Can I get a bit of clarification on how closely you are defining the "Art of the Garden" theme?
While vased flowers are not ruled out, as "garden produce", I would definitely lean toward actual gardens displayed. However, I will also be keeping in mind that closeups of flowers are popular with the audience.
Wild flowers? Many gardens include wildflowers, and certainly in a closeup one would not know whether the wild flowers are in the wild or in a cultivated garden. A garden in the wild, though uncultivated, might be someone's perfect garden and give them the same pleasure as a cultivated garden.
Rural landscape scene with wild flowers as an element?A rural landscape could qualify if there is a garden in it, no matter how small.
Woodland landscapes with some garden element like a bench or gazebo ?Woodland landscapes with some garden element like a bench or gazebo can work.
Agricultural scenes that show barns, tractors or livestock ? Agricultural scenes showing barns, tractors and livestock are more farm oriented than garden oriented. I think it depends on the image.
Yard scenes without formal landscaping but showing trees and shadows? In Britain, the term "garden" is often used to refer to the yard, the place that might well be the scene of a garden party.
Snow scenes with some natural or garden elements ?Snow scenes with garden elements could add interesting variety to the show.
A single tree or leaf or pinecone?A single tree or leaf or pinecone are not going to enthuse me as much as seeing a real garden in all its glory for this particular show, but I wouldn't rule them out altogether on any fixed idea that they don't pertain.
A bird and birdhouse? While a bird and birdhouse would seem more appropriate in a bird show, the same applies as what I said about the natural elements you asked about above.
Also, do you have a target price range for the artwork? The target audience, while upscale, may not be inclined in today's economy to make huge purchases. The price range that promotes the most sales is what the Friends of the HTFL have in mind. That is why the commission is low, at 20%, to promote sales. A lower-priced piece may sell more easily than a higher priced one in this market. Many people are looking to aid the library by buying artwork in this show but may be working from a certain budget.
What would be the lowest price that would be acceptable, and what would be the highest?The price should be high enough to reflect the value of the work and not so high as to discourage sales. There is no ceiling (or floor) either way.