different rootstocks, called the "NC-140" trials. These test many pome rootstocks in many different sites across the United States and thereby provide growers a clearer picture of what to expect when growing fruit trees on specific stock, in specific planting methods in their specific area of the. This information has the potential to create economic benefits to both growers and consumers, as well as helping to reduce the need to spray pesticides as frequently as is currently required.
The following are a selection of apple tree rootstocks. They are referred to by numbers prefixed by letters indicating the developer of the rootstock.
|Very Small||Small||Medium||Large||Very Large|
|P22, M27, G65||G11, G41, M9, G16, Bud 9, Mark||M26, G935, G202, G30, MM102, Interstems, M7, M116||MM106||MM111, Bud 118, M25|
|6 ft/2m||8 ft/2.5m||10 ft/3m||14 ft/4m||18 ft/5m|
"Bud 118" A winter hardy early bearing replacement for MM111 bred in the Soviet Union. Full sized tree unless allowed to bear young which will stunt its growth. Hardy to USDA zone 3.
"Bud 9" A winter hardy, early bearing replacement for M9 bred in the Soviet Union. Dwarf tree resistant to crown rot and less susceptible to drought than most other dwarfing stocks. Produces large fruit, is precocious and hardy to USDA zone 3.
"M" designates Malling series developed stocks. East Malling Research is a pioneer in the development of dwarfing rootstocks. East Malling Research Station in Kent, England collected clones of the Paradise stocks from France in 1912 from which 24 "M" were designated with no particular order to the rootstock characteristics other than where they were located in the garden at the time the numbers were assigned. In other words, M.2 is a larger tree than M.9, while M.27 is smaller than M.26.
"MM" designates Malling-Merton stocks developed from joint breeding program by John Innes Institute, in Merton, England, & East Malling Research Station in the early 1950s. The "MM" series was developed primarily to provide resistance to Woolly Apple Aphid(Eriosomatinae) infestation.
"EMLA" designates East Malling / Long Ashton research stations who took the "M" stocks and developed virus free versions. For example., EMLA 7 is M 7 with a guaranteed virus-free stock. EMLA characteristics are often different from the parent "M" rootstock. Note that nearly all the apple rootstocks in the industry are now virus free.
"CG" or "G" designates Cornell-Geneva stocks which are those developed via the Cornell and USDA collaboration at the New York Agricultural Experiment Station in Geneva, NY. The "G" is the old designation. All newer stocks are "CG" followed by numbers that actually provide some information about the stock. As one might surmise, this is a huge improvement in the classical naming scheme which has no identification method at all.
"P" designates Poland. The Polish apple rootstocks were designed specifically for winter hardiness.
Seedling: Very vigorous trees produced on a rootstock grown from seed. There is greater variability than with the vegetatively propagated rootstocks. Apples used for production of seedling rootstocks include "Dolgo" and "Antonovka", which are both extremely hardy and vigorous.
Pears are usually grafted onto quince rootstocks, which produce small to medium-sized trees. Some varieties, however, are not compatible with quince, and these require double working. This means that a piece of pear graft-work compatible with both the quince rootstock and the pear variety is used as an intermediate between the two. If this is not done the pear and the rootstock could eventually separate at the graft. Varieties that require double working include "Bristol Cross", "Dr Jules Guyot", "Doyenné d' été" and "Williams Bon Chrétien".
Until the 1970s, cherries were grown on the vigorous "Malling F12/1", "Mazzard" (Prunus avium), or "Maheleb" (P. maheleb) rootstocks, which required much space and time before cropping began, thus the growing of cherries was not a realistic option on a garden scale. The introduction of the rootstock "Colt" enabled trees reaching a maximum height of 12 to 15 ft (3.7 to 4.6 m) to be grown, and if trained as a pyramid it is possible to restrict growth to about 10 ft (3.0 m). The popular sweet variety "Stella" can even be successfully grown in a pot on the patio when grafted onto a "Colt" rootstock. A newer rootstock, "Gisela 5", is slowly becoming available to gardeners and produces a tree 20% smaller than "Colt" and 45% smaller than "Mahaleb" and "Mazzard", making netting for bird protection much easier. Furthermore, German Nurseries Consortium (Consortium Deutscher Baumschulen — CDB®) is introducing across the EU their newest most dwarfing "Gisela 3" cherry rootstock that has 50% dwarfing qualities in comparison with "Mahaleb" and "Mazzard" and is 10% smaller still than "Gisela 5" rootstock.
Many species of fruit, e.g., fig, olive, and pomegranate, are commonly grown on their own roots, as there may be no great advantages to using a special rootstock, or suitable rootstocks may not be readily available. However, even for fruit trees that usually are grown grafted on a rootstock, there can be advantages in growing them on their own roots instead, particularly in the traditional coppicing systems advocated in both sustainable agriculture and permaculture. Disadvantages of using own-root trees can include excessive size and excessive production of wood (thus very long times until the start of fruit production), although training branches horizontally and limiting pruning to summer only may help encourage fruit production at an earlier age. There is a lack of research on the use of the own root method in large-scale systems. British horticulturalist Hugh Ermen was a prominent advocate of own root fruit trees, particularly Apple trees, following his work at the Brogdale research centre in Kent. The research undertaken at Brogdale did not continue but there has been renewed academic interest in own root fruit trees since the turn of the century.
In addition to propagating trees on rootstocks designed to control size/vigour and confer disease resistance, grafting above the rootstock can be used to provide multiple cultivars of a single species, known as a family tree, or, within certain limits, cultivars of different fruit species on one tree, often known as a fruit salad tree. Family trees typically combine several cultivars (two or three being most common) of apple, pear or a given species of stonefruit on a single rootstock, while fruit salad trees typically carry two or more different species from within a given genus, such as plum, apricot, and peach or mandarin orange, lemon, and lime. Certain combinations, including sour cherry (prunus cerasus)/sweet cherry (prunus avium), although from the same genus, are known to be difficult, although successes have sometimes been reported. Other grafts of this kind can produce the Pomato.