Plant 43

Chapter: 12 · Side Street and Blackthorn Station
Where: Seedlings: Train Carriage 3 (Blackthorn Station, B1) and the Subterranean Passage (B3). Boss: the ARK Service Entrance ledge, with two side tendrils and the central bud.

Weak point

Only the central bud when it's fully open. The seedlings when their mouths open; the tendril flowers when they unfurl.

A Connections plant B.O.W. ("Stubborn weed, aren't you?", says Leon). Before the boss, 9 seedlings along the train and the cave give the Not Today, Buds! trophy (500 CP): it's missable, because the cave is one-way. Writhing vines on the floor are grab traps: shoot them to lure the plant out.

The boss has less health than the Super Tyrant: the Requiem isn't needed, nor are grenades. SMG (the best per Game8 and PCGamesN) or shotgun, in short bursts.

Reward

The way into ARK. Not Today, Buds! (500 CP) if you destroyed all 9 seedlings first.

No published HP. Insanity: Evil Resource lists only 7 seedlings (the train + 6 in the cave): the northeast exit has 2 Raccoon City BSAA Zombies instead of seedlings #8 and #9. It's a single source and nobody says whether the trophy counts per run: earn it on Casual or Standard.

Step by step

Seedlings: they sway, then open up (vulnerable) and strike fast. A few quick rifle or pistol taps when the mouth opens, or a tight shotgun blast up close. A perfectly timed parry severs them instantly.
Seedling order: #1 in Train Carriage 3 (unmissable); on B3, #2 near the start (crouch tunnel on the right), #3 and #4 below a dangling car (close range, use the shotgun), #5 on the left side path past the blue tarp (the only one off the main path), #6 in the flooded area with tangled wrecks, #7 directly left of #6, and #8 and #9 at the northeast exit, towards the light.
ARK Service Entrance: first two large vines rise with flowers at their base. Shoot the flowers as soon as they open, before they slam. Then drop down to the boss ledge (2 crates on the east side, one with a Med Injector).
Central bud: stay in the middle and shoot it only when it's fully open.
Side tendrils: one rises and slams straight down onto its side, often both back to back. They can't be parried. Watch which one lifts and step sideways (not backwards, into the bite zone).
Bud lunge: it closes up and pulls back slightly, then launches and can reach anywhere in the arena. Sidestep.
Finish: after enough damage it thrashes, vomits and dislodges a truck or container of explosive canisters under or opposite it. Shoot the canisters immediately: the fight ends.