A. Grafting for Clonal Selection and Propagation of Otherwise Difficult-to-Clone Plants
1. When a plant must be clonally propagated to maintain a selected genotype (cultivar , new sport ), but is difficult to propagate vegetatively by cuttings or other means, it is often grafted or budded.
a. Shade tree cultivars of several difficult-to-root species are routinely budded:
- Norway Maple (e.g. Acer platanoides 'Crimson King')
- Green Ash (e.g. Fraxinus pennsylvanica 'Marshall's Seedless')
- Honeylocust (e.g. Gleditsia triacanthos inermis 'Moraine')
- Littleleaf Linden (e.g. Tilia cordata 'Greenspire')
b. Other ornamental cultivars
(1) Cultivars of selected Pinaceae (Pine Family) species with unusual growth forms
(2) Bloodgood Japanese Maple (A. japonicum 'Bloodgood')
(3) Taxus bacatta 'Repandans'
(4) Upright Juniper cultivars
c. Find out about these and other ornamental plants. The Nursery Web has links to many Plant Identification websites
2. Economics - sometimes grafting is less expensive than cuttage
This is a corollary to A.1. above, since if a selection is difficult to root, grafting is usually cheaper than cuttage.
a. Although labor for grafting per se is usually more costly than cuttage (more time-consuming per unit, and more skilled), the cost of materials and equipment may be lower if cuttage requires long periods in a heated greenhouse, with bottom heating, mist, etc.
b. Cost analysis: Flowering dogwood cultivars: This is the result of an economic analysis of costs for production of flowering dogwood cultivars by either budding or cuttings ( Badenhop, 1986).
3. Budding for delayed self-rooting of slow-to-root species / nurse (root) grafting (NRG)
a. Some species are difficult to root from cuttings, because a conventional cutting cannot stay alive long enough for rooting to occur. Such "cuttings" may be grafted to a piece of root to keep them alive long enough for them to become self-rooted. This is called nurse root grafting. The graft union is planted below the soil line (unlike most grafting), and eventually the scion becomes self rooted. Afterwards, the rootstock can either be deliberately removed or it will die off, especially in cases where the scion and rootstock are not closely related, resulting in a delayed graft incompatibility. Incompatibility is discussed in the section on Compatibility.
In what way is the process of nurse grafting similar to layering?
(1) Lilac (Syringe vulgaris), until the advent of micropropagation, was commonly nurse root grafted to California privet (Ligustrum ovalifolium). Both of these genera are in the Oleaceae family.
- The root piece is typically whip and tongue grafted at the bench during winter, stored in a cool place where graft union formation occurs, and then lined out in the field in the spring, where scion rooting occurs.
- Eventually the graft union fails due to delayed incompatibility, and the privet root piece dies. The likelihood, overall (for any kind of plant), of an intergeneric graft like this being a compatible scion/stock combination is low.
- Alternatively lilacs may be nurse root grafted onto one year old root pieces of seedling Green Ash (Fraxinus pennsylvanica, also in the Oleaceae).
- In recent years, most lilacs are propagated by tissue culture (micropropagation). Nurse root grafting has lost popularity because sometimes the graft union fails before the scion becomes self rooted, or it will not fail at all, or the rootsystem will sucker, eventually outgrowing the lilac scion. In the image shown here, this nurse root grafted lilac apparently did not self root, and after several years, the lilac/privet graft union broke apart (delayed incompatibility), killing the lilac.
(2) Avocado. Nurse seedling graft of avocado rootstocks by the Frolich method and modifications are described by Reuben Hofshi in the Subtropical Fruit News, (vol. 4, no. 2, Spring, 1997). The method was developed as a means of cloning avocado rootstock varieties. Avocado is very difficult / slow to root from cuttings; hence, grafting. The method involves grafting a scion, from a clone that is ultimately intended to be used as a rootstock, onto a nurse seedling. This nurse seedling will serve as a temporary root system for the scion of this rootstock variety. New growth from the scion is then etiolated, and then air layered, in order to induce its own adventitious root system. The rooted layer is then detached from the nurse seedling and grown on. Subsequently a scion of a fruiting variety is grafted onto the rootstock clone.
