GRAFTING

When thinking of graft, one immediately thinks of people taking bribes for services rendered, but grafts have had, and continue to have, an important role to play in horticulture. For example, grafting (and it’s subset, budding) has been an important technique to control plant vigor for fruit trees for many years. It provides a means to establish specific varieties (clones) of the majority of fruit trees (such as cherries and apricots), which do not breed true from seed and cannot easily form roots from cuttings. The development of dwarfing rootstocks in apples has had a major impact in the way in which apples are produced, compared with, say, 50 or 100 years ago.

Grafting also has a role to play in vegetable production, particularly (but not solely) in controlled environment agriculture. It is not a new technology; in fact, information on the benefits of using grafted vegetable seedlings was first published in the 1920s. But like most new technologies, it was not taken up immediately, and it was not until the 1960s that it started to become popular.

The major vegetable crops being grafted are tomato, cucumber, eggplant, melon, pepper, and watermelon. The main purpose of the grafting is to provide pest and disease tolerance, but tolerance to low soil temperatures and high salts can be additional advantages. In the 1960s it also became a method to provide resistance to soil-borne pathogens such as Fusarium, Verticillium, and nematodes for a range of vegetable crops, including tomatoes, cucumbers and melons.

Grafting has become a particularly popular practise in Asian countries, where the small farm size reduces the opportunity for long rotations and, therefore, increases the potential for pest and disease buildup in the soil. The introduction of protective cropping in these countries using plastic-film-clad houses resulted in plants being more stressed and being cropped for a longer period than in the field. Grafted seedlings were useful in overcoming some of these stresses.

Information in 2000 from Japan and Korea (Table 1) demonstrates the relative importance of grafting for a range of vegetables. Strangely, very few tomatoes were grafted in Korea; yet this is one of the more popular grafted plants in Europe.

My first practical involvement with the grafting of vegetable seedlings was at Massey University in the mid-60s, when we had some problems with Fusarium in greenhouse cucumbers. Using fumigants to "sterilize" the soil prior to planting was not proving successful, so we decided to graft the cucumbers onto the rootstock of the Malabar gourd (Cucurbita ficifolia). The method worked well, so the following year we used the method again, this time using a new variety of long (telegraph) cucumber, the recently developed only-female-flowering (gynoecious) variety "Princess." Removal of male flowers or restricting pollinating insects was not required. (Note, however, that telegraph cucumbers develop a bulge at the end of the fruit if they are pollinated!)

It was an interesting experience because at that time cucumber grafting involved beheading the rootstock, cutting the roots off the scion, making a V-shaped cut in the rootstock, and inserting the scion into the "V". Because the C. ficifolia stem is hollow it looks easy just to stick the scion into the center of the rootstock, but, in fact, no true graft occurs because the scion develops adventitious roots, which grow down the stem into the soil, without any graft effect. It is essential that the scion be inserted into the solid stem of the rootstock so that a true graft occurs. This is a trap for inexperienced growers. The operation normally requires mist propagation facilities because there is a major check to the growth of the scion. The check was quite clear with our grafted Princess cucumbers, because the plants reverted to producing only male flowers on the main stem, which made for an interesting lesson. These days the female-only characteristic in greenhouse cucumbers is much stronger, so that is unlikely to occur.


At about the time we were learning about cucumber grafting, the grafting of tomatoes became popular in some parts of the world. There were a range of rootstocks available, which incorporated resistance to Fusarium, Verticillium, corky root and nematodes — the so-called KVNF rootstocks. The so-called "supertom" so successfully promoted by Alan Naish from Kakanui in New Zealand was a grafted plant. Grafting at this time was a tedious business, as it involved growing both the scion and rootstock and then making a small V-shaped cut in both stems and sliding the cut pieces together (inarching). Once the plants were properly callused and joined, the top of the rootstock and the roots of the scion variety could be removed. Planting had to always (of course) be undertaken with the scion well above soil level, so that no scion rooting could occur.

