diff --git a/docs/briefs/captures/terrain_showcase_boulders.png b/docs/briefs/captures/terrain_showcase_boulders.png
index b9d3d92..1b15a61 100644
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diff --git a/docs/briefs/captures/terrain_showcase_canyon.png b/docs/briefs/captures/terrain_showcase_canyon.png
index d1d1b42..f97ca8c 100644
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diff --git a/docs/briefs/captures/terrain_showcase_crevasse.png b/docs/briefs/captures/terrain_showcase_crevasse.png
index 40b6d4c..ea47899 100644
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diff --git a/docs/briefs/captures/terrain_showcase_spurs.png b/docs/briefs/captures/terrain_showcase_spurs.png
index 89a4a28..a99580c 100644
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diff --git a/docs/briefs/captures/zone1_vigilant_plateau.png b/docs/briefs/captures/zone1_vigilant_plateau.png
index 6f2b594..82fc2f2 100644
Binary files a/docs/briefs/captures/zone1_vigilant_plateau.png and b/docs/briefs/captures/zone1_vigilant_plateau.png differ
diff --git a/docs/briefs/captures/zone1_vigilant_tombant.png b/docs/briefs/captures/zone1_vigilant_tombant.png
index fd1b0e7..8c9d3a6 100644
Binary files a/docs/briefs/captures/zone1_vigilant_tombant.png and b/docs/briefs/captures/zone1_vigilant_tombant.png differ
diff --git a/docs/briefs/terrain-site-zone1-gameplay.md b/docs/briefs/terrain-site-zone1-gameplay.md
index 5d58e57..d20889b 100644
--- a/docs/briefs/terrain-site-zone1-gameplay.md
+++ b/docs/briefs/terrain-site-zone1-gameplay.md
@@ -255,6 +255,17 @@ de contacts existant les sépare du fond.
   assemblés ensuite sur le thread principal.
 - Props low-poly posés à la main, en nombre borné : blocs d'éboulis au pied
   du tombant, têtes de corail sur les patch reefs, rien d'animé.
+- Flore et encroûtantes (2026-10-07, demande d'Olivier) : `SeabedFloraActor`
+  sème, depuis le champ de hauteur et une graine dérivée de l'identifiant du
+  site, des touffes d'herbier sur le sable plat de la bande éclairée (pleines
+  à 20 m, absentes à 36 m, en prairies par bruit) et des gorgones sur la roche
+  de 15 à 60 m, cap croisé sur la houle ; une `MultiMesh` par tuile et par
+  espèce, maillages et textures à découpe alpha procéduraux
+  (`SeabedFloraAssets`), ondulation en vertex shader, aucune collision ni
+  présence sonar. Le shader roche ajoute gazon algal sur les faces éclairées,
+  croûtes corallines par plaques et éponges encroûtantes à l'ombre, le tout
+  s'éteignant entre 40 et 75 m. L'optique de la colonne d'eau est partagée par
+  `water_light.gdshaderinc`.
 - Brouillard, lumière directionnelle et particules de la scène de test
   actuelle sont conservés et réglés par site.
 - Passe lumière du 2026-10-06 (`dive_site.tscn`) : l'ambiance vient d'un ciel
diff --git a/game/domain/sites/terrain_height_field.gd b/game/domain/sites/terrain_height_field.gd
index e20e5cb..4a2c139 100644
--- a/game/domain/sites/terrain_height_field.gd
+++ b/game/domain/sites/terrain_height_field.gd
@@ -64,6 +64,11 @@ func elevations_m() -> PackedFloat32Array:
 	return _data.elevations_m
 
 
+## Row-major sonar materials, same layout as elevations_m.
