Is there any expectation that people need to respond to this in numbers in order to get the new forum? In other words, is there anything I can do to make it more likely that we could get this forum?
Since this Suggestions forum doesn't get many visitors, you might want to post something about it on the Perennials forum. I would guess that's where your largest audience would be. If you get enough of a response there to support a separate forum, then you can send a link to that posting to GardenWeb or add it here so everyone can see that it's supported.
Keep in mind that many people have requested new forums, and in prior years many of these requests were quickly honored, only to discover that barely a dozen or so people consistently posted there. You'll reach more people and get more replies to posts on forums with lots more viewers. Even if you don't want to read and reply to messages about any other plant, one of the larger forums may be a good place for discussions about delphiniums. Perennials or maybe your regional forum or the Butterfly forum.
Soltice is right in that often with new forums there is not enough traffic to make it successful.... Lucky Bamboo and Sansevieria Forums comes to mind to name just 2.
Example...while there are a lot of folks who love, grow and collect Daffodils, as well as there being many Daffodil Societies, I sincerely doubt that there would be enough traffic to warrant having a Daff Forum, since there is an existing Bulb Forum where they are discussed.
If you are truly passionate about Delphiniums you might consider joining the Delphinium Society at the link below.
Here is a link that might be useful: Delphinium Society
laurels 14: If you do get a delphinium forum going, count me in! I am a long time member of the Delphinium Society and revere delphs.