The Lausanne Jardins event is in full swing and continues on until mid October - have a look on line for details of each garden and explore the interactive map
https://lausannejardins.ch/en/

Pro Specie Rara host two autumn fairs, one at the Cantine de Sauvabelin on the 9 September in Canton Vaud and one in October, on the 29th at the Heiternplatz in Zofingen. www.prospecierara.ch/fr/decouvrir/manifestations.html

Other events worth checking out include the European Heritage Days in the middle of the month, which this year include an open day at the beautiful Ermitage in Arlesheim and an activity day at the alpine garden in Champex Lac www.nike-kulturerbe.ch/fr/venezvisiterch-journees-du-patrimoine/