+func materials() -> PackedByteArray:
+	return _data.materials
+
+
 func elevation_at_cell(column: int, row: int) -> float:
 	return _data.elevations_m[_data.index_of(_clamp_column(column), _clamp_row(row))]
 
diff --git a/game/world/sites/dive_site.gd b/game/world/sites/dive_site.gd
index 2884b63..2343711 100644
--- a/game/world/sites/dive_site.gd
+++ b/game/world/sites/dive_site.gd
@@ -7,11 +7,13 @@ const SitePlacement = preload("res://game/domain/sites/site_placement.gd")
 const BathymetricTerrainActor = preload(
 	"res://game/world/sites/bathymetric_terrain_actor.gd"
 )
+const SeabedFloraActor = preload("res://game/world/sites/seabed_flora_actor.gd")
 
 const METAL_REFLECTIVITY := 1.0
 const BEACON_COLOR := Color(0.2, 0.78, 0.9)
 
 @onready var _terrain: BathymetricTerrainActor = %Terrain
+@onready var _flora: SeabedFloraActor = %Flora
 @onready var _placements_root: Node3D = %Placements
 
 var _definition: DiveSiteDefinition
@@ -29,6 +31,11 @@ func configure(definition: DiveSiteDefinition) -> String:
 	var terrain_error := _terrain.configure(field)
 	if not terrain_error.is_empty():
 		return terrain_error
+	if _flora != null:
+		# The flora is seeded by the site id, so a site always grows the same meadows.
+		var flora_error := _flora.configure(field, hash(String(definition.site_id)))
+		if not flora_error.is_empty():
+			return flora_error
 	_definition = definition
 	for child in _placements_root.get_children():
 		_placements_root.remove_child(child)
@@ -47,6 +54,10 @@ func terrain() -> BathymetricTerrainActor:
 	return _terrain
 
 
+func flora() -> SeabedFloraActor:
+	return _flora
+
+
 func spawn_transform() -> Transform3D:
 	return _definition.spawn_transform() if _definition != null else Transform3D.IDENTITY
 
diff --git a/game/world/sites/dive_site.tscn b/game/world/sites/dive_site.tscn
index 736222d..05bfb51 100644
--- a/game/world/sites/dive_site.tscn
+++ b/game/world/sites/dive_site.tscn
@@ -1,8 +1,9 @@
-[gd_scene load_steps=9 format=3]
+[gd_scene load_steps=10 format=3]
 
 [ext_resource type="Script" path="res://game/world/sites/dive_site.gd" id="1_site"]
 [ext_resource type="Script" path="res://game/world/sites/bathymetric_terrain_actor.gd" id="2_terrain"]
 [ext_resource type="Shader" path="res://game/world/sites/water_column_fog.gdshader" id="3_water_column"]
+[ext_resource type="Script" path="res://game/world/sites/seabed_flora_actor.gd" id="4_flora"]
 
 [sub_resource type="ShaderMaterial" id="WaterColumnMaterial"]
 shader = ExtResource("3_water_column")
@@ -96,5 +97,9 @@ material = SubResource("WaterColumnMaterial")
 unique_name_in_owner = true
 script = ExtResource("2_terrain")
 
+[node name="Flora" type="Node3D" parent="."]
+unique_name_in_owner = true
+script = ExtResource("4_flora")
+
 [node name="Placements" type="Node3D" parent="."]
 unique_name_in_owner = true
diff --git a/game/world/sites/seabed_flora.gdshader b/game/world/sites/seabed_flora.gdshader
new file mode 100644
index 0000000..10bc6c4
--- /dev/null
+++ b/game/world/sites/seabed_flora.gdshader
@@ -0,0 +1,60 @@
+shader_type spatial;
+render_mode cull_disabled, depth_draw_opaque;
+
+// Seagrass tufts and sea fans as alpha-cut billboards instanced by MultiMesh. The mesh
+// is unit sized (y from 0 to 1); the instance transform gives size and heading, the
+// instance COLOR the species tint, INSTANCE_CUSTOM.x the sway phase and
+// INSTANCE_CUSTOM.y the sway weight (grass bends, fans barely). Daylight follows the
+// same water column as the seabed; the submarine's projectors are not attenuated.