(3) In the past, apples were sometimes nurse root grafted before the use of size controlling clonal rootstocks became common. This illustration is from Liberty Hyde Bailey's Standard Cyclopedia of Horticulture, written in the early part of the 20th century ( Bailey, 1924). NRG is sometimes still used to "bulk up" (rapidly increase the numbers of) newly selected apple rootstock selections in rootstock breeding programs such as the one at the New York Agricultural Research Station at Geneva.(4) Other examples: Large-flowered Clematis hybrids, Peony, and Catalpa cvs.
B. Grafting for repair
1. Grafting to repair a girdled stem - Bridge Graftinga. Young bridge graft
b. Older bridge graft
c. Bridge grafting described in a UConn bulletin on repairing mouse damage
How is bridge grafting like double working (described in the section on Concepts and Definitions)?
What time of year (season) is bridge grafting performed? (see Seasonal Considerations in the section on Required for Successful Grafting and Budding)
2. To replace a damaged (girdled) trunk base - inarching or bridge grafting
- Inarching, an in-ground version of approach grafting, is described in the University of Georgia Extension Web site, Propagating Deciduous Fruit Plants Common to Georgia
3. To replace a damaged or diseased root system - inarching
- The picture is from the cover of a 1933 extension bulletin by Thomas & MacDaniels, which described the use of inarching to repair damage caused by freezing.
4. To overcome a delayed incompatibility - bridge grafting or inarching
C. Grafting to create unusual growth forms - Highworking (see Grafting by Position)
1. To obtain a tree-like form high working of otherwise naturally shrubby plants several feet up on a tall straight trunk.
a. Tree (standard) Roses
- Roses that typically grow as low shrubs or climbers can be given an arborescent (tree-like) appearance by grafting them at the top of a long straight interstock which is in turn grafted onto a suitable rose rootstock such as R. multiflora. This would be an example of both highworking and double working .
b. Tree peonies
- Peonies from China Web site
c. Weeping Higan Cherry (a cultivar of Prunus subhirtella)
( Picture from Edgar Joyce Nurseries)
- Prunus subhirtella is normally an arborescent (tree) form, but the variety pendula "weeps," and would grow as a prostrate shrub. It is grafted ~ 4 - 6' high on a P. subhirtella understock to give a "weeping tree."
- Picture shows high graft union (right), and root suckers from the rootstock (left).
Why are these suckers upright instead of weeping?
- This ancient Chinese art is, perhaps, the ultimate in tree "engineering." If the grower wants a branch in a particular location where none exists, it can be grafted into place, as is described at the Bonsai Primer Web page
f. Living sculptures created by grafting
- One of the most unusual applications of grafting is its use to create living sculptures such as chairs, tables, and a variety of strange abstractions.
- The Arborsmith Studios has many examples of the work of its owner, Richard Reams, and images of other creative designs from the past.
D. Grafting to change fruit varieties
- Replacing an old variety on an established tree with an new one for economic or other reasons is known as Reworking, which is a form of topworking (see Grafting by Position spatial diagram)
- Seedling fruit trees can take 7 years or so to flower, and even grafted nursery stock can take several years. An alternative to waiting this long was, and still is, to a limited extent, to cleft graft a new variety up into the crown of an established tree. This could hasten production of the new fruit variety by several years.
E. Grafting to put multiple scion varieties on a single tree.
This is an example of Topworking (see Grafting by Position spatial diagram)1. An enjoyable home gardening option. Examples are:
- Apple with Macintosh, Granny Smith, & Red Delicious, etc. all on one tree
- Citrus tree with orange, lemon, grapefruit all on one tree.
- Hibiscus with several cultivars differing in flower color.
b. Cleft grafting would typically be used for this topworking objective.
2. According to Ian Merwin, a pomologist from Cornell University, top-working to shift from a low priced to high priced apple variety is quite common recently, especially in Washington state. There are professional grafters who do it relatively cheaply, with a high percentage take. This practice creates a bearing tree relatively rapidly, especially if the trunk is not too old. Although some experts caution about the spread of viruses using this method, Prof. Merwin states that virus infestations are unlikely today due to the availablity of virus-free certified scion wood.
