An effective way to make these (in 15mm or 20mm) is to use garden wire (the stuff covered with green plastic which you can get in DIY and gardening shops) to form the trunk and branches, with paper fronds:

 
• First, take three or four lengths of wire, about twice as long as your required tree height, and twist them together. This should give a central "trunk" with splayed strands at either end.
• Then spread out the strands at one end, so that they lie at roughly 90 degrees to the trunk - these are the "roots" of the tree, and should be glued to a base (cardboard, MDF, thin plywood, coin or other stiff material).
• The strands at the other end are the branches, and should be stripped of their plastic coating (back to the "bole" of the trunk)
• Take some bandage (strip type) and soak it in dilute filler/plaster (Tetrion, Polyfilla, Plaster of Paris, etc) then wrap this around the trunk.
• Cut out some rectangles of paper, as long as the "fronds" are required (i.e. to cover the stripped wire of the branches) and twice as wide as necessary. Fold these in half, flatten out again, cover one half with PVA glue and fold back over the wire branches.
• Once dry, trim the paper rectangle into a "leaf" shape, then make cuts at a slight diagonal along either side of the central wire "spine", to give that palm frond effect.
• Spray the whole thing dark brown, then paint the fronds green, dry-brush the trunk with a lighter brown to bring out the texture, and flock the base. Finished!

Other Vegetation

Lichen and other model-railway material such as static grass is also of use. Some plastic aquarium plants are good for tropical foliage.