Of course, working in a museum, I love old photographs. Lantern slides, glass plate negatives, gelatin silver prints: they're all good; and when the subject is gardens, even better.
The Archives of American Gardens at the Smithsonian Institution contains 80,000 images documenting contemporary and historic gardens. The core collection is 3,000 glass lantern slides from the 1920's and 1930's. Bliss! On a winter's evening, how pleasant to wander through the searchable database and stop to smell the virtual flowers. Best of all, you can direct your path by searching for gardens in particular places and times.
On my first trip to this database some years ago, I found several images of the Chevy Chase, Maryland, garden of renown rosarian, Whitman Cross. From the marginalia for this photograph, the rambler so happily sprawling over the stone wall on a sunny afternoon in late May, 1930, appears to be "Dr. W. van Fleet." When I showed these photographs to my mother, she told me that Mr. Cross, who lived just around the corner from her, had 2,000 roses growing in his half-acre suburban yard. Among them was the dark red rambler, "Chevy Chase," and I've wondered since if the deep crimson rose that my mother trellised by our back door was an homage to her childhood neighbor.