F. To provide a pollinizer branch for self-incompatible fruit tree species
1. Apples, cherries and some other fruit tree species are self-incompatible within a clone.
2. In commercial apple production, one pollinizer (often crabapple) tree is usually planted at the end of each row, but grafting a pollinizer branch of another variety may be practical (and fun) in the home fruit garden.
- e.g. Macintosh apple will not self-pollinate, but it will cross-pollinate with another domestic apple or a crabapple.
G. Grafting to Influence Growth Phase
1. Grafting to avoid rejuvenation
a. Generally a grafted tree will come into bearing sooner than a seedling.
(1) This is because the (adult) growth phase of the scion tends to be maintained; whereas, a seedling is naturally rejuvenated by the process of embryogenesis (seed formation) compared to the seed-bearing parent tree.
(2) Furthermore, dwarfing rootstocks tend to induce scion precocity, i.e. they cause a scion to come into flowering one or more years sooner than it would on its own roots or grafted onto a non-(or less) dwarfing rootstock.
b. This avoidance of rejuvenation is exploited by tree fruit growers, to avoid the long delay (several years) associated with orchard establishment from seedlings.
- e.g. Avocado would come acceptably true-to-type from seed, but several years of bearing would be lost. Hence top wedge grafting of a scion from a mature bearing tree onto a seedling understock is commonly practiced in the nursery production of this tropical crop.
- See autotutorial slide set on Top Wedge grafting of avocado.
- e.g. Many Citrus varieties would come true-to-type from apomictic seed, since apomixis is a natural form of asexual propagation. Apomictic seedlings, however, like zygotic seedlings, are juvenile, and flowering would be delayed for several years, as explained above. Hence, one reason for T -budding citrus is to avoid seedling rejuvenation.
- see autotutorial slide set on T-budding of citrus.
- It should not be inferred that avoidance of rejuvenation is the only or even the principal reason for grafting fruit trees. Grafting is also practiced simply because many fruit tree species (e.g. apple, avocado, etc.) are difficult to root from cuttings (described above), and in order to take advantage of specific rootstock effects (described below)
2. Grafting for scion rejuvenation to facilitate subsequent rooting of cuttings ( Serial Grafting)
a. Ease of rooting is a general property of juvenile compared to adult growth.
b. Even though a scion from a mature tree tends to retain its adult growth phase.
As pointed out in the previous section, an adult scion will be slightly rejuvenated by grafting onto a juvenile (seedling) understock. Because this rejuvenating effect is only slight a scion may have to be sequentially regrafted onto a series of juvenile rootstocks before a useful degree of rejuvenation has been achieved. Cuttings taken from this rejuvenated growth tend to root more easily than from mature growth.
c. This rootstock-influenced gradual rejuvenation of the scion is called serial grafting.
d. Serial grafting is an extreme (last resort) method for facilitating the rooting of cuttings from the mature wood of extremely difficult-to-root species such as 100 year-old Sequoia ( Tranvan, et al., 1991).
H. Grafting for Virus Detection (Graft indexing)
1. Essentially, all viruses are graft transmissible. This is, of course, a disadvantage of grafting unless the objective is viral detection via graft indexing (see below).
2. In a given crop species, a virus may or may not cause obvious disease symptoms. Those virus-infected plants which are asymptomatic have no apparent symptoms may still exhibit decreased vigor and yield.
3. Graft Indexing. Grafting a scion from a tree of an asymptomatic species suspected of harboring a virus onto a more sensitive (symptomatic) indicator species will result in viral transmission from asymptotic scion to susceptible stock, which will then develop visible symptoms (mottling, streaking).
- Why is it important that the indicator is used as understock, not as the scion?
- Note: for many viruses there are other newer, more specific and/or more sensitive virus indexing techniques such as ELISA (enzyme linked immuno sorbant assay). Hence, graft indexing tends to be used less frequently.
(1) Strawberry - petiole wedge graft - terminal leaflet of trifoliate leaf removed, and small split made, where rachis of suspect terminal leaflet is inserted.
