"Garden Leave" is a fairly common British term.
According to wikipedia:
Garden leave describes the practice whereby an employee who is leaving
a job (having resigned or otherwise had his or her employment
terminated) is instructed to stay away from work during the notice
period, while still remaining on the payroll.
The article goes on to state a brief explanation of its origin…
The term originated in the British Civil Service where employees had
the right to request special leave for exceptional purposes.
and that it came into common usage in 1986 (although ngrams suggests a little earlier)…
The term came to widespread public attention in 1986 when it was used
in the BBC sitcom Yes, Prime Minister, episode "One Of Us".
… but fails to fully explain its name. What does special leave have to do with gardens?
Is it because the employee has been "kicked out of the house" and must wait in the garden like a dog?
Or perhaps it is because an employee not allowed in work is expected to go home and sit in their garden?
Best Answer
It's gardening leave in British English, and yes, it's because that's all the employee is allowed to do.
The employee's contract of employment will state that they can't work for a competitor — either concurrently with their employment or subsequently for a certain period — and this leave ensures that they remain on the payroll for a certain time.
The employee cannot work for a competitor while they are on the payroll, and it extends the time until they can do so.
The reason for doing this is so that any information they have about clients or practices ages while they are away from the workplace and eventually becomes worthless to competitors.
The leave is called gardening leave because that's all the employee can do: they can't come in to work and they can't work for anyone else. All they can do is work in or sit in their garden.