OK, here is the first 3/5 of the query:
Here is my latest query, which features 5 different proposals. We had held a poll on ENWorld to determine which types of monsters people had seen too many of, and which ones people wanted to see more of. One of the creature types that stood out the most to me was plants and plant creatures; not only were people not tired of seeing plant creatures, but they wanted more of them! Most of these creatures are conversions of old plant creatures, but we do have a few new ones, and can make more if we need to fill in some gaps somewhere. Many of these creatures could be shifted around from one article to another, for a better fit.
In any of these articles, we can feature a "plant index" much like the one we created for the Monsters of the Mind article (people seemed to like that).
1 & 2. "The Garden of Unearthly Terrors":
The idea strikes me that one way to present deadly plant creatures is as guardian creatures. A druid may grow and control such creatures to make his own little garden of dangerous plants, a local lord may use them in defense of his castle, or an assassin may use them as nasty traps for trespassers.
1. Garden of Unearthly Terrors v1.
Version #1 presents a sampling of creatures that can be used for defense with minimal personal risk, if handled properly. Some of these are dangerous but not carnivorous, and some may wind up as "hazard" creatures in a sidebar if no appreciable combat stats can be made. Thus since many entries are likely to be brief, I think all of these creatures could conceivably be included in a single article.
Razorvine
This creeper/ivy is harmless to anyone that doesn't touch it. If you fall into a patch or try to move through it, though, you can lose limbs or bleed to death on the iron-hard razor-sharp leaves.
Originally found in the Planescape setting.
Explodestool
These mushrooms explode like firecrackers when touched, injuring and deafening nearby creatures. One explosion sets off a chain reaction in a patch of mushrooms.
Originally found in Dragon #89.
Boring Grass
This patch of grass is made of hard corkscrew blades that dig aggressively into material or creatures touching it, and is coated in paralytic poison.
Originally found in Monstrous Compendium Annual #2.
Grab Grass
This patch of tall grass wraps around victims and squeezes, strangling prey and feeding on the remains as they decompose.
Originally found in the D&D (Mystara) setting.
Retch Plant (CR 2)
This palm tree drops coconut sized fruits that burst on contact, leaking a fluid that causes creatures to vomit. The fruit's odor is strong and attracts other monsters to finish off unlucky victims.
Originally found in Monster Manual II.
Witherweed
This plant appears as grass, but actually has many moving fronds to attack. When a frond hits on the touch attack, it deals Dex damage and the weed feeds on the creature's ability loss.
Originally found in Fiend Folio.
Amber Lotus Flower
These golden water lilies spray a cloud of pollen that puts creatures to sleep. The plant does not kill its own victim, but in its symbiotic relationship with other plants, the others kill the victim and all benefit from fertilization of soil.
Originally found in D&D (Mystara) setting.
Burnflower
This patch of grey-green vines has closed flower bulbs that absorb sunlight, releasing it in a deadly heat ray.
Originally found in Dark Sun Monstrous Compendium 1.
Firethorn (CR 3)
This squat, broad rose bush has thorny scarlet blooms that give off a scent which causes confusion. Confused victims may wander about and bump into the bush; touching the thorns causes fire damage and delivers a deadly poison.
Originally found in Corsairs of the Great Sea/Monstrous Compendium Annual #2.
Pilfer Vine
This largely harmless mass of tangled and twisted creeper vines steals shiny objects from passersby. It is semi-intelligent, and somewhat mobile, and can defend itself with its vines by constricting/strangling.
Originally found in Dragon #101.
Sword Grass
This plant has a number of huge blades of grass, which slice through anyone walking through the grass. It feeds on the blood and decaying bodies of victims.
Originally found in Dragon #167.
2. Garden of Unearthly Terrors v2.
Version #2 presents a sampling of creatures which can be used as guardians, though with some notable personal risk. These plants are immobile and not intelligent , but are all carnivorous and can easily turn on someone trying to make use of them.
Tri-Flower Frond (CR 1)
When a creature get close to this plant, it launches its tendrils; the first one puts the creature to sleep, the second dissolves the creature with acid, and the third sucks the liquefied fluids out of the victim.
Originally found in Monstrous Manual II.
Forester's Bane (CR 3)
This huge shrub has tough outer leaves that trap a creature, while its inner saw-like leaves chop the creature up like a blender.
Originally found in Monstrous Manual II.
Giant Venus Fly-Trap
This flytrap is large enough to swallow a human, with up to 8 sets of jaws. When it swallows a vitim, the victim drowns in the plant's water supply or dissolves from its acid.
Originally found in Dragon #89.
