We had some good responses from the John Cabot Academy in Bristol UK.
Hannah and Mary wrote:
Jess and Katie added:
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.