In many SF games the local animal life is a distinct threat to adventurers. Whether it's a water moccasin in your moccasin or a grizzly leaving you grisly you're well advised to pack some firepower on those long trips referees seem to love to railroad you into. After all some of those animals living in the Traveller encounter table weigh as much as a T-rex and lack its charm.
"Feed me or your next survival roll will be a b!tch!"
I'm not talking about them today. I'm talking about another kind of enemy that bullets are useless against: plants. Even on Earth a plant can mess you up. Plants have evolved defense mechanisms over millions of years. If they hadn't the animals would have eaten them all by now.
Thorns on a plant are a no brainer. If the plant has resources animals require or just like it will have some sort of objectionable cover: thorns, nettles, brambles etc. Having a plant that can shoot their thorns is regarded as cheating by some players. However, some Earth plants, like cucumbers, use gas to fire their seeds at new spawning fields. Why not a rose analogue that shoots thorns to annoy pesky animals? Some seed distributers could be more energetic than Earth cukes. Imagine walking through some innocent plants only to have them erupt and pepper you with seeds or thorns. The garden variety (hah!) rose produces thorns that are exceptionally sharp and painful as I have found out over the years.
Instead of a tomb or plunder site being guarded by robots (that need maintenance and power) a savvy culture might seed such plants around the entrance.
The part you were waiting for. Amiright? Plants emit any number of deadly substances: allergens, poisons and worse (much worse) . The results could range from a stuffed nose to horrendous rashes to anaphylactic shock or burns. This may vary by species so it is possible some hard pressed locals might pay a group of (relatively) immune offworlders to clear a deadly species from vital areas. Just bring plenty of skin lotion. Okay itchy skin is kind of weak when it comes to driving a story forward but it's perfect to teach a player character some humility (or at least why you should get some Survival skill).
Many decorative plants are also poisonous or have poisonous oils or sap. Some are quite pretty (rhododendrons, oleander, wisteria) so be careful crawling through that garden.
Adventurers should be wary what they use for their campfires. Oleander for one is so toxic even the smoke from burning it will cause poisoning. Other alien plants may cause any number of hallucinations or worse. See M. Night Shyalmalan's The Happening for more on this.
Plants move. They usually do it slowly. Bamboo, though, can grow nearly a meter a day. Bamboo analogues could result in a path that was easily traversed a day ago to becoming nearly impassable or hide the entrance to the latest loot site. Or imagine the looks on your players' faces when they emerge from their latest foraging underground to find their ATV lofted two meters in the air by what looked like grass a few days ago.
Some plants move faster and can actually catch animals or have trap mechanisms. So far as modern science knows there are no plants capable of trapping a human (sadly the Man Eating Tree of Madagascar is only a legend). That does not have to hold true on other planets. Even so a thorn lined mouth closing on a foot will convince characters not to wear open toed shoes. Having a group of characters wake up with their sleeping bags held down by vines is also fun,
Some plants are vital for some animal species for food or shelter. Intelligent enough animals (or one manipulated by pheromones released from flora) may rush to attack people damaging the plants. Imagine a killer bee hive primed to attack you because you picked a flower (mind the thorns!) Suddenly people are not so blasé about cutting through a jungle, are they?
Bizarre Plant Life
Let your imagination soar. Within reason (or not). Psionic plants? Sentient plants? Why not?! Solar energy may limit their movements (especially at night or in the cold seasons). They may have to spend some time rooted in order to 'eat' and get water ( a hibiscus bending over for a drink at a creek is a great way to tell people they are on an alien world.)
A Word to the Players
I may seem to pull for the referees and that is because I was a referee and often felt outnumbered (though my players years later claimed the same.) Here are a couple of tips to deal with carnivorous vegetation and other local color.
1) Always, always, always read up on the worlds you visit. If possible speak to a traveler who visited or is from there. Buy them a drink and you won't be able to shut a referee mouthpiece up.
2) Find means other than combat to achieve your goal (though combat is a good secondary option in many cases). Perhaps in the examples above dangerous plant life can be rendered dormant with pheromones, darkness or other means. The flora may be inactive and safe at different times of day and seasons. Learn about the locals. Some filter masks and disposable hazmat suits are easy enough to pack and transport.
3) Equip yourselves. Again talking to locals is the way to go hear. Not necessarily the ones selling you gear though. Various travel societies and hiring halls may be able to get you a licensed guide or accredited academic. Not so much Crazy Ivan of Crazy Ivan's Trigger Happy Firearm Palooza (LLC). Scout Surplus (tm) is where you want to go (yeah I just made these businesses up.)
4) Reconnoitre. If you're a Scout or a Free Trader playing Final Frontier there will not be any library data to look at or guides. Look before you leap is good. Look before you land is even better. Even in modern day Earth many nations will kill for orbital reconaissance.
5) When all else fails shotguns are pretty effective all around. Laser cannons do a fine job combining cutting and burning.