I'm currently on a mission to create a tabletop's worth of jungle terrain and the first installment is a small forest of slightly alien looking trees. Back in the nineties i had a disperate collection of home made flora for my battlefields inspired largely by Rogue Trader and 2nd edition 40k, and I hope to recapture the feel from those days.
The basis for the trees was a mixture of electrical core wire and florists wire which was either scavenged from the garage or available from.... You guessed it the florists (or hobbycraft,llll they stock it too). The wire was twisted up into armatures and hot glued onto the bases. In my case the bases were 3mm sign board as I find that it has a good tendency not to warp and it keeps glue and paint.
The wire armatures were wrapped in masking tape then coated with a textured paint made up from pva glue, fine sand and brown acrylic paint. Once they were dry i had the basic tree which is good enough to game with if you prefer the blasted and lifeless look which seems to be in vogue these days. However, being of a naturalistic ilk (BSc honours Zoology thankyou very much) then i want plenty of excuse for xeno fauna on my tabletop, and that means greenery, or primary producers if i'm going to get all scientific on you. (Actually you could argue for any colour of flora pigment, it depends on local conditions and the evolution on your chosen planet as well as things such as the type of light the local star throws out, but i'm going to be conservative here and opt for good old chlorophyll. But kudos to anyone who makes a purple or orange jungle. It wpuld look awesome.)