Fruit tree propagation

Fruit tree propagation is usually carried out through asexual reproduction by grafting or budding the desired variety onto a suitable rootstock.

Perennial plants can be propagated either by sexual or vegetative means. Sexual reproduction occurs when male pollen from one tree fertilises the ovules (incipient seeds) of the flower of another, stimulating the development of fruit. In turn this fruit contains a seed or seeds which, when germinated, will become a new specimen. However, the new tree will inherit many of the characteristics of both its parents, and it will not grow 'true' to the variety from which it came. That is, it will be a fresh individual with many unpredictable characteristics of its own. Although this is desirable in terms of increasing biodiversity and the richness of the gene pool (such sexual recombination is the source of most new cultivars), only rarely will such fruit trees be directly useful or attractive to the tastes of humankind. A tendency to revert to a wild-like state is common.

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.

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Methods

The essentials of our present methods of propagating of fruit trees date from pre-Classical times. Grafting as a technique was first developed in China from where it was imported to Greece and Rome. Classical authors wrote extensively about the technical skills of fruit cultivation, including grafting techniques and rootstock selection. The oldest surviving named varieties of fruits date from classical times.

The simplest method of propagating a tree asexually is rooting. A cutting (a piece of the parent plant) is cut and stuck into soil. Artificial rooting hormones are sometimes used to assure success. If the cutting does not die of desiccation first, roots grow from the buried portion of the cutting and it become a complete plant. Though this works well for some plants (such as figs and olives), most fruit trees are unsuited to this method.

Root cuttings (pieces of root induced to grow a new trunk) are used with some kinds of plants. This method also is suitable only for some 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. These are varieties selected for characteristics such as their vigour of growth, hardiness, soil tolerance, and 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. Grafting is the process of joining these two varieties, ensuring maximum contact between the cambium tissue (that is, the layer of growing plant material just below the bark) of each so that they grow together successfully. 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 July and August.

Bud grafting

1. Cut a slice of bud and bark from the parent tree.
2. Cut a similar sliver off the rootstock, making a little lip at the base to slot the scion into.
3. Join the two together and bind.
4. In time, the scion bud will grow into a shoot, which will develop into the desired tree.

Whip and Tongue grafting

1. Make a sloping cut in the rootstock with a 'tongue
2. Make a matching cut in the scion wood with a 'tongue' pointing downwards.
3. Join the two, ensuring maximum contact of the cambium layers. Bind with raffia or polythene tape and seal with grafting wax.

Apple Rootstocks

Another reason for grafting onto rootstocks is that this enables the grower to determine the tree's eventual size. Apple tree size classes number one to ten in increasing height and breadth. ^[1] A "1" is a dwarf which can be productive and as short as three(3) feet with proper pruning. A "10" is the standard sized tree with no dwarfing and will grow to twenty(20) or more feet tall and wide, 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.

Apple tree rootstocks are referred to by numbers prefixed by letters indicating the developer of the rootstock.

"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 larger tree than M.9 while M.27 is smaller than M.26. ^[2]

"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. ^[3] The "MM" series was developed primarily to provide resistance to Woolly Apple Aphid(Eriosomatinae) infestation. ^[4]

"EMLA" designates East Malling / Long Ashton research stations who took the "M" stocks and developed virus free versions. E.g., EMLA 7 is M 7 with a guaranteed virus free stock. ^[5] EMLA characteristics are often different from the parent "M" rootstock. Note that nearly all the apple rootstocks in the industry are now virus free. ^[6]

"CG" or "G" designates Cornell-Geneva stocks which are those developed via the Cornell & 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. ^[7]

• M.27 Malling 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. ^[8] Trees can be grown three to four feet tall and produce about 45 fruit, roughly 2 pecks, depending on fruit cultivar. ^[9]

• M.9: Very dwarfing - Reaches a height of 8 to 10 ft (3 m), coming into fruit after 3-4 years, reaching full capacity of 50 to 65lb (20 kg) 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.^{[citation needed]} Trees on this rootstock always require leader support. The rootstock is very susceptible to fire blight and can develop burr knots. ^[10]

• G.41 Geneva 41, released in 2005, produces trees the size of M.9. The rootstock was developed from a cross between M.27 and Robusta 5 made in 1975. Resistant to Crown|Collar|Root rot(Phytopthora) and fire blight. ^[11]

• M.26: Dwarfing - Similar to M9 in effect, although somewhat more vigorous and generally stronger, with a higher expected eventual yield of 65-75 lb (35 kg) and height of 8 to 10 ft (3 m). A good choice where soil quality is average and compact growth is required. Comes into fruit after 3-4 years, reaching full cropping capacity after 5 to 6 years. Staking needed for first five years of its life.^{[citation needed]} 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). ^[12]

• G.11 Geneva 11 is the second release of the Cornell breeding program similar in size to M.26(Class 4) but more productive. Has the advantage of being resistant to fire blight and crown rot as well as only rarely producing suckers or burr knots. ^[13]

