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You might find some moss or lichen growing wild but it's not native.
Anything else would be grown in the cities in imported soil.

If you ever get a chance to see 'Lightyears' (an obsurce, heavily edited and butchered american version of Rene Laloux's animated masterpiece of the same name ), it sorta has a few ideas of what Wormwood could have as imported flora / fauna etc... who's to say that the planet coudn't make it's own symbiote war machines or even plant-like growths.
That's the problem; there is so much potential in the Wormwood setting, but it was never expanded upon (officialy, at any rate).