My random forest monster chart includes a bunch of plant monsters, and in the Mearls sidebar game we ran into most of them. Here are the details of the dangerous flora of my elven forest.
Glowing pollen sifts from the tree. Its dim golden halo can be seen 200 feet away at night or 20 feet during the day. There are humanoid and animal bones under the tree, and there may also be treasure (1 chance in 3) and/or passive living creatures (roll d6. 1: normal animal, 2: magical animal, 3: humanoid, 4-6: none).
If you go beneath the tree’s leaves, you feel peaceful and happy. You must make a Wisdom check/save (against enchantment) any time you try to leave the tree, initiate combat, or stand up. If you fail the save, you sit beneath the tree and won’t willingly perform any actions for an hour.
When you first sit down, and after every hour sitting under the tree, you have some spiritual breakthrough (DM: come up with a wise-sounding epigram!) and gain 30 XP.
After six hours spent beneath the tree, you make a death/Will/Wis saving throw (depending on edition). If you succeed, you permanently gain a point of Wisdom. Furthermore, you gain serenity and resolution about a sorrow or dilemma you’re facing. From now on, this tree has no effect on you. If you fail the save, you die.
Red bell ring: – All the trees around a clearing are hung with red bell-shaped flowers. If any creature larger than a fox passes into the ring, or touches a bell, all the bells ring (stealth and invisibility has no effect). The ringing of the bells forces a wandering monster check, prevents surprise, and wakes sleepers (including magical sleepers). Red bells are easy to spot unless you are hurrying.
Cobra vine: You’ve ventured from the path and are wandering through the elven forest. As your foot touches the ground, 2 glowing eyes snap open on an otherwise normal-looking vine. It instantly readies an action to strike (it has the stats of a poisonous snake), at anyone within 10 feet who moves or attacks it. You can try to move out of range slowly enough not to trigger it (5 consecutive rounds of easy dex checks, moving one foot per turn) or try to dodge/attack it (it gets the first attack) or someone outside the range can throw something large enough to distract it (you can escape while it’s attacking something else).