I'm looking for information on the Robert Craig Nursery -- do you have anything?
Yes, we have the 1856-1956: A Century of Growing booklet in the catalogue collection, along with catalogues for the Robert Craig Company for 1910, 1911, and 1916. There is quite a bit of nursery history in the booklet, as well as family history.
See the accompanying uploaded scan of A Century of Growing.
Perhaps in your online research you saw this blog post mentioning Robert Craig in the Growing History blog.
There are several mentions of Robert Craig in James Boyd’s A history of the Pennsylvania Horticultural Society, 1827-1927 since he was at various times a Vice President, on Council, gave lectures, served on committees, and was elected an honorary member. See pp. 329, 349, 353, 356, 376-77, etc. in this digitized copy.
The introduction of the Janet Craig Dracena is mentioned here:
According to the 1956 booklet A Century of Growing, Janet was the daughter of Robert A. Craig who was the son of the Robert Craig mentioned in Boyd.
We have a very brief newspaper obituary of William Patterson Craig (brother of Robert A. Craig); he's also mentioned in the 1956 booklet.
Since Robert Craig was also active in the Philadelphia Florists' Club, the American Dahlia Society, and the Society of American Florists, there may be more information about him available online. – Elinor Goff