QRF have a range of '15mm' scale (Actually standing about 17-18mm tall) plants, various types including spine shooters, stranglers and grapplers.
Hydra Miniatures also have their new "plant men" - just the thing for Venus I think!
The 'Mouth-arm' by Emperor Miniatures reminds me a lot of the nasties at the bottom of the crevice in the new King Kong movie
Fenryll offers these nasty looking pot-plants:
And Kilroy Industries has these nice looking 'spider' plants