By Marnie Aasheim
Apple trees can be propagated by many different means.Grafting, budding, and layering are among the most popular and widely used methods of propagation.Selecting the right rootstock is an essential part of apple tree propagation.
Apple trees (Malus x domestica) are one of the most popular fruit trees grown in the world today.There are thousands of cultivars of this popular fruit tree.Methods of propagation of the apple are numerous and can vary widely.Some of the most popular methods of propagation are grafting and budding.Layering is also done for apple rootstocks.Most apple production today is from clonally propagated fruiting varieties, which include MacIntosh, Crispin and Red Delicious, which are grafted and budded onto clonal rootstocks.These rootstocks are specially selected to affect the size and disease resistance of the tree.
Methods of Propagation
Grafting involves joining two pieces of genetically different living plant tissue together so they grow together as one plant.The lower part of the compound plant is known as the stock or rootstock.This is the part that produces the root system.The upper part is referred to as the scion and produces the shoot system of the plant.For grafting, a short piece of the tree of a desired variety is taken and placed onto a limb of the rootstock. Grafting is usually done during the dormant season and must be done on dormant scion and stock wood.There are many methods of grafting, but the main objective is to unite the cambium layers from the scion and the stock together so the plant grows as one (California Backyard Orchard 3).
The whip-and-tongue graft is usually done on small diameter material (6-13mm) and is one of the strongest types of grafts.High success rates are seen with this graft because the cambium layers have a lot of surface contact.When selecting material for this graft, it is important to have the scion and stock of somewhat equal diameter.A section of scion with two or three buds should be used.Identical cuts are made at the top of the rootstock and bottom of the scion, so the two pieces fit together nicely.The first cut is about 1-2.5 inches long and made at an angle with one long and continuous motion of the knife.This first cut should be made on the scion in the internode area below the two or three buds.Smooth cuts are essential so that good cambium layer contact will occur.
An identical cut is then made on the rootstock.A second cut is started about a third of the way down from the tip of the first cut.This second cut should be about half as long as the first cut and made parallel to the first.After this second cut is made on both the scion and stock, the two pieces are interlocked.They should fit together perfectly, with no gaps.At this point, it is very important that the vascular cambium layers come into contact on one or both sides of the cuts.The tips of the cuts should also not overlap the material they are grafted onto to avoid callus knots.The graft can be held together by budding rubber strips or grafting tape. To keep the moisture in the tissues, sealing the graft with grafter’s wax or Parafilm is sometimes done.The grafts can also be placed in very high relative humidity until the graft begins to take (Hartmann et al. 439). (See Figure 1 below)
Figure 1-Whip and Tongue Graft
(Source: Reproducing Fruit Trees by Graftage, Leonard P. Stolz)
Whip grafts are very similar to whip-and-tongue grafts.The only difference is that the second cut is not made when doing the whip grafts. The first cut is again identical to the whip-and-tongue cut (smooth, sloping and 1-2.5 inches long).The cut is made on both the scion and stock and the two pieces are fitted together, making sure good cambium contact occurs.The graft is wrapped as described above.It is common for this type of graft to be done on apple tree material with specialized machines (Hartmann et al. 439).
Budding is a form of grafting in which only a bud and a small piece of bark from a stock is used.For budding to be done, the bark on the rootstock must be “slipping”, meaning the bark can be easily separated from the wood.When the bark is slipping, the plant is actively growing and the cambium cells are dividing.