• G.202 Geneva 202(CG 5202) is a semidwarfing rootstock that produces a tree in class 5 slightly larger than M.26(Size Class 4) and is more productive than M.26. It was developed from a cross of M.27(Size Class 1) and Robusta 5 to be fire blight and Phytopthora resistant as well as having resistance to woolly apple aphids. In a 9-year study with the scion cultivar of the "Liberty" apple, G.202 was about 50 percent smaller than M.7 but had much greater production efficiency. ^[14]

• M.7 Malling 7 rootstock produces a semidwarf tree of Class 6 ^[15] 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(Phytopthora). ^[16]

• MM.106: Semi-dwarfing - Sometimes referred to as semi-vigorous, this is the most widely used of rootstocks.^{[citation needed]} It is probably the best choice for the average garden under average conditions, being tolerant of a wide range of soils, and producing a tree with an eventual size of 14 to 18 ft (5 m).^{[citation needed]} Trees on this stock begin producing fruit within three to four years, and yield up to 90 to 110 lb (50 kg) after some seven or eight years.^{[citation needed]} MM106 is very suitable for use with weaker varieties that would produce under sized bushes with more dwarfing rootstocks. Can be trained as a half standard tree, but is rather too vigorous for cordons unless the soil is poor. Requires staking for the first five years or so of its life.^{[citation needed]} Trees on MM.106 are highly susceptible to collar rot especially when planted in soils that remain wet(poor percolation). ^[17]

• M.111 : Vigorous - Not generally suitable for garden scale growing, being both too large and spreading (18-25'), and too slow to come into cropping. They are however suitable for growing as specimen standards in the large garden, or for producing medium sized bushes on poorer soils. Begins to fruit after six or seven years, reaching full capacity of 160 to 360 lb (80 to 180 kg) after eight to nine years.^{[citation needed]} It is the most cold-hardy rootstock readily available. Planting depth of this rootstock is critical. The union should be no higher than 1 to 2 inches above the final soil line. ^[18]

• M.25: Very vigorous - Suitable for a grassed orchard, and to grow on as a full standard. Plant 20 ft (7 m) apart, makes a tree of 15 to 20 ft (5 m) or more height and spread, eventually yielding 200 to 400 lb (100 to 200 kg) per tree.^{[citation needed]} This rootstock is primarily used in UK and is rarely seen in USA where M.111(size Class 8) ^[19] is used for this size tree.

• 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.^{[citation needed]}

That is only a sample of some of the more important current apple rootstocks that are available. There are at least a hundred more that have been developed to either provide enhancement or prevent potential damage from one kind of pest or another.

The problem with growing fruit trees, especially apple trees, is that they are subject to many different types of damage from bacteria, fungi and insects. The general approach of the commercial industry has been to use as many chemicals as necessary to insure attractive and marketable fruit. The attitude, still prevalent, has been "Who cares? Nobody eats a tree!" ^[20] but as environmental problems increase and the general public pushes for low or no-spray fruit, there has become a commercial need for fruit that does not require such intensive spray programs. This is being achieved, albeit slowly, by rootstocks and trees that are bred to have natural disease and pest resistance.

The Malling series and clones have been standard rootstocks for apples for many years and remain the standard "workhorses" for the commercial industry(in USA). ^[21] However, since most of them are susceptible to disease some Malling rootstocks are being replaced by new breeds, including the Cornell-Geneva series. One of the newest rootstocks, only released commercially in 2004, is CG5202(G.202) which adds resistance to the woolly apple aphid(WAA) for the "CG" series of stocks which already has resistance to the major problems preventing quality production of apples utilizing organic control systems. Combined with highly resistant trees such as "Liberty" it is showing great potential. ^{[22] [23]}

That leads to another characteristic of rootstocks that is or can be bred into them: environmental adaptability. This may be tolerance to wet|dry soil conditions, acidity|alkalinity of soil or even hot|cold air temperature. ^[24]

Some new rootstocks based on Siberian Crab Apple are being used in colder areas for more cold tolerance.^{[citation needed]}

The ability of rootstocks to modify or augment characteristics of fruit trees is limited and often disappointing in the final results. 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 (apple) rootstock was such a stock and has now fallen mostly into disfavor.^[25] 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 "Gala" apple(and possibly others) so that it is now recommended to be staked and wired.^[26]

To get a clear picture and push the industry forward, a consortium was founded and the so-called "NC-140" trials of rootstocks began. ^[27] These test many pome rootstocks in many different sites across the USA and thereby provide growers, be they backyard or commercial, a clearer picture of what to expect when growing fruit trees on specific stock, in specific planting methods in their specific area of the USA. As one can imagine, this has the potential for a large economic benefit to both growers and consumers as well as going a long way to eliminating the need to spray pesticides as frequently as is currently required. ^[28]

Pear Rootstocks

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'.

