We had some good responses from the John Cabot Academy in Bristol UK.

Hannah and Mary wrote:

We think the triangle format would work better with the circular fence, you would be able to have more triangle arranged trees and you would have more land if you wanted to put up more fences. Also, you would pollute the earth less by having more trees in one fence.

Connor, Ben, Ray, Natty, Aimee and Laura said that $14$ in triangles was best.
But Emily and Megan said that their solution was to build a fence around the trangular trees, so you can get at least $16$ trees in there. I guess that many more ideas came to people's minds but were not sent in. Remember we are very interested in whatever comes out of these explorations.