+
+#include "res://game/world/sites/water_light.gdshaderinc"
+
+uniform sampler2D albedo_map : source_color, filter_linear_mipmap;
+uniform vec3 water_absorption_per_m = vec3(0.11, 0.04, 0.032);
+uniform float reference_depth_m = 24.0;
+uniform float minimum_daylight = 0.05;
+uniform float sway_amplitude = 0.09;
+uniform float sway_speed = 0.8;
+uniform float alpha_threshold = 0.5;
+uniform float translucency = 0.7;
+
+varying vec3 world_position;
+varying float daylight;
+varying vec3 chroma;
+
+void vertex() {
+	float phase = INSTANCE_CUSTOM.x * TAU;
+	float weight = INSTANCE_CUSTOM.y * VERTEX.y * VERTEX.y;
+	float swing = sin(TIME * sway_speed + phase) + 0.35 * sin(TIME * sway_speed * 2.3 + phase * 1.7);
+	VERTEX.x += swing * sway_amplitude * weight;
+	VERTEX.z += cos(TIME * sway_speed * 0.7 + phase) * sway_amplitude * 0.5 * weight;
+	world_position = (MODEL_MATRIX * vec4(VERTEX, 1.0)).xyz;
+	vec3 transmittance = water_transmittance(-world_position.y, water_absorption_per_m, reference_depth_m);
+	daylight = water_daylight(transmittance, minimum_daylight);
+	chroma = water_chroma(transmittance, water_absorption_per_m, reference_depth_m);
+}
+
+void fragment() {
+	vec4 sample = texture(albedo_map, UV);
+	ALPHA = sample.a;
+	ALPHA_SCISSOR_THRESHOLD = alpha_threshold;
+	ALBEDO = sample.rgb * COLOR.rgb * chroma;
+	if (!FRONT_FACING) {
+		NORMAL = -NORMAL;
+	}
+	AO = daylight;
+	AO_LIGHT_AFFECT = 0.35;
+	ROUGHNESS = 0.85;
+	METALLIC = 0.0;
+}
+
+void light() {
+	float sun = water_sun_factor(LIGHT_IS_DIRECTIONAL, daylight);
+	// Thin blades and fan nets let light through and are lit from both faces: the
+	// lambert term is wrapped around the blade, so a vertical blade under an
+	// overhead sun is not black.
+	float facing = dot(NORMAL, LIGHT);
+	float lambert = clamp(facing, 0.0, 1.0);
+	float wrapped = clamp(abs(facing) * 0.5 + 0.5, 0.0, 1.0) * translucency;
+	DIFFUSE_LIGHT += (lambert + wrapped) * ATTENUATION * LIGHT_COLOR * ALBEDO * sun / PI;
+}
diff --git a/game/world/sites/seabed_flora_actor.gd b/game/world/sites/seabed_flora_actor.gd
new file mode 100644
index 0000000..38b2a0b
--- /dev/null
+++ b/game/world/sites/seabed_flora_actor.gd
@@ -0,0 +1,286 @@
+class_name SeabedFloraActor
+extends Node3D
+
+## Scatters the seabed flora of a site from its height field, deterministically from a
+## seed: seagrass tufts on gently sloping sand in the lit band, sea fans on rock. One
+## MultiMeshInstance3D per terrain tile and per kind, with a visibility range, no
+## collision and no sonar presence. Placement is pure and runs on the worker thread
+## pool; the multimeshes are built afterwards on the main thread.
+
+const TerrainHeightField = preload("res://game/domain/sites/terrain_height_field.gd")
+const SonarMaterial = preload("res://game/domain/sonar/sonar_material.gd")
+const SeabedFloraAssets = preload("res://game/world/sites/seabed_flora_assets.gd")
+const FLORA_SHADER = preload("res://game/world/sites/seabed_flora.gdshader")
+
+enum Kind { SEAGRASS, SEA_FAN }
+
+const TILE_CELLS := 64
+# Seagrass: meadows on flat sand, dense to 20 m, gone by 36 m (Caribbean Syringodium
+# and Halophila thin out with the light).
+const GRASS_SPACING_M := 2.0
+const GRASS_DEPTH_FULL_M := 20.0
+const GRASS_DEPTH_NONE_M := 36.0
+const GRASS_SLOPE_COSINE := 0.9659  # cos(15 deg)
+const GRASS_MEADOW_THRESHOLD := 0.12
+const GRASS_VISIBILITY_M := 110.0
+# Sea fans: on rock from 15 to 60 m, heading across the swell.
+const FAN_SPACING_M := 5.0
+const FAN_DEPTH_MIN_M := 15.0
+const FAN_DEPTH_MAX_M := 60.0
+const FAN_KEEP_PROBABILITY := 0.45
+const FAN_VISIBILITY_M := 170.0
+const FAN_HEADING := Vector2(0.8, 0.6)
+const FAN_COLORS: Array[Color] = [
+	Color(0.55, 0.35, 0.62), Color(0.85, 0.42, 0.2), Color(0.8, 0.7, 0.3), Color(0.6, 0.3, 0.45),
+]
+
+
+## Placements of one tile for one kind, computed off the main thread.