• Quince C: Moderately vigorous- Makes a bush pear tree about 8 to 18 ft (3 to 6 m) tall, bearing fruit within @ four to eight years. Suitable for highly fertile soils and vigorous varieties, but not where conditions are poor. Used for bush, cordon and espalier growing. Old stocks of Quince C may be infected with a virus, so care should be taken to obtain certified virus free stock. If in doubt, use Quince A as there is not a great amount of difference in vigour between the two.

• Quince A: Medium vigour- Slightly more vigorous than Quince C, this is the most common variety upon which pears are grafted. Bears fruit between four to eight years, making a tree of some 10 to 20 ft (3 to 7 m) in height and spread. Suitable for all forms of pear trees except standards.

Pear stock: Very vigorous- Pears grafted onto pear rootstocks make very large standard trees, not suitable for most gardens.

Cherries

Until the 1970s, cherries were grown of 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' enables trees reaching a maximum height of 12 to 15 ft (4 to 5 m) to be grown, and if trained as a pyramid it is possible to restrict growth to about 10 ft (3 m). The popular sweet variety 'Stella' could even be grafted onto a 'Colt' rootstock and successfully grown in a pot on the patio.

Plums

Plum rootstocks include;

• Pixy - A dwarfing rootstock, suitable for bush trees planted 8-10 (3 m) apart.

• St. Julien A - A semi vigorous rootstock suitable for bush and half standards planted 12 to 15 ft (4 to 5 m) apart. Also suitable for peaches, nectarines and apricots.

• Brompton or Myrobalan B- Suitable for half standards planted 18 to 22 ft (6 to 7 m) apart. Also suitable for peaches, nectarines and apricots.

• Myro-29C - Semi-dwarf rootstock. Shallow, vigorous, good choice for hard soils. Somewhat drought tolerant.

• Citation - Semi-dwarf rootstock. Shallow, vigorous, good choice for hard soils. Prefers a wetter soil.

Own-Root Fruit Trees

Some species of fruit are commonly grown on their own roots; new plants are propagated by rooting, layering, or modern tissue-culture techniques. In these cases there are may be no great advantages to using a special rootstock or improved rootstocks are not available. Fig, filbert, olive, pomegranate, gooseberry, bramble, and other fruits are commonly grown without any special rootstock.

Though vegetative propagation of apple, pear, stone fruits, and many other species is a nearly universal practice, it does have some detractors. Here is an account of one advocate of own-root fruit trees.

Own-root apples in a Permaculture design

Permaculture designer and fruit nurseryman Phil Corbett has for a number of years been working on an innovative project involving growing fruit trees on their own roots. His work is based on research carried out by Hugh Ermen of the Brogdale Horticultural Research Station, home of the National Apple Collection. Corbett writes; "Hugh discovered that there are several advantages in growing apples on their own roots- that is, not grafted onto a rootstock. The graft union, which is a union between two genetically different individuals, always creates a degree of incompatibility. Not having this incompatibility, own-root trees were found to have better health and better fruit quality. The only disadvantage of own-root trees is that most varieties are more vigorous than is usually wanted. This means that trees may make a lot of wood at the expense of fruit bud production, giving big trees that take a long time to come into crop. Conventionally this vigour is controlled by grafting onto a dwarfing rootstock. With trees on their own roots, however, a number of traditional techniques can induce early cropping. Once cropping begins, the tree's energies are channelled into fruit production and growth slows down to a controllable level. The techniques that are usually sufficient to bring about cropping are:

• Withholding nitrogen, which would stimulate growth, and withholding irrigation, except in serious drought;
• Tying down one and two year old branches to the horizontal. This induces fruit bud formation;
• Prune in summer to stimulate fruit buds to form, and avoid winter pruning which stimulates regrowth.

Once cropping has begun, a normal feeding and watering regime can begin. The average cropping own-root tree can be maintained at a size very similar to the same variety on MM106 rootstock" (Phil Corbett, from The Common Ground Book Of Orchards, pub Common Ground, 2000).

He is also conducting research into the 'coppice-ability' of own-root fruit trees, including an experimental 'Coppice Orchard' project, wherein own-root trees are planted in north-south rows; "When the canopy of the orchard closes, a north-south row will be coppiced and the land in the row used for light demanding crops while the trees regrow. The trees on either side of the glade will have higher light levels on their sides and produce more fruit buds". Over time a series of parallel sheltered glades will be created which will be coppiced on a rotational basis, allowing for multifunctional use of land in order to produce not only fruit but also small wood products, soft fruit, vegetables and even possibly cereals, fungi and the more traditional bees and poultry. This is a project with much promising potential, and deserving of attention from the wider organic growing movement for the valuable lessons that will no doubt be provided over time.

References

See also

• Fruit tree forms
• Pruning fruit trees
• Fruit tree pollination
• Orchards
• Rootstocks