I suspect this is not the answer you had hoped for, but I hope that giving you more of the story helps you to understand why the Catholic Church has decided to require weddings to take place in “sacred places.” Parish churches are, of course, “sacred places” but there are other possibilities.
The commentary in the code of canon law says: “Marriages may also be celebrated in churches or oratories other than the parish church, including private oratories.
So, while bishops can and do grant exceptions (e.g.
if your mother is an invalid and confined to bed, it is easy to get permission to have the wedding in your mother’s home so she can be present at the wedding), they are not likely to give permission for a “garden wedding” out of this concern that such settings give the wrong message about the seriousness and sacredness of the vows.
Personally, I was involved in a few outdoor weddings in the 1970s (before the stricter rules were made), and I know that a couple may have serious and sacred intentions about an outdoor wedding.
But I also understand where the bishops are “coming from” because outdoor weddings have to deal with many more difficulties and distractions.
Between 1100AD and the present, there has been a gradual shift toward more standardized ways of doing things in the Catholic Church.