+class TilePlacements extends RefCounted:
+	var cells := Rect2i()
+	var transforms: Array[Transform3D] = []
+	var colors := PackedColorArray()
+	var custom := PackedColorArray()
+
+
+var _field: TerrainHeightField
+var _seed := 0
+var _assets: SeabedFloraAssets
+var _grass_material: ShaderMaterial
+var _fan_material: ShaderMaterial
+var _meadow_noise: FastNoiseLite
+var _fan_noise: FastNoiseLite
+var _counts := {Kind.SEAGRASS: 0, Kind.SEA_FAN: 0}
+var _samples := {Kind.SEAGRASS: [], Kind.SEA_FAN: []}
+
+
+func configure(field: TerrainHeightField, seed: int) -> String:
+	if field == null:
+		return "flora field is missing"
+	_clear()
+	_field = field
+	_seed = seed
+	if _assets == null:
+		_assets = SeabedFloraAssets.build(seed)
+	_grass_material = _material(_assets.blade_texture)
+	_fan_material = _material(_assets.fan_texture)
+	_meadow_noise = _noise(seed + 11, 0.012)
+	_fan_noise = _noise(seed + 23, 0.03)
+	var tiles := _tiles()
+	var grass: Array[TilePlacements] = []
+	var fans: Array[TilePlacements] = []
+	for tile in tiles:
+		var grass_tile := TilePlacements.new()
+		grass_tile.cells = tile
+		grass.append(grass_tile)
+		var fan_tile := TilePlacements.new()
+		fan_tile.cells = tile
+		fans.append(fan_tile)
+	var group := WorkerThreadPool.add_group_task(
+		_place_tile.bind(grass, fans), tiles.size(), -1, true, "seabed flora"
+	)
+	WorkerThreadPool.wait_for_group_task_completion(group)
+	for index in tiles.size():
+		_add_multimesh(Kind.SEAGRASS, grass[index])
+		_add_multimesh(Kind.SEA_FAN, fans[index])
+	return ""
+
+
+func instance_count(kind: Kind) -> int:
+	return int(_counts[kind])
+
+
+## World transforms of the first few instances of a kind, for inspection and tests.
+func sample_transforms(kind: Kind) -> Array[Transform3D]:
+	var result: Array[Transform3D] = []
+	for transform in _samples[kind]:
+		result.append(transform)
+	return result
+
+
+func _clear() -> void:
+	for child in get_children():
+		remove_child(child)
+		child.free()
+	_counts = {Kind.SEAGRASS: 0, Kind.SEA_FAN: 0}
+	_samples = {Kind.SEAGRASS: [], Kind.SEA_FAN: []}
+
+
+func _tiles() -> Array[Rect2i]:
+	var tiles: Array[Rect2i] = []
+	var last_column := _field.columns() - 1
+	var last_row := _field.rows() - 1
+	var start_row := 0
+	while start_row < last_row:
+		var end_row := mini(start_row + TILE_CELLS, last_row)
+		var start_column := 0
+		while start_column < last_column:
+			var end_column := mini(start_column + TILE_CELLS, last_column)
+			tiles.append(Rect2i(start_column, start_row, end_column - start_column, end_row - start_row))
+			start_column = end_column
+		start_row = end_row
+	return tiles
+
+
+func _place_tile(tile_index: int, grass: Array[TilePlacements], fans: Array[TilePlacements]) -> void:
+	var tile := grass[tile_index].cells
+	var cell := _field.cell_size_m()
+	var origin := _field.origin_m()
+	var x_min := origin.x + float(tile.position.x) * cell
+	var z_min := origin.y + float(tile.position.y) * cell
+	var x_max := origin.x + float(tile.end.x) * cell
+	var z_max := origin.y + float(tile.end.y) * cell
+	var generator := RandomNumberGenerator.new()
+	generator.seed = _seed * 7919 + tile_index
+	_place_grass(grass[tile_index], generator, x_min, x_max, z_min, z_max)
+	_place_fans(fans[tile_index], generator, x_min, x_max, z_min, z_max)
+
+
+## Cheap rejections first, straight from the field arrays (cell reads, one noise read),
+## random draws only for accepted candidates: the draw sequence stays deterministic
+## because every decision is.