T-budding is usually done on rootstocks that range from 6 to 25 mm in diameter.The first cut on the rootstock is a vertical cut through the cambium and just into the wood tissue, about 5 to 25 cm above the soil level.Then a horizontal cut just above the vertical cut is made to form a “T” in the bark.The knife is twisted when this horizontal cut is made to slightly open up the “T” so the flaps formed can be peeled back.Removing a bud on the scion is done by making an upward slicing cut about 13 mm below the bud and then cutting upward to about 2.5 cm above the bud.The bud is removed then by a horizontal cut about 1.3 to 1.9 cm above the bud.This bud “shield” is then inserted into the T-shaped cut flaps and slid under the bark flaps until the top edges are even.The bud union area is then wrapped with budding rubbers or wrapping tape, being careful not to cover the actual bud itself.It is important to not tie the tape or budding rubbers too tight or too loose.The top part of the seedling rootstock is removed after growth from the bud occurs (Hartmann et al. 491). (See Figure 2 below)
Figure 2- T-Bud
(Source: Propagation of Woody Ornamentals by Grafting and Budding, David Hamilton)
Chip budding is done on 13 to 25 mm in diameter material.For this type of budding, a small chip of bark is cut out from the rootstock near the base and an identical size and shape chip from the scion is put back in its place.Identical procedures for cuts are used for both the scion bud removing and the rootstock chip removing.A 30-45 degree angle is cut below the bud and into the wood about ¼ of the way.Then another cut is made about 1 inch above the bud and sloping downward, cutting behind the bud.This cut is made all the way down until it reaches the first cut, thereby completely removing the bud “chip” from the scion.As stated previously, identical cuts are made on the rootstock.So the chip from the scion can then be inserted onto the area of the rootstock where the chip was removed.Again, care must be taken to insure the cambium layers from the rootstock and the scion chip are meeting in the graft area.Contact is better if the cambium layers meet on both sides, but contact on only one side is really needed.Wrapping the chip bud with nursery adhesive tape or transparent plastic tape holds the two pieces together firmly and keeps the moisture in the bud area. The tape is cut when the bud start to grow and the top of the rootstock is removed when the graft has taken, meaning the union is complete (Hartmann et al. 487). (See figure 3 below)
Figure 3-Chip Bud
(Source: Reproducing Fruit Trees by Graftage, Leonard Stoltz)
Mound layering is used to propagate apple clonal rootstocks. In this method, soil is mounded around shoots that have been cut back, thereby stimulating roots to grow at the base of the shoots.A year before propagation begins, eight to ten millimeter diameter stock plants are planted in rows and then cut back to 45-60 cm.They are then grown for one year.In the spring, the plants are again cut back, this time to 2.5 cm above the ground. New shoots gradually form and more soil and bark is added in mounds around the plants. This cycle may continue on through the growing season (Hartmann et al. 509). Then the shoots are harvested by cutting close to the bases.The mother stool beds are then left exposed until further growth of the new shoots has occurred and another cycle of hilling begins (Hartmann et al. 510).
When propagating apple trees, a great deal of attention is put on the roots, probably more than any other species.Rootstocks are used for grafting and propagation of apple scion varieties (Apple Tree Rootstocks 1).
When rootstocks grown from seed are used, the resulting trees are often large, slow developing and susceptible to dying because of not being able to adapt to the soil or climate in the orchard (California Backyard Orchard 4).Some cultivars such as ‘Golden Delicious’, ‘McIntosh’, ‘Winesap’, ‘Yellow Newtown’ and ‘Delicious’ have actually been quite successful though, producing uniform apples and no incompatibilities.In the Dakotas and Minnesota, where winters are cold and harsh, the Siberian crab apple (Malus baccata) and the cultivar ‘Antonovka’ are widely used.Some example of apples not recommended for use as seedling rootstocks include ‘Arkansas’, “Bramley’s Seedling’, ‘Jonagold’, ‘Tompkins King’, and ‘Mutsu’.All these are triploid cultivars and have low viability (Hartmann et al. 640).
Clonal rootstocks are widely used today to get around adaptability problems.Clonal rootstocks are rootstocks that are grafted onto rootstocks that are genetically identical clones of a parent rootstock.These rootstocks are special because they have certain desirable characteristics including tolerance to winter cold, poorly drained soil, heavy clay or salty soil, or very dry soil.The ultimate tree size can be controlled by the rootstock also.Apple rootstocks are available for dwarf trees all the way through gigantic trees.By selecting the right rootstock for the conditions in the area, the grower greatly enhances productivity of the apple trees(Apple Tree Rootstocks 2).
