PLANT OF THE WEEK

Dr. T. Ombrello - UCC Biology Department

 

GRAFTED CACTI

grafted Cactus A grafted cactus growing in one of Union County College's greenhouses.

As a rule, cacti graft quite readily. Almost any two cacti can be successfully grafted, and can produce some interesting forms. Cactus grafting is often tried by beginning grafters, while learning the basic techniques. It is a great confidence builder, because with a little practice one can become quite good at it. But first, what is grafting?

Grafting:

 The art and science of connecting two pieces of living plant tissue together to grow as one composite plant.

Grafting Terminology

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 Rootstock (stock, understock) – The lower portion of the graft, becoming the base of the stem and the root system of the composite plant.

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Scion – The upper part of the graft. A short shoot with one or more buds that will develop into most of the aboveground portion of the composite plant.

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Budding – A type of grafting where the scion consists of a single bud and a small section of bark.

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Vascular Cambium – Meristematic cells found in association with the vascular tissues of stems. For success in grafting, the vascular cambiums of the stock and the scion must be placed in contact with each other.

 Why are Plants Grafted?  

People graft plants for a number of reasons.

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To perpetuate clones that do not propagate well by cuttings or other asexual propagation techniques.

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To obtain the benefits of certain rootstocks for insect resistance, disease resistance, dwarfing, and tolerance of unfavorable growing conditions.

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To change the cultivars of established plants by “topworking”.

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To produce special growth forms such as “standards” and weeping trees.

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To repair damaged trunks of trees.

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To study plant development, physiological processes, and viral diseases.

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To hasten the reproductive maturity and fruit production in fruit breeding programs.

  A Brief History of Grafting

While no one really knows when people first started to graft plants, a good guess would be that our ancestors mimicked what they observed in nature. Natural grafting occurs regularly. Branches and especially roots of woody trees and shrubs, when held in close contact for prolonged periods of time, will graft. As the stems or roots expand in girth over time, the bark between them is crushed. Cambial contact is made, and a connection between the separate vascular systems differentiates.

The Chinese were grafting plants by 1560 BC. Both Aristotle (384-322 BC) and Theophrastus (371-287 BC) wrote about grafting on a level suggesting intimate knowledge of the techniques and results. For example, Aristotle wrote, “Grafting of one on another is better in the case of trees which are similar and have the same proportions”. That advice on graft compatibility is still good today! During the reign of the Roman Empire, grafting came into common horticultural use. Paul the Apostle discussed grafting “good” olives onto “wild” olives in Romans 11:16-24.

During the Renaissance, many plants were brought back to Europe, and were in many cases maintained by grafting. The need to match the cambiums of stock and scion was realized by this time, but of course a real appreciation of meristems was hundreds of years away. An understanding of plant circulatory systems developed in the 1700’s and the formation of graft unions were the subject of much research. By the 1800’s over 100 different grafting techniques had been described, including those that are in wide use today. Many of these techniques have remained unchanged over all that time.

Formation of a Graft Union

In order for a graft to be successful, meristematic tissue must develop between the stock and scion, and differentiate into vascular tissues (xylem and phloem). Initially, undifferentiated “callus” cells grow from the vascular cambiums of the stock and the scion. These cells form a callus bridge between the stock and scion and intermingle as they proliferate. If the cells are incompatible, this intermingling does not occur and the graft fails. If the cells are compatible, this bridge of callus differentiates into vascular cambium and vascular tissues. The vascular connection between the stock and scion allows for the translocation of water, mineral nutrition, carbohydrates and other metabolites. Well-formed graft unions are structurally sound, and are no more likely to break than other portions of the stem.

 Graft Compatibility

Taxonomic relationships are good guides to graft compatibility. The closer the genetic backgrounds are between the stock and scion, the better the chances of success.

 Relationship                                                  Grafting Success

 Within a species                                             Excellent

Two species within a genus                           Possible

Two genera within a family                             Rare

Two families within an order                          Very rare

 

Other considerations on graft compatibility include:

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Monocots, in general, are not good candidates for grafting. The scattered vascular bundles in their stems are difficult to match between the stock and scion. In contrast, grafting success is common amongst dicots and amongst conifers. Their vascular tissues and vascular cambium are arranged in easily discernable rings.

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A few families are very compatible, with good grafting success amongst diverse species within families. Examples: Cactaceae, Rosaceae, and Rutaceae.

Grafting Cacti

Almost any two cacti can be successfully grafted, and produce some interesting forms. Cactus enthusiasts graft to:

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Save plants severely rotted or diseased, by grafting the remaining healthy portion of the plant onto another cactus.

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Ensure better growth and flowering by grafting scions of slow growing species onto vigorous, fast-growing stocks.

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Develop unusual growth forms.

Nurseries, especially in Japan and Europe, have used grafting to bring unusual growth forms of cacti to the market. The most common of these novelty cacti are the “Moon Cacti”. With brightly colored scions of red, orange, yellow, or white atop green stocks, the “Moon Cacti” are quite striking. The sources of the colored scions are mutant seedlings lacking the green chlorophyll pigment. These seedlings would not live by themselves for more than a few weeks since the absence of chlorophyll prevents them from making food by photosynthesis. As tiny seedlings they are grafted onto vigorous green stocks, which provide the materials to support the colored scions. These “Moon Cacti” grow for years, but when the green tissues of the stock begin to cork over from old age, regrafting to a new stock is necessary or the scion will slowly starve to death. Interestingly, many people think that “Moon Cacti” are giant, brightly colored flowers. In reality, they are just brightly colored stems. Cactus grafts made between green scions and green stocks are much more vigorous than the “Moon Cacti”, and can live indefinitely.

 A simple technique for grafting cacti follows:

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With a sharp knife, cleanly cut off the top of a small, upright cactus, several inches above the soil surface. This will be the stock. A plant growing in a three-inch pot, with a one-inch diameter stem would be ideal.

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Remove a small (approximately one-inch diameter), spherical stem from a barrel cactus, and cleanly cut it across the bottom. This will be the scion.

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Set the scion on top of the stock in such a way that at least some of the vascular cambium of each part is in contact. This may require the scion to sit atop the stock a bit off center. The vascular cambium region can be readily seen as a distinct ring on the cut surfaces of the stock and the scion.

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Take two rubber bands of appropriate size, and affix them over the scion and under the bottom of the pot. They will exert a steady pressure on the scion, pressing it against the stock. The two rubber bands should go over the scion at 90 degrees from each other, in order to prevent the scion from shifting. Rubber bands that are too loose will not hold the scion tight enough. Rubber bands that are too tight will cut through the scion.

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Grow the plant as you would any other cactus.

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After a month or two, remove the rubber bands. The scion should be firmly attached to the stock.

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If the graft union failed to knit, cut a new surface on the stock, prepare a new scion, and try again. Practice makes perfect!

 

Grafting cacti

Here are 2 cacti ready to be grafted.  The potted plants will be stocks.  The scions will be the small stems next to the stocks.

The stocks and the scions have been cut with a clean, sharp knife.
Note the vascular rings in this view of the prepared scion and stock.  The vascular cambium is located in the ring.  When put together, the rings must be in contact.  In these specimens, the vascular ring of the stock (right) is wider than that of the scion (left).  In order to match up the vascular cambiums, the scion should be set slightly off center on the stock.
The scions are held tightly in place with appropriate rubber bands.
This is what your grafted cactus should look like a year later.