+func _place_grass(out: TilePlacements, generator: RandomNumberGenerator, x_min: float, x_max: float, z_min: float, z_max: float) -> void:
+	var origin := _field.origin_m()
+	var cell := _field.cell_size_m()
+	var columns := _field.columns()
+	var rows := _field.rows()
+	var elevations := _field.elevations_m()
+	var materials := _field.materials()
+	var sand := SonarMaterial.Value.SAND
+	var slope_limit := (1.0 / (GRASS_SLOPE_COSINE * GRASS_SLOPE_COSINE) - 1.0) * (2.0 * cell) * (2.0 * cell)
+	var z := z_min
+	while z < z_max:
+		var x := x_min
+		while x < x_max:
+			var px := x + generator.randf() * GRASS_SPACING_M
+			var pz := z + generator.randf() * GRASS_SPACING_M
+			x += GRASS_SPACING_M
+			var column := int(roundf((px - origin.x) / cell))
+			var row := int(roundf((pz - origin.y) / cell))
+			if px >= x_max or pz >= z_max or column < 1 or row < 1 or column >= columns - 1 or row >= rows - 1:
+				continue
+			var index := row * columns + column
+			var depth := -elevations[index]
+			if depth >= GRASS_DEPTH_NONE_M or materials[index] != sand:
+				continue
+			var meadow := _meadow_noise.get_noise_2d(px, pz)
+			if meadow < GRASS_MEADOW_THRESHOLD:
+				continue
+			# Slope by central differences: (dz_dx^2 + dz_dz^2) against tan^2 of the limit.
+			var rise_x := elevations[index + 1] - elevations[index - 1]
+			var rise_z := elevations[index + columns] - elevations[index - columns]
+			if rise_x * rise_x + rise_z * rise_z > slope_limit:
+				continue
+			var density := 1.0 - smoothstep(GRASS_DEPTH_FULL_M, GRASS_DEPTH_NONE_M, depth)
+			if generator.randf() > density * clampf((meadow - GRASS_MEADOW_THRESHOLD) * 4.0, 0.3, 1.0):
+				continue
+			var yaw := generator.randf() * TAU
+			var size := generator.randf_range(0.6, 1.1)
+			var phase := generator.randf()
+			var tint := generator.randf_range(0.85, 1.15)
+			var elevation := _field.elevation_at(px, pz)
+			var basis := Basis(Vector3.UP, yaw).scaled(Vector3(size, size * 0.9, size))
+			out.transforms.append(Transform3D(basis, Vector3(px, elevation - 0.03, pz)))
+			out.colors.append(Color(0.9 * tint, 1.0 * tint, 0.85 * tint))
+			out.custom.append(Color(phase, 1.0, 0.0, 0.0))
+		z += GRASS_SPACING_M
+
+
+func _place_fans(out: TilePlacements, generator: RandomNumberGenerator, x_min: float, x_max: float, z_min: float, z_max: float) -> void:
+	var origin := _field.origin_m()
+	var cell := _field.cell_size_m()
+	var columns := _field.columns()
+	var rows := _field.rows()
+	var elevations := _field.elevations_m()
+	var materials := _field.materials()
+	var rock := SonarMaterial.Value.ROCK
+	var heading := atan2(-FAN_HEADING.y, FAN_HEADING.x)
+	var z := z_min
+	while z < z_max:
+		var x := x_min
+		while x < x_max:
+			var px := x + generator.randf() * FAN_SPACING_M
+			var pz := z + generator.randf() * FAN_SPACING_M
+			x += FAN_SPACING_M
+			var column := int(roundf((px - origin.x) / cell))
+			var row := int(roundf((pz - origin.y) / cell))
+			if px >= x_max or pz >= z_max or column < 1 or row < 1 or column >= columns - 1 or row >= rows - 1:
+				continue
+			var index := row * columns + column
+			if materials[index] != rock:
+				continue
+			var depth := -elevations[index]
+			if depth < FAN_DEPTH_MIN_M or depth > FAN_DEPTH_MAX_M:
+				continue
+			if generator.randf() > FAN_KEEP_PROBABILITY or _fan_noise.get_noise_2d(px, pz) < -0.1:
+				continue
+			var yaw := heading + generator.randf_range(-0.45, 0.45)
+			var width := generator.randf_range(1.0, 2.4)
+			var height := generator.randf_range(0.9, 2.0)
+			var phase := generator.randf()
+			var color: Color = FAN_COLORS[generator.randi_range(0, FAN_COLORS.size() - 1)]
+			var elevation := _field.elevation_at(px, pz)
+			var normal := _field.normal_at_cell(column, row)
+			# Stand up, leaning a little with the rock face.
