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.  

    Question 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 Grafting

    a. 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

    3. To replace a damaged or diseased root system - inarching

    4. To overcome a delayed incompatibility - bridge grafting or inarching

    C. Grafting to create unusual growth forms - Highworking (see Grafting by Position)

    D. Grafting to change fruit varieties

    1. 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)
    2. 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:

    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

    G. Grafting to Influence Growth Phase

    1. Grafting to avoid rejuvenation

    a. Generally a grafted tree will come into bearing sooner than a seedling.

    b. This avoidance of rejuvenation is exploited by tree fruit growers, to avoid the long delay (several years) associated with orchard establishment from seedlings.

    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.

    a. Examples:

    (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:

    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:  

    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.

    b. Effects of rootstock on precocity (early flowering) of scion

    c. Pathogen resistance

    4. Specific Interstock Benefits

    5. Summaries of Specific Rootstock Characteristics for Specific Crops