Photo shows the autumn flowering Sedum ' Autumn Joy'.
Wait until winter to harvest the dead flower stalks then PVA glue some together to form a posy that looks like a tree. Some sprigs are set into the bunch at an angle to help form the tree shape. I glued one sprig at a time waiting for the glue to set between placements. It takes quite some time to complete. A different, instant setting glue would speed up the process.
Spray the dead flower heads with green acrylic paint, maybe a dark shade and then lighter shade where the light falls. I made a wooden template same size as the area destined for the trees and arranged five trees in a group. I drilled holes in the template to accept the tree trunks. (I made the template because the trees were made miles away from the train set.)
The template was placed on the train set and holes drilled into the baseboard through the template holes. The trees were then pushed into the holes without the need for glueing.