(2) Prunus subhirtella is used as an indicator for cherry, plum, etc. - compatible union doesn't even form, but virus is transmitted, and detection can be made.
I. Grafting to achieve independent optimization of component genotypes - Specific Rootstock / Interstock Benefits
1. Grafted Plants are Compound Genetic Systems
a. The rootsystem and the shoot system of a plant exist in different environments. Each has a different role in plant development and each makes a different contribution to agricultural productivity. Given the long generation time of trees (years), it could take a very long time, using standard plant breeding methods, to breed a tree to genetically optimize both the root and the shoot systems. Grafting, on the other hand, has allowed agriculturists to mix and match different genotypes in the root and shoot systems, resulting in a genetically compound plant that performs better overall than either genotype alone.
b. Of course, in modern times, genetic engineering, is another way to "construct" a plant with genes from more than one organism. However promising, genetic engineering is still in its infancy with respect to "designer" trees.
Do you think genetic engineering will ever make traditional grafting obsolete?
2. What is the Difference Between "Specific" Rootstock Effects and Non-Specific Rootstock Effects?
a. "Specific" rootstock (or scion, or interstock) benefits, in the context of this discussion, are advantages gained by grafting that are due to the specific genotype of stock or interstock. For example:
- Grafting Macintosh apple (scion) onto an M9 (dwarfing) rootstock, results in size control (dwarfing) of the scion because hormonal or other aspects of M9, under genetic control, are translated to the scion, affecting its vigor. Size control and other specific rootstock benefits in apples discussed in the section on Clonal Apple Rootstocks.
- Grafting Arabica Coffee (Coffea arabica, higher quality but nematode susceptible) onto another species of coffee, C. robusta, which is nematode resistant.
b. Non-specific rootstock effects would be grafting to achieve an objective that could be achieved by any compatible rootstock, regardless of its genotype. This includes many of the reasons for grafting & budding stated above such as the following examples in which the scion but the rootstock genotype is important:
- Grafting onto a seedling rootstock merely to propagate a difficult-to-root clone.
- Grafting onto a seedling rootstock to produce a plant with an unusual growth form, such as a weeping cherry.
What are some other non-specific rootstock effects?
3. A List of Specific Rootstock Benefits
a. Size control of the scion.
"Size control" in this context refers to some degree of dwarfing (or in some cases invigoration) of the scion by the rootstock, especially in the case of fruit trees.
Apple - the use of clonal rootstocks for size control (and other reasons) is a major part of modern apple production. The Malling and Malling-Merton apple rootstocks, introduced in the early 20th century, revolutionized apple production. Progress has been made since then by a number of other apple rootstock breeding programs around the world. Since apple is perhaps the best example of crop improvement through selection of clonal rootstocks, this topic is discussed at length in the section on Clonal Rootstocks.
Pears are sometimes dwarfed by grafting them onto quince rootstocks
b. Effects of rootstock on precocity (early flowering) of scion
Flowering and fruiting of an adult phase scion occurs more rapidly (precociously) when grafted on some rootstock genotypes than on others. In particular, the more dwarfing caused by the rootstock, the sooner the scion will flower and "come into bearing" from the standpoint of fruit production.
c. Pathogen resistance
Many rootstocks have been selected for disease or pest resistance but in most cases the resistance is not transmitted to the scion (in contrast to dwarfing). For example:
(1) Fungal pathogens
- Fusarium sp.
- Fusarium causes a wilting disease of many species, caused by fungal plugging of host xylem.
- e.g. Passion fruit (Passiflora edulis), purple-fruited hybrid varieties that are Fusarium wilt-sensitive, are grafted onto resistant seedlings of P. edulis forma flavicarpa
- Phytophthora root rot
- Resistance to root rot is one of the major selection criteria for the apple rootstock breeding program at the NY Agriculture Experiment Station at Geneva, NY.
