Grafting is not usually thought of as a method for the home gardener. However, the basic techniques involved are not difficult and are well within the scope of anyone who has mastered propagation by cuttings.
Grafting is normally used when propagating plants which are not hardy on their own roots or which do not reliably form roots with other propagating methods. The graft consists of a top portion (the scion), which is the desired plant, growing on the roots of another plant (the stock). Grafting is a well developed practice in commercial horticulture and is used for propagation of most fruit trees and roses as well as being used with camellia and hibiscus in certain areas.
There are many different kinds of graft (not all of which are are related to horticulture!) but only the most common ones will be mentioned here. These are the Top Wedge Graft and the Approach Graft. Other methods are described in the listed references.
The basic principle in grafting is to ensure contact between the cambium of the scion and that of the stock. The cambium is the thin layer of actively dividing cells located between the bark and the sapwood.
The closer the relationship between the stock and the scion, the greater will be the chance of success. Plants of the same species will normally graft easily; different species of the same genus, a little less reliably; and different genera within the same family, a lesser chance again. Trying to graft plants from two different botanical families is unlikely to achieve much apart from improving your technique!
The materials needed are fairly simple...secateurs: a clean, sharp blade or knife; strips of teflon "plumber's" tape for wrapping the graft union; and waterproof mastic or petroleum jelly to protect the union until the graft has taken. A "cold frame" to protect the newly grafted plants is also desirable but not absolutely essential.
By far the greatest amount of success with the grafting of Australian plants has been with the genera Grevillea, Hakea, Prostanthera (mint bushes) and Macadamia. In the latter case, selected forms of Macadamia tetraphylla and M.integrifolia are grafted onto seedlings of the same species. This enables robust plants to be produced which will fruit much more reliably than seedlings and at an earlier age.
Many grevilleas have been successfully grafted onto the common 'Silky Oak' (Grevillea robusta) but another good stock is the robust hybrid G. "Poorinda Royal Mantle". Basically, any Grevillea which is hardy and robust in a particular district would be worth a try as a stock plant for the difficult to grow grevilleas.
Hakeas graft quite well onto the commonly grown species H. salicifolia and spectacular species such as H.francisiana and H.bucculenta are very reliable on this rootstock.
For those starting out in grafting a good confidence boosting graft to try is a mint bush, such as Prostanthera nivea, grafted onto the related Coast Rosemary (Westringia fruticosa) or the hybrid Westringia "Wynyabbie Gem". Mint bushes are often short lived on their own roots but are very reliable as grafted species.
Other grafts that could be attempted include some of the Western Australian Banksias onto hardy eastern Australian species such as Banksia integrifolia or B.spinulosa. Banksias, however, are more difficult to graft successfully although some success has be demonstrated by experienced propagators.
This is probably the simplest and most convenient method for the beginner. For best results the stock and scion should be of the same stem diameter (this facilitates the matching up of cambium layers). The stock would need to be propagated by seed or cutting some months prior to the graft being made (unless a cutting graft is used, as described below). The stock is cut off about 50 -100 mm above the soil level leaving a few leaves on the stock until the graft has been successfully established. A simple slit about 5-15 mm long is then made in the centre of the stem.
The scion is cut to a suitable length (say 75mm) and the lower end cut in the shape of a wedge of the same length as the slit in the stock. A very sharp blade will be needed to ensure that the cuts are clean. Leaves need to be removed from both the stock and the scion in the area to be joined so that the binding tape can be applied more easily. Handling of the freshly cut surfaces should be avoided as oil from the fingers may coat the freshly cut surfaces and hinder the successful union.
After the scion is inserted into the slit in the stock, the join needs to be tied firmly with the teflon tape, wrapping from the bottom up (this gives an overlap which sheds water). A coating of grafting mastic or petroleum jelly is applied over the tape to prevent water seeping into the grafted area. If the tape gives good coverage, the mastic need only be applied at the top and bottom of the taped area to give a good seal in those areas.
The grafted plant should be placed in a propagating "cold" frame until the scion shows new growth. At this stage the tape can be removed very carefully and the graft union inspected. If the join has callused well and the join looks to have taken well on both sides of the wedge, the tape can be left off but a coating of mastic or petroleum jelly over the join would be useful. The grafted plant should not be exposed to drying conditions for several more weeks.
If it can be obtained, pharmaceutical tape is preferable to the teflon tape as the former stretches as the graft union develops and eventually splits. This means that it is unnecessary to risk damaging a fragile union by untying the tape.
Sometimes it is not practical to arrange that stock and scion be the same diameter. If the scion is thinner, it needs to be set into the slit in the stock so that the cambium layers match on one side.
This is carried out in exactly the same way as the top wedge graft except that in this case the stock has no roots and the graft is treated as a cutting. In this way both grafting and propagation of the stock are carried out at the same time.
This method is useful in cases where the stock is known to strike reliably from cuttings. In other cases it is quite possible to have a situation where the graft is successful but the stock refuses to form roots. This is enough to drive most propagators to the bottle!
The advantage of the approach graft is that it doesn't require a cold frame for protection while the graft is developing. A shady position in the garden is usually sufficient.
With this method both the stock and scion grow on their own roots until the graft has taken. The stems of the two plants are brought next to each other. Slices 25-30mm long (or more, depending on the sizes of the stems) and about 1/3 the thickness of each stem in depth are removed from each stem at a convenient height above soil level and at the same height above soil level on each stem. The two stems are then joined so that their cambium layers are touching on at least one side of the stems. The join is then tied with tape (as shown in the diagram) and covered with mastic or petroleum jelly at the join.
When the graft has taken, the top growth of the stock plant is cut back (Cut 1). Two to three weeks later or when the scion shows vigorous growth, the scion is severed from its roots to leave the scion growing solely on the rootstock (Cut 2).
This final cut can cause a shock to the plant and it is advisable to cover the scion with a clear plastic bag for a few days to maintain humidity. If the scion shows sign of wilting after the bag has been removed, it should be replaced for a further period.
Once the plant has acclimatised, the tape can be removed from the join.
The time for a graft to "take" is variable but would rarely be less than six to eight weeks in warm weather. Often success will be indicated by a flush of new growth from the scion and a general thickening of the graft union. When checking the progress of a graft it's necessary to untie the tape around the join very carefully - the union will be fragile even if successful.
With all successfully grafted plants, any shoots that occur from the stock below the join must be removed or the stock may grow away very vigorously and overwhelm the scion.
Dawson, Ian (1996); Grafting Australian Native Plants. Proceedings of the IV National Workshop for Australian Native Flowers, 28-30 September 1996, University of Western Australia, Perth (includes comprehensive list of scion/stock combinations).