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?