(2) Bacterial pathogens
- Fire blight (Erwinia amylovora)
- A disease of pear, apple, etc. Rosaceous fruits (link to Geneva Experiment Station website)
- Fireblight resistance is one of several selection criteria in modern apple rootstock breeding program at the NY Agriculture Experiment Station
- Characteristics of Apple Rootstocks and Interstem Combinations by Paul Domoto (including resistance to fireblight) is part of the NC-140 Regional rootstock breeding program Web site.
(3) Viral pathogens
- Tristeza virus
- Tristeza causes greening disease in citrus which is a serious problem in Africa and other parts of the world.
- Resistance to tristeza is conferred by rough lemon rootstock. An article on citrus rootstock resistance to Tristeza and other diseases is presented on this Australian government website.
d. Pest resistance
(1) Insect pests
- Wooly aphid (WA) (Eriosoma lanigerum) is an insect pest of apple. The Malling-Merton series rootstocks were developed by crossing wooly aphid-susceptible East Malling selections with WA-resistant Northern Spy apple (Information sheet from UC Davis IPM Pest Management project).
- Phylloxera, described in an Information sheet from Univ. of California Integrated Pest Management project, is an aphid-like sucking insect pest of grape which parasitizes the root system. Phylloxera infestation in the wine regions of France in the 19th century virtually destroyed production of European (wines) grapes (Vitus vinifera) until they began grafting them onto resistant American grape rootstocks (Vitus lambrusca).
- Nematodes are microscopic "eel worms" which parasitize the root systems of many agricultural crops
- Almond (Prunus amygdala) scions are grafted on Mariana plum 2624 rootstocks which are nematode-resistant. ( Information sheet from University of California IPM Pest Management Project)
- Arabian coffee (Coffea arabica) produces a higher grade of coffee than Canefera coffee (Coffea robusta, the kind used to make instant coffee), but the former is nematode-susceptible while the latter is resistant. Scions from seedlings of C. arabica are grafted onto seedling understocks of C. robusta for coffee plantations in Guatemala and other parts of Latin America.
e. Cold hardiness
(1) Trifoliate orange (Poncris trifoliata) is deciduous (unlike the citrus species cultivated as fruit crops) citrus species. It is sometimes used as a rootstock for citrus. Orange or other citrus grafted on trifoliate rootstock are better suited for more northerly Florida growing conditions because of the cold hardiness of the trifoliate orange. This is one of the few examples of evergreen/deciduous grafting.
(2) Apple - for a summary of apple rootstock (cold) hardiness consult Paul Domoto's Characteristics of Apple Rootstocks table.
For what (other) reason is it surprising that Poncris trifoliata is used as a rootstock for Citrus sp. (hint, check the section on Requirements for Successful Grafting and Budding)?
f. Tolerance of specific soil types
(1) The apple rootstock M7 is tolerant of wet soil conditions; conversely MM104 is tolerant of dry soil conditions. For a summary of apple rootstock soil adaptability consult Paul Domoto's Characteristics of Apple Rootstocks table.
4. Specific Interstock Benefits
Just as single working (scion/understock grafting) allows the grafter to combine the best possible scion genotype with the best possible rootstock genotype, double working (scion/interstock/understock grafting) allows for further optimization of each of the three components of a tree - root system (nutrition, anchorage, dwarfing, etc.), trunk (support), canopy (fruit). See Grafting by Position
a. Size control
(1) A genotype that causes dwarfing when used as a rootstock has a similar dwarfing effect (but to a lesser extent) when used as an interstock.
(2) In addition, the degree of dwarfing by a given interstock genotype is proportional to the length of the interstock, i.e. a relatively long section of M9 used as an interstock has a greater dwarfing effect than a shorter section of the same genotype.
Why use an interstock for size control of a double worked tree rather than using the same genotype as a rootstock in a single worked tree? (Hint: see Apple Grafting autotutorial slide set)
b. To achieve an arborescent growth form of an otherwise shrubby scion variety (see Grafting to achieve special growth forms in this section, above)
c. Avoid incompatibility of an otherwise incompatible stock / scion combination by inserting a mutually compatible interstock. (see discussion of Bradford pear/Quince incompatibility in the section on Compatibility)
5. Summaries of Specific Rootstock Characteristics for Specific Crops
b. Stone fruits