+			var up := (Vector3.UP * 0.6 + normal * 0.4).normalized()
+			var forward := Vector3(cos(yaw), 0.0, -sin(yaw))
+			var right := forward.cross(up).normalized()
+			forward = up.cross(right).normalized()
+			var basis := Basis(right * width, up * height, forward)
+			out.transforms.append(Transform3D(basis, Vector3(px, elevation - 0.05, pz)))
+			out.colors.append(color)
+			out.custom.append(Color(phase, 0.12, 0.0, 0.0))
+		z += FAN_SPACING_M
+
+
+func _add_multimesh(kind: Kind, placements: TilePlacements) -> void:
+	var count := placements.transforms.size()
+	if count == 0:
+		return
+	var multimesh := MultiMesh.new()
+	multimesh.transform_format = MultiMesh.TRANSFORM_3D
+	multimesh.use_colors = true
+	multimesh.use_custom_data = true
+	multimesh.mesh = _assets.tuft_mesh if kind == Kind.SEAGRASS else _assets.fan_mesh
+	multimesh.instance_count = count
+	for index in count:
+		multimesh.set_instance_transform(index, placements.transforms[index])
+		multimesh.set_instance_color(index, placements.colors[index])
+		multimesh.set_instance_custom_data(index, placements.custom[index])
+	var instance := MultiMeshInstance3D.new()
+	instance.name = "%s_%d_%d" % [
+		"Seagrass" if kind == Kind.SEAGRASS else "SeaFans",
+		placements.cells.position.x / TILE_CELLS,
+		placements.cells.position.y / TILE_CELLS,
+	]
+	instance.multimesh = multimesh
+	instance.material_override = _grass_material if kind == Kind.SEAGRASS else _fan_material
+	instance.visibility_range_end = GRASS_VISIBILITY_M if kind == Kind.SEAGRASS else FAN_VISIBILITY_M
+	instance.visibility_range_end_margin = 20.0
+	instance.visibility_range_fade_mode = GeometryInstance3D.VISIBILITY_RANGE_FADE_SELF
+	instance.cast_shadow = (
+		GeometryInstance3D.SHADOW_CASTING_SETTING_OFF if kind == Kind.SEAGRASS
+		else GeometryInstance3D.SHADOW_CASTING_SETTING_ON
+	)
+	add_child(instance)
+	_counts[kind] = int(_counts[kind]) + count
+	var samples: Array = _samples[kind]
+	for index in mini(count, 4):
+		if samples.size() < 16:
+			samples.append(placements.transforms[index])
+
+
+func _material(texture: ImageTexture) -> ShaderMaterial:
+	var material := ShaderMaterial.new()
+	material.shader = FLORA_SHADER
+	material.set_shader_parameter(&"albedo_map", texture)
+	return material
+
+
+static func _noise(seed: int, frequency: float) -> FastNoiseLite:
+	var noise := FastNoiseLite.new()
+	noise.noise_type = FastNoiseLite.TYPE_SIMPLEX_SMOOTH
+	noise.seed = seed
+	noise.frequency = frequency
+	noise.fractal_octaves = 3
+	return noise
diff --git a/game/world/sites/seabed_flora_assets.gd b/game/world/sites/seabed_flora_assets.gd
new file mode 100644
index 0000000..8f01a52
--- /dev/null
+++ b/game/world/sites/seabed_flora_assets.gd
@@ -0,0 +1,122 @@
+class_name SeabedFloraAssets
+extends RefCounted
+
+## Procedural meshes and alpha-cut textures for the seabed flora, built synchronously
+## from a seed so no asset is needed and every build is identical: a seagrass tuft
+## (three crossed unit quads carrying a strap-blade texture) and a sea fan (one unit
+## quad carrying a branching net texture). Meshes are unit sized, y from 0 to 1.
