Most plants, with the exception of the dreadblossom swarm, are sessile, and the weakest have only passive attacks. Some, however, are able to multiply, whether by spreading to adjacent tiles (razorvine) or by releasing motile spores that explode, causing damage and creating new plants (dungeon fern and swamp fern).
Others, like the sunflower and viper tree, compensate for their inability to move with ranged active attacks.
Razorvine grows quickly, and if left unattended can take over entire dungeon floors. It has trouble spreading through narrow passages, however.
Only razorvine patches at full HP spread, and new patches are created at 1/2 HP. Razorvine patches only grow into adjacent empty squares, so only the patches at the edge of a group grow. Razorvines on deeper dungeon levels grow more quickly than razorvines on shallow levels.
Razorvine's a fact of life in Sigil and on some of the Lower
Planes. It's a black-leaved creeper or ivy with an exceptionally
sharp-edged stem hidden under the lush foliage. The plant's
capable of surviving almost any conditions, and flourishes in
most environments - regardless of the quality of the soil, atmos-
phere, rainfall, or light. Razorvine can grow several feet in a
single day, and can cover a small building or untended wall in
a week. There are few creatures as can stomach razorvine, so
its growth is often unimpeded by natural means.
Merchants and other cutters interested in extra security have
been bringing razorvine cuttings with them to the Outlands,
planting the vines on whatever they wanted kept safe, and then
learning just how virulent ravorvine growth really is. Chant is
they recently had a sod drawn and quartered in Ribcage for trying
to smuggle cuttings in after they'd just finished clearing the
town of the stuff.
[ Planescape Monstrous Compendium II, by Rich Baker ]
Sunflowers attack by focusing ambient light, so only sunflowers in lit squares can attack. The sunflowers' attack does fire damage, as well as blinding the character.
... A single species of plant evenly dispersed across the land,
from here to the infinity-horizon. Each plant had a single blossom,
and each blossom turned to follow Louis Wu as he dropped. A
tremendous audience, silent and attentive.
He landed and dismounted beside one of the plants. The plant stood
a foot high on a knobbly green stalk. Its single blossom was as big
as a large man's face. The back of that blossom was stringy, as if
laced with veins or tendons; and the inner surface was a smooth
concave mirror. From its center protruded a short stalk ending in
a dark green bulb.
All the flowers in sight watched him. He was bathed in the glare.
Louis knew they were trying to kill him, and he looked up somewhat
uneasily; but the cloud cover held.
"You were right," he said, speaking into the intercom. "They're
Slaver sunflowers. If the cloud cover hadn't come up, we'd have
been dead the instant we rose over the mountains."
Dreadblossoms swarms only move if the player character can see them. Blind yourself to render them harmless. However, once the swarm has engulfed you, it will continue to attack until you escape. Players killed by one will arise as one in their bones file.
An individual dread blossom looks like a foot long
crimson flower, speckled with gold and black pollen,
that ends in a 6-inch-long hollow thorn surrounded at
its base by a frill of inch-long roots. Few creatures can
examine a dread blossom in such detail, as a dread blossom
swarm at rest is usually firmly planted in the bodies
of its latest victims.
[ Monster Manual III, Dungeons and Dragons 3.5 ]
This creature might be mistaken for a white beech tree at a
distance. A closer look, however, reveals reptilian skin and
branches tipped with fanged viper heads.
[ Expedition to the Demonweb Pits, by Wolfgang Baur
and Gwendolyn F.M. Kestrel ]
These dreadful weeds have been driven to extinction on the
surface, yet deep within the dark and damp of the dungeon
they still flourish. When it detects the motion of nearby
creatures, the dungeon fern releases a deadly, poisonous
spore, which quickly detonates to propagate the species.
Residents of the dungeon have learned how not to disturb
the dungeon fern, and will attack its spores on sight to
keep them from overtaking their home.
More than one variety of this plant is known to exist.
When a mandrake dies, it emits a deadly shriek. This shrieks kills all living (i.e. not undead or nonliving) monsters on the level with less than 101 HP (including you!). Be careful not to hit one if you can't resist the shriek, as it is instadeath otherwise.
"Stop moving!" Hermione ordered them. "I know what this is --
it's Devil's Snare!"
"Oh, I'm so glad we know what it's called, that's a great help,"
snarled Ron, leaning back, trying to stop the plant from curling
around his neck.
"Shut up, I'm trying to remember how to kill it!" said Hermione.
"Well, hurry up, I can't breathe!" Harry gasped, wrestling with
it as it curled around his chest.
"Devil's Snare, Devil's Snare . . . what did Professor Sprout
say? -- it likes the dark and the damp --"
"So light a fire!" Harry choked.
"Yes -- of course -- but there's no wood!" Hermione cried,
wringing her hands.
"HAVE YOU GONE MAD?" Ron bellowed. "ARE YOU A WITCH OR NOT?"
[ Harry Potter and the Philosopher's Stone, by J. K. Rowling ]