My first experience with grafting tomatoes was in the late 60s, when I decided to show the technique to a group of students from the Cook Islands. My demonstration was (to put it mildly) a little ham-fisted, as I had never grafted tomatoes before. It was, therefore, very traumatic for me when the students demonstrated to me how they did the tomato grafting in the Cook Islands. It was a standard technique that they had learned "on their mother’s knee" to overcome bacterial wilt!

Following the interest in the 60s, grafting became of less importance, probably because of the growing interest in hydroponics and the idea that it was a totally "sterile" system. It was not until the 90s that the industry again became interested in this technology again.

In the Netherlands and Japan one of the main uses of grafted rootstocks is for eggplant (aubergine). Grafting onto a resistant rootstock not only provides greater vigor, but also offers resistance to both viruses and root diseases. Interestingly, a paper recently presented by Palada, from AVRDC, at the recent Singapore hydroponics symposium showed how grafting tomatoes onto eggplant increased tomato tolerance to flooding and to bacterial wilt diseases in Taiwan, Philippines, and Vietnam! Perhaps all things are relative and the enhanced productivity obtained by grafting onto eggplant in these tropical areas would be considered an unacceptably low productivity in an intensive temperate greenhouse situation.

Since the 60s the technology has changed and the need to retain the roots on the scion after grafting is now considered unnecessary. Grafting now involves slicing the rootstock and the scion at a 45-degree angle and holding them together using a plastic clip. The grafted seedlings are then placed in high humidity (a mist propagation chamber) for five to seven days to keep the scion turgid while the graft takes, before they are slowly weaned back into normal conditions.

Of course, grafting costs money. Not only is it necessary to grow two sets of plants, but also there is also the labour required to undertake the grafting, as well as the cost of mist propagation facilities. So why graft? I guess there are two main reasons. The first is that no matter how sterile the medium in which the crop is to be grown, over time pathogens will inevitably be introduced. By using a resistant rootstock the reduction in productivity is minimized, so that the crop can be grown for a longer time before it is necessary to replace it. Every time a tomato crop, for example, is removed there is about an eight-week turnaround before harvest commences again. The second reason is that because grafted plants tend to have greater vigor (see my comments below, however) it is possible to grow at least two stems on every plant; thus, the cost of propagating each plant is greatly reduced.

As one might anticipate, there is a delay from sowing the seed to first harvest due to the check involved in grafting. However, because this occurs during the propagation stage it is of little importance in the cropping house.

How to Graft
Obviously, it is necessary to sow two sets of seeds, namely the rootstock and scion varieties. Experience has suggested that the rootstock should be sown several days before the scion, because not only is germination a little more erratic, the seed also takes a little longer to germinate.

The ideal size for grafting is when the stems of both scion and rootstock are 1.5 mm in diameter. Almost certainly this will mean some need for grading the rootstock seedlings after emergence, to ensure that all the rootstocks in a tray are the same diameter. Growing the seedlings in plug trays makes it possible to grade, but this must be done at least two days prior to grafting, to give the plants time to get over the check. If the rootstock is germinated at 25ºC (77ºF), it takes about 17 days to reach the 1.5 mm diameter. The growth of the scion can be controlled with temperature, to ensure that that the scion and rootstock are at the correct size at the same time.

It is essential that the grafting area is hygienic and all equipment clean. Knives should be disinfected regularly to avoid the possible spread of viruses. The first stage of grafting is to remove the heads of the rootstock and throw away the heads to ensure that they do not get mixed up with the scion. The cut is made at a 45-degree angle at a height of 2 to 2.5 cm (~1 in.) above the pot. Too low and there is the risk of scion rooting; too high and the graft might become too heavy and fall over. The grafting clips are then attached to the rootstock.

The scion is prepared by cutting the seedling heads to a length of 1 to 1.5 cm (~1/2 in.) Again, a 45-degree-angle cut is made. This provides the maximum possible surface area for the rootstock and scion fusion. The scion is then inserted into the grafting clip until the cut surfaces of the rootstock and scion cut make full contact.

