Hmmm...I've seen series about farming and such but never gardening. This might prove an interesting subgenre to check out.
Some of the books I mentioned are focused on individual (sometimes poisonous) plants, others - like THE SAVAGE GARDEN - feature a specific landscaped garden. And still others have a central theme of gardening, as in planting, growing and harvesting flowers, fruits and vegetables and/or ornamental plants.
I've come across a couple more:
Elizabeth Ashby - A DEATH IN THE FLOWER GARDEN, a Danger Cove Farmer's Market Mystery
Ellery Adams - MURDER IN THE SECRET GARDEN (A Book Retreat mystery)
Marty Wingate - THE GARDEN PLOT (a Potting Shed mystery)
Dorothy St. James - FLOWERBED OF STATE (A Whitehouse Gardener Mystery)
Kayte Nunn - THE BOTANIST'S DAUGHTER
London Lovett - PEONIES AND POISON (and other Port Danby mysteries)
Gillian Larkin - MURDER IN THE FLOWER BEDS
Carola Dunn - THE WINTER GARDEN MYSTERY
Ruby Loren - GARDENIAS AND A GRAVE MISTAKE (Diana Flowers Floriculture mysteries)
Ellis Peters - THE ROSE RENT (a Brother Cadfael book)
Rebecca Rothenberg - THE SHY TULIP MURDERS
Lauren Willig - THE SECRET HISTORY OF THE PINK CARNATION (series)
Anthony Eglin - English Garden mystery series
Rosemary Harris - Dirty Business garden mysteries
There are lots more suggestions here:
https://www.cozy-mystery.com/blog/gardening-cozy-mysteries.html