In the 1900s, the East Malling Research Station in England started classifying apple rootstocks based on size.Researchers, or specifically pomologists, gathered and characterized the clonal rootstocks that had been used for hundreds of years in Europe by farmers.For each clone, they assigned a number.Rootstocks have now been created by breeding, or hybridizing, these clones with other species and varieties of apples.As a result, these “Malling” rootstocks are compatible with most apple cultivars around the world.In later years, a “Malling-Merton” series of rootstocks was developed to provide resistance to wooly aphids and a wider range of vigor.Both the Malling-Merton and Malling rootstock have high yield and excellent propagation qualities (Hartmann et al. 640).
Dwarfing Clonal Rootstocks
The greatest dwarfing affect occurs with this stock.One and one half to two meter tall trees are the average size for this stock.This stock can be used as an interstock between the scion and another rootstock for dwarfing.
‘Malling 9’ (‘Juane de Metz’)
This is a popular variety in North America, producing small, dwarfed trees ideal for small areas.Trees wont grow over 3 meters usually and can be planted relatively close together. After planting, these trees will produce fruit within one or two years.This rootstock needs fertile soil and support with staking and trellising.The roots are thick, brittle, fleshy, and sensitive to cold temperatures. Crown gall, fire blight, mildew and wooly apple aphid can be a problem with this rootstock.It can be used as an interstock and is usually propagated by stooling.
This is also a popular variety in North America, with better anchorage and ultimate larger size than the ‘Malling 9’.It is a cross between ‘Malling 16’ and ‘Malling 9’.Methods of propagation include softwood cuttings and hardwood cuttings.This stock usually needs staking and is relatively winter-hardy.Poorly drained or heavy soils can be a problem in this rootstock, as well as fire blight and collar rot (Hartmann et al. 641).
Semi-dwarfing Clonal Rootstocks
Trees on this stock are larger than ‘Malling 26’ trees.The original ‘Malling 7’ was the first of this type, and then a clonal selection, ‘Malling 7a’, was produced to be free of certain viruses that were in ‘Malling 7’.The root system of ‘Malling 7a’ is deeper and stronger than ‘Malling 9’.The tree produced with ‘Malling 7a’ is semi-dwarfed and bears fruit early.Good anchorage and tolerance to excessive moisture are advantages of this stock.But it is susceptible to crown gall, tends to produce numerous suckers and is not very winter hardy.Propagation methods of ‘Malling 7a’ included stooling or leaf cuttings.
This stock is the result of crossing ‘Northern Spy’ and ‘Malling 1’.Some advantages of this stock include good anchorage, limited sucker production and resistance to fire blight.Crown rot problems have limited the use of this stock, as well as susceptibility to collar rot.It is also not resistant to early fall freezes.Propagation methods of this stock include hardwood cuttings, softwood cuttings and stooling.
Vigorous Clonal Rootstocks
This stock came from a cross of ‘Northern Spy’ and “Merton 793’.An advantage of this stock is that it does well on most soil types and is resistant to wooly aphid and collar rot.It is susceptible to mildew.Stooling, hardwood cuttings and softwood cuttings are all propagation methods (Hartmann et al. 641).
Future Work Needed
Future work is needed in developing even more disease resistant rootstocks and cultivars of apple.With recent advances in biotechnology and breeding, new genetically modified cultivars will likely soon be developed.A great deal of research will need to be done on these genetically altered apple cultivars, making sure they are safe for food consumption.These genetic alterations may not happen until years down the road, but I think future work in propagation of apple trees will likely revolve around genetics.I think a lot of research will also be needed on developing disease resistant cultivars, possibly also by genetic engineering.
“Apple Tree Rootstocks-Gardening Resources, Cornell College of Agriculture and LifeSciences.” 14. Apr. 2002<http://www.hort.cornell.edu/gardening/fctsheet/egfactsh/rootstoc.html>
Hamilton, David F. and James T. Midcap.“Propagation of Woody Ornamentals by
Grafting and Budding.” 21. Apr. 2002 < http://edis.ifas.ufl.edu/EP031>
Hartmann, Hudson T., et al. Plant Propagation: Principles and Practices. New Jersey:
Prentice Hall, 1997.
Stoltz, Leonard and John Strang. “Reproducing Fruit Trees by Graftage.” 21. Apr. 2002
“The California Backyard Orchard: A University of California Resource for Fruit and Nut Crops.” 24. Mar. 2002