The grafted seedlings must remain in the high humidity environment (mist propagation) for at least four days to ensure the scion remains turgid and the graft takes. Then, over several days the humidity should be slowly reduced to glasshouse levels. Normally, full ventilation should be possible after day seven. It is not necessary to remove the grafting clips; they fall off naturally. In fact, removing them by hand could damage the plants.

Currently, the most common rootstock used for grafting tomatoes is Beaufort, from the Dutch seed company De Ruiter. It is resistant to Corky Root, Fusarium, Verticillium, nematodes, and TMV, but it is rapidly being replaced by Maxifort, also from De Ruiter. Other rootstocks are Eldorado from Enza and 61-063 from Rijk Zwaan.

Rootstocks vary in their vegetative/ generative characteristics, and it is really a question of selecting the appropriate rootstock for your scion/ production system/ planting date. Where vigor is desirable then a highly vegetative rootstock should be selected, but if growing a crop into the winter, a generative rootstock might be favored.

One of the advantages of a vegetative rootstock is that it introduces the opportunity to grow two or more main stems to a single root system, with a consequential reduction in propagation costs. This might almost negate the additional cost of producing grafted seedlings. The bonus, then, would be a potentially more productive plant that would resist soil-borne pathogens and grow better during cool conditions.

Of course, there are pros and cons surrounding any technology and they must be carefully weighed up by the individual grower for his or her specific situation. In general there are many advantages to grafting, not the least being the improved productivity due to resistance to disease. It also appears that grafted plants (via the rootstock) are more tolerant to poor water quality (salinity), an increasing problem in some countries. Tolerance to low temperature is also another potentially important advantage. Tomatoes grafted onto the rootstock KNVF grow well at low soil temperatures (10ºC to 13ºC or 50ºF to ~55ºF) when compared to non-grafted plants. In the same way, grafted watermelon performs better than non-grafted at low temperature; grafted eggplant is similar. Of course, it is essential to have the appropriate rootstock, as some rootstocks are not suitable.

Edelstein (2004) found that there was a marked difference in productivity when growing cucumbers non-grafted and on four different rootstocks. As one might anticipate, the non-grafted plants were a little more precocious in cropping, as they had not gone through the grafting check, but this advantage was soon eliminated and the grafted plants were the most productive. In fact, by the end of the experiment, the non-grafted plants had only produced 26 kg/m2 (~57 lb. per ~1 sq. yd.) and the best treatment (grafted onto the rootstock Shintoza), nearly 35 kg/m2 (77 lb. per ~1 sq. yd.) The plants grafted onto three other rootstocks produced 30 kg/m2 (66 lb. per ~1 sq. yd.).

The take-home lesson from this is that grafting has the potential to markedly increase productivity, but there is probably a need to match the rootstock to the scion for best results. In terms of productivity, it appears that grafting will not only increase yield (for melons there is a reported increase of 50-60 percent), but also the more vigorous root systems might also increase water efficiency and nutrient absorption.

Of course, nothing in this world is perfect, and there can be disadvantages to grafting. The first one to consider is cost. Clearly, it is going to be more expensive to produce a grafted seedling than a non-grafted seedling. There is also a question of having the technical skills available to undertake the grafting. Incompatibility is another potential problem. This is failure of the scion to unite properly with the rootstock, causing poor growth or premature death of the plant. This has been noted on occasion with Cucurbita ficifolia and melon, and also when tomato is grafted onto Datura tatula. However, this should not really be a problem, provided that care is taken to use rootstocks and scions with a good history of success. There have also been some examples of grafting causing fruit shape and taste differences, but in general the benefits of using grafted plants far outweigh any risks.

References:
Echebarria, P.H. (2001) Influence of Different Rootstocks on the Yield and Quality of Greenhouse Grown Cucumbers. Acta Hort. 559: 139-143.

Edelstein, M, (2004) Grafting Vegetables-Crop Plants: Pros and Cons. Acta Hort. 659: 235-238.

Lee, J.M. (2003) Advances in Vegetable Grafting. Chronica Horticulturae 43 (2): 13-19.

Palada, Practical Hydroponics 85: 40.