Fruit tree propagation is usually carried out vegetatively (non-sexually) by grafting or budding a desired variety onto a suitable rootstock.
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.
Methods 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 vigor 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.
Bud grafting
Cut a slice of bud and bark from the parent tree. Cut a similar sliver off the rootstock, making a little lip at the base to slot the scion into. Join the two together and bind. In time, the scion bud will grow into a shoot, which will develop into the desired tree.
Whip and tongue grafting
Make a sloping cut in the rootstock with a "tongue" pointing upwards. Make a matching cut in the scion wood with a "tongue" pointing downwards. Join the two, ensuring maximum contact of the vascular cambium layers. Bind with raffia or polythene tape or wind around with a 5mm wide strip of elastic band (this is particularly successful because it keeps pressure on the cambium layers to be joined and eventually falls away without cutting into the bark as the tree grows) and seal with grafting wax.
Rootstocks 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 rootstocks 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 with proper pruning. A "10" is the standard sized tree with no dwarfing and will grow to 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 country. 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, A2 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.
M.2: Produces a semidwarf to semistandard freestanding tree, depending on scion variety. Trees are strong, crop well, and do not have collar rot problems. M.7: Produces a semidwarf tree of Class 6 that is freestanding in deep well drained soils but in rocky, steep, or shallow soils, it tends to lean. The rootstock may sucker profusely and is susceptible to collar rot (Phytophthora). M.9: Very dwarfing — Reaches a height of , coming into fruit after 3 to 4 years, reaching full capacity of after 5 to 6 years. It will grow under average soil conditions, but needs a good rich soil to thrive. A good choice where space is limited and fertility is high. Permanent staking is required, as is routine feeding and watering. Trees on this rootstock always require leader support. The rootstock is very susceptible to fire blight and can develop burr knots. M.25: Very vigorous — Suitable for a grassed orchard, and to grow on as a full standard. Plant apart, makes a tree of or more height and spread, eventually yielding per tree. This rootstock is primarily used in UK and is rarely seen in the United States where MM.111(size Class 8) is used for this size tree. M.26: Dwarfing — Similar to M9 in effect, although somewhat more vigorous and generally stronger, with a higher expected eventual yield of and height of . A good choice where soil quality is average and compact growth is required. Comes into fruit after 3 to 4 years, reaching full cropping capacity after 5 to 6 years. Staking is needed for the first four or five years of its life. It is susceptible to collar rot and fire blight and should not be planted in a wet site. Certain varieties when grafted onto this rootstock may exhibit signs of graft union incompatibility, i.e., the union breaks. M.27: A very dwarfing rootstock. Unless the central leader is supported, the tree will be very small. Often only used as an intermediate stem piece on MM.106 or MM.111. If handled and spaced properly, it can be a very productive stock for a vertical axe system. Trees can be grown three to four feet tall and produce about 45 fruit, roughly 2 pecks, depending on fruit cultivar.
"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.