Oh, for heaven's sake, do the BBC have some sort of one-arm-bandit-based title-generator with dicky wheels that stick on "secrets", (also "wonders").
viz a viz
Secrets of the Universe .Secrets of the Sexes .Secrets of Quantum Physics,The Secret History of Our Streets, Archaeology: A Secret History, Secrets of the Superbrands, Secrets of the Arabian Nights ..
There are many other titles of a similar ilk across television and wireless stations None really introduced anything that wasn't widely known to those with an interest in the topic presented. The science-literate expect no such dishonest claims in the titles of the physical sciences programmes.
From the OED
"Not known or seen or not meant to be known or seen by others::Something that is kept or meant to be kept unknown or unseen by others. Something that is not properly understood; a mystery:
These programmes do not present anything that meets any definition of secret.
There are numerous published works on sociological histories of gardens and gardening. The knowledge is already there for the finding.
Is someone in BBC Factual who is paranoid / delusional with an obsession about secrets making up these programme titles? If not, do they have the impression that inserting the word "Secret" (or a derivative) will make the programme more appealing? It does the opposite for many of those who would otherwise watch or listen.
Stick to the facts, BBC, in the programme titles as well as the content. You are very probably not presenting any novel insights nor any secrets. Furthermore as there are numerous opinions and interpretations in the fields of professional history and professional sociology based on the same pool of evidence or lack of, it would be "A" Secret History of not the arrogant "The" History of.
I'm unlikely to watch this.
"A History of the British Garden" or "A History of British Gardens" would be more intellectually honest title, and one which I believe would be more appealing title for viewers with an interest in factual television.