“Hooper’s Law” gives us a way to estimate the age of a hedgerow (Max Hooper is a scientist who studied and wrote about hedgerows and biodiversity.)
The simplest method is to count the number of species of tree or shrub found in a 100 ft length of hedge. This number (averaged over three or more sample stretches) multiplied by 100 gives a rough estimate of the age of the hedge. So a hedge with an average of five woody species might be tentatively dated to the 16th century. (There are more complicated versions, but they give similar results).
However, it is important to bear in mind that Hooper’s Law is a rule of thumb and can only be used alongside other dating techniques such as local history, old maps, study of the field patterns, other flora in the hedge and so on.
An example of hedges where Hooper’s Law gives a misleading result comes from the area around the Stiperstones in Shropshire – squatters and free miners of the 18th and 19th century planted a lot of domestically useful trees in their smallholding hedges, including fruit trees, spindle, laburnum (for the wood) and gooseberries, so there are more woody species than one would expect.