Grafting is the practice of splicing one plant onto another, so that they fuse together to become one plant – the new plant is known as a ‘chimera’^ref. Most of the grafting you’ll see out in the horticultural industry is putting a different root stock together with a named variety above ground, as a form of clonal propagation of the above ground plant.

But the same principle is used in bonsai to add new branches or roots to an existing tree. According to Garner (see references) two main forms of grafting exist – approach grafting and scion grafting. Approach grafting is when two plants (or two parts from the same plant) are held together for long enough that they fuse – but neither are detached from their parent until the union is made. Scion grafting is when the stem to be added is removed from its donor plant before the union takes. Bottle grafting appears to be halfway between the two, where the scion (the stem being added) is sustained by standing it in a bottle of water until the union is made.

I am certainly not the person to be informing you about the techniques for good grafting (check out The Grafter’s Handbook by Garner for that), but I do want to look into the science behind grafting, how and why it works, and what you can do to make it more successful. First some terminology – the plant which is being grafted onto is the ‘stock’, and the piece of plant which is being grafted onto the stock is the ‘scion’.^ref

The basic idea behind grafting is that the vascular systems (the xylem & phloem) of both plants become connected – this is needed so the scion can obtain the water and nutrients it needs to survive, having been separated from its parent plant.

The first requirement for this is to have genetically compatible plants. If you are using the one plant to graft to itself, obviously this will be compatible. If you are using two plants of the same species, known as ‘homografts’, they will be compatible also. Otherwise rootstock and scion belonging to the same botanical species are nearly always compatible, rootstock and scion belonging to different species of the same genus are usually compatible, intrafamilial (within the same family) grafts are rarely compatible, and interfamilial (between different families) grafts are essentially always incompatible.^ref

To find out the genus of a particular plant, you can search on http://www.theplantlist.org/ – for example Pinus is a genus and Pinus sylvestris is a species. So in theory you should be able to graft any pinus onto another one. The Pinus genus is a member of the Pinaceae family and there are examples of intrafamilial grafting working in this family – for example grafting Cedrus atlantica scions onto Pinus strobus stock.^ref

In order to create connections between the two vascular systems, each stem needs to be cut to expose the vascular tissue, then the vascular cambiums of both plants are aligned as closely as possible and held tightly together with tape or a rubber band (or similar). Since the vascular cambium can be extremely narrow (depending on the species, but 3-10 cells if you look at the images in The Plant Stem by Schweingruber & Börner – see References for details) – it can be extremely challenging to get the positioning correct. After this the graft is ‘sealed’ to the extent possible – beeswax has traditionally been used.

Below is a diagram showing the sequence of events in a successful graft. It’s not the case that the vascular systems just line up and start working, like you’ve connected pipes together. When plant cells are wounded they die (see repairing damage), so these can’t just reconnect to another plant. Initially there is “a necrotic layer of one or two damaged cells” between the wound sites. When the two wound sites are placed together, the plant activates a process known as autophagy^ref – incidentally this is a similar process which is invoked when humans fast – it clears away and recycles dead or damaged cell material. Although their vascular systems are not connected, there is some communication between cells at the graft boundary, otherwise they would not detect each other and activate autophagy (which isn’t activated if another plant is not present at the wound site).

At the same time auxins and sugars start to accumulate at the wound site (since there is nowhere for them to go) and callus tissue starts to form – this is what ultimately joins the two plants together. Callus tissue is a mass of unorganized cells that forms in response to wounding – this can then regenerate the entire plant based on the plant growth regulators which are present.^ref The callus tissue differentiates into vascular tissues which act as a bridge between the two plants.

A key point with grafts is that even after the graft is completed, you will always have two genetically distinct individuals with a joining layer between them (which apparently includes transfer of DNA between the individuals, but only for a short distance)^ref. The upward supply of water and mineral nutrients as well as the downward flow of photosynthates are modified and so is the root–top interchange of hormonal signals.^ref This can result in graft failure many years afterwards, due to more long-term genetic incompatibilities. The best way to reduce this risk is use the closest genetic match as possible – the same plant (best), variety (good), species (good but not if very different varieties) or genus (OK but risky).

To optimise the chance of success of a graft there are a few factors which contribute to better outcomes (aside from compatibility), according to studies^ref:

• Grafting technique – some types of graft work best with specific plants – for example in conifers terminal fissuring and lateral plating are used.
• Use vigorous stock and scions
• Use younger stock and scions, unless you are bud grafting, which seems to be also successful from older plants
• In some species winter grafting is more successful
• Temperature can affect success – depending on the species – you don’t want it too hot or cold
• The graft union needs to be held together, and protected as best as possible from drying out or from pathogens. One study found that paraffin wax was effective.^ref This might be one situation when the slightly dodgy tree ‘wound sealants’ would actually be useful.

So bonsai nerds, what does it all mean? If you are considering using grafting techniques, my first piece of advice is to find someone or a book which has proper detail in it about the process. As noted I have the Grafter’s Handbook by Garner. Brent Walston also has instructions for grafting pines on his website.

Do some research about the species you are looking to graft, to find out the most successful techniques for that species.

Consider using bud or chip grafting – it’s supposed to be one of the easier forms, doesn’t disfigure your tree and allows you to use the same plant as the donor, reducing incompatibility issues.