Mantrap (CR 2)
This relative of the venus flytrap attracts prey with its fascinating pollen. Fascinated creatures voluntarily climb into a leaf, which trapping the character inside and allows it to deal acid damage.
Originally found in Monstrous Manual II.
Twilight Bloom (CR 2)
This tall trunked-plant has no branches, though it has fern-like foliage on top. Bell-like purple flowers hidden on top of the plant drop deadly poisonous juice on victims; these decomposing remains feed the plant.
Originally found in Monstrous Manual II.
Alternates:
Bloodflower: bushes contain white flowers that become deep red as the plant feeds, narcotic perfume puts creatures to sleep, and leaves drain blood. Not intelligent, immobile. (Dragon #167)
Dew fronds: a quickly-growing plant with large barbed fronds that cut into a creature and stick in to drain blood. Not intelligent, immobile. (Dark Sun MC #1)
Pitcher Plant, Giant: giant green vase-shaped plant filled with water and acid grabs prey with a whiplike organ, dropping prey into the mixture, which either drowns or dissolves. not intelligent, immobile. (Dragon #89)
Sirenflower: large fungus has big leaves that close and crush on a creature while dissolving it. Foes are drawn in using scents and lights. Not intelligent, immobile. (Mystara)
Somnastis Plant: this large rosebud exudes an inhaled poison that puts creatures to sleep, allowing the plant to place the victim on its acidic flower bud. Not intelligent, immobile. (Mystara - Hollow World)
Thornslinger: this low-lying blossom-covered plant fire a volley of thorns at anything that moves. Anything touching the plant is caught by its powerful adhesive, and is also subject to its digestive acid. Not intelligent, immobile. (FR MC2/Monstrous Manual)
3. Carnivorous Hunter Plants
Unlike the "Garden" variety above, most of these creatures are mobile, and all are looking for a good meal or two. Anyone trying to use these creatures for his own end is likely to end up as one of those meals.
Hangman Tree (CR 5)
This carnivorous tree has sinewy, ropey noose-like vines that strangle prey, and lift a victim to drop it into the acid-filled barrel of the trunk.
Originally found in Monster Manual II.
Whipweed (CR 2)
This animal/plant hybrid is spherical with eight small legs, and two whip-like stalks that it attacks with. It can eat animals, but it derives part of its sustenance by rooting in the ground.
Originally found in Fiend Folio.
Vampire Rose (CR 3)
This mobile white rose bush has thorny stalks that drain blood from a creature. The bush emits a hypnotic fragrance to keep its prey still.
Originally found in D&D (Mystara) setting.
Archer Bush (CR 1)
This wild bramble bush shoots a spray of thorns as a ranged attack. It devours devours downed prey with its exterior mouth.
Originally found in D&D (Mystara) setting.
Black Willow (CR 8)
This intelligent, evil tree likes eat humanoids, and can attack with its whiplike branches. It can causes tired creatures to fall sleep, which the willow can pick up and put inside its body cavity to digest them quickly with acid.
Originally found in Monster Manual II.
Alternates:
Mandragora: vegetable monster that hungers for human flesh, vaguely shaped like a human, lurks in the dark but comes out to strangle prey with tentacles. (Monster Manual II)
Giant Sundew: Mound of tarry ropey hairs lashes out with sticky acidic tendrils which slow a character down and can block its nose and mouth to suffocate. Semi-intelligent, semi-mobile. (Monster Manual II)
Choke Creeper: Enormous 25-HD vine with numerous creepers that strangle and constrict. Not intelligent, slightly mobile (basically, the big daddy of the assassin vine). (Monster Manual II)
Jupiter Bloodsucker: Big leafy plant that can hold a victim with one leaf, smother it with another, and drain blood as it holds the victim. Not intelligent, somewhat mobile. (Mystara)
Singing Tree: Tree's lovely voice compels creatures to approach, drops leaves on creature which drains blood through tubes connected to tree. Barely intelligent, immobile. (Monstrous Compendium Annual 1)
Killer Tree: Ordinary-looking tree with branch-tentacles that can grab creatures to drag to its mouth to bite. Barely intelligent, immobile. (Mystara)
Sashalus: Mushroom-like fungus that fires poisonous spines (effect variable, including unconsciousness, confusion, or nausea) and eats downed or helpless prey by dissolving it with acid. Semi-intelligent, somewhat mobile. (Dragon #119)
Vampire Moss: Netlike mass of pale green moss hangs from trees, drawing life energy from nearby creatures (physical contact not necessary!); on a failed Fort save creature loses 1 hp per round. Not intelligent, immobile. (Monstrous Compendium Annual 1)