Although some propagation occurs through shoots rising from the stem and roots of the original most plant life continues mainly by creating seeds that either fall to the ground or are carried by the wind, birds or other animal life. .  Most new olive trees come from the seeds (olives) from the wild trees. The DNA in a newly formed tree comes from the olive fruit, normally from the original wild tree, which produces small black and very bitter olives. The fruit that is available for both table olives and oil does not come from the growth of the wild trees but has been grafted into the wild form from chosen stock.   Previous generations of farmers discovered choice forms of olive and learned that a twig or branch from these trees could be inserted into a wild tree, (the process of grafting) and the result is propagation of the popular form of olive.

The olive tree has been cultivated this way since the bronze age and has been developed to provide mankind with a fruit that is regarded as one of the healthiest either in its full form as the table olive or as an oil. 

Unfortunately only one cultivar can be eaten off the tree itself in Thassos in the Eastern Mediterranean and all the others have to be processed either preserved in brine or crushed to make the free oil.  The commonest variety in Portugal is Galega Vulgar but there are many others, like Mancillenha and Picaul, which are also propagated and harvested throughout Portugal.Throughout the Iberian Peninsula there is  a mixture of normal farms where generations of farmers have introduced grafting and improved their stock over the centuries and plantations showing intensive olive growing practices.

If you look at the fruit of 'wild' olive trees you will see that it is small and hard and nothing like the table olive.  Most of the olive trees on Silves course are essentially 'wild' i.e. propagated by bird life but some have been grafted and with a little practice you can see which ones have the abrupt change in bark pattern indicating that they have been grafted with a different type. 

So how does grafting work?

When a tree is well established the tree uses the central part of the trunk and its branches for strength but not used for carrying nutrients to the leaves and branches.  This function is carried out by the outermost layer of the tree, just under the bark.  It is called the cambium layer and is so vital that if you 'ring' the tree i.e. run a knife round  the trunk only at this depth, you will kill it.  It also means that if you can insert a graft so that its own cambium layer joins up with the host tree's cambium layer the graft will survive and subsequently bear fruit.  The principle is easy to describe but the skill is acquired and treasured by the farming families.  Disappointments occur and much of the skill is in choosing the place to graft and how many to make.  Fortunately it can be done over several seasons and there is no shortage of  upcoming new trees thanks to the activity of birds and soil type.  The amateur must accept a 50% failure rate and be grateful for anything better.  There are several rules of thumb to guide you and you must have a sharp knife.

Olive grafting is done in March.

Probably the most contentious rule is that it must be done when the moon is full.  If anyone has the time, enough samples and the patience to do the statistics it may be found to be only a  folk superstition and the phase of the moon is not relevant.  However a time in the month has to be chosen to do this and generations of farming families and monks have had success with it so why not follow the crowd and do it then?

There are two trees involved.  The donor tree must be identified and, if not your own, permission must be given to take a branch. Keeping it in water to take to the graft site may be needed.

All action thereafter is on the recipient tree.   You must chose the limb to be grafted and saw it off just above the desired level so that if the bark tears the stripped part is above the level desired.  The cut is the made  at the optimum level and the cut surface cleaned with a paring action of the knife.

A piece of twig is prepared to act as a separator to cleave the bark from the cambium of the host tree.  Nothing special but it should be a narrow 'V' shape 10 to 14 cm long.   A vertical cut is made down the branch at one, two or three points around the cut surface according to how many grafts are to be inserted.  The slit is then widened along the cleavage of the cambium to take the graft.

The graft is then prepared.  It is cut from the branch taken from the donor tree and should have four or five buds above the level inserted.  Below these buds a tapering sliver of wood is sliced to leave a  length of bark exposing the cambium. This is inserted into the slit already prepared.

The wound is bound firmly with cord.  The open cut area is then covered with clay and a retaining cloth with holes taking the grafts is strapped over the whole graft site.

This is the end of the human activity.  The rest is for nature.  In a few weeks it will become apparent if the graft has taken.  Small buds will develop followed by leaves and a healthy growth.

As the summer progresses the graft swells and it has to be  watched.  The top may develop too fast and need to be trimmed while the union is strengthening.  It may also be wise to release the tightness of the binding to prevent damage from constricting the expanding tissues and a splint may be needed to support the graft when the wind blows.

All this is a matter of judgement and experience, but that is the wisdom acquired from the traditional farmer or your own instincts.  Failure one year is the basis for improving techniques next year.