+
+const BLADE_TEXTURE_WIDTH := 128
+const BLADE_TEXTURE_HEIGHT := 256
+const BLADE_COUNT := 5
+const FAN_TEXTURE_SIZE := 256
+
+var tuft_mesh: ArrayMesh
+var fan_mesh: ArrayMesh
+var blade_texture: ImageTexture
+var fan_texture: ImageTexture
+
+
+static func build(seed: int) -> SeabedFloraAssets:
+	var result := SeabedFloraAssets.new()
+	result.blade_texture = _texture_from(_blade_image(seed))
+	result.fan_texture = _texture_from(_fan_image(seed + 1))
+	result.tuft_mesh = _crossed_quads(3)
+	result.fan_mesh = _crossed_quads(1)
+	return result
+
+
+## Vertical unit quads crossed around the y axis, each 1 wide and 1 tall, UV mapped
+## so the texture's bottom row sits on the ground.
+static func _crossed_quads(count: int) -> ArrayMesh:
+	var vertices := PackedVector3Array()
+	var normals := PackedVector3Array()
+	var uvs := PackedVector2Array()
+	var indices := PackedInt32Array()
+	for quad in count:
+		var angle := PI * float(quad) / float(count)
+		var right := Vector3(cos(angle), 0.0, sin(angle)) * 0.5
+		var normal := Vector3(-sin(angle), 0.0, cos(angle))
+		var base := vertices.size()
+		vertices.append_array([-right, right, right + Vector3.UP, -right + Vector3.UP])
+		for _corner in 4:
+			normals.append(normal)
+		uvs.append_array([Vector2(0.0, 1.0), Vector2(1.0, 1.0), Vector2(1.0, 0.0), Vector2(0.0, 0.0)])
+		indices.append_array([base, base + 2, base + 1, base, base + 3, base + 2])
+	var arrays: Array = []
+	arrays.resize(Mesh.ARRAY_MAX)
+	arrays[Mesh.ARRAY_VERTEX] = vertices
+	arrays[Mesh.ARRAY_NORMAL] = normals
+	arrays[Mesh.ARRAY_TEX_UV] = uvs
+	arrays[Mesh.ARRAY_INDEX] = indices
+	var mesh := ArrayMesh.new()
+	mesh.add_surface_from_arrays(Mesh.PRIMITIVE_TRIANGLES, arrays)
+	return mesh
+
+
+## A few strap-shaped blades rising from the bottom edge, each with its own curve,
+## width and length, darker towards the edges and the base. Alpha outside the blades.
+static func _blade_image(seed: int) -> Image:
+	var generator := RandomNumberGenerator.new()
+	generator.seed = seed
+	var image := Image.create(BLADE_TEXTURE_WIDTH, BLADE_TEXTURE_HEIGHT, false, Image.FORMAT_RGBA8)
+	image.fill(Color(0.0, 0.0, 0.0, 0.0))
+	for blade in BLADE_COUNT:
+		var root_x := (float(blade) + 0.5) / float(BLADE_COUNT) * float(BLADE_TEXTURE_WIDTH)
+		var bend := generator.randf_range(-22.0, 22.0)
+		var curve := generator.randf_range(0.6, 1.4)
+		var length := generator.randf_range(0.72, 1.0)
+		var half_width := generator.randf_range(3.5, 6.0)
+		var hue_shift := generator.randf_range(-0.05, 0.05)
+		for y in BLADE_TEXTURE_HEIGHT:
+			var t := 1.0 - float(y) / float(BLADE_TEXTURE_HEIGHT - 1)  # 0 at the base, 1 at the tip
+			if t > length:
+				continue
+			var centre := root_x + bend * pow(t, curve)
+			var width := half_width * (1.0 - 0.6 * t * t)
+			for x in range(int(floorf(centre - width)), int(ceilf(centre + width)) + 1):
+				if x < 0 or x >= BLADE_TEXTURE_WIDTH:
+					continue
+				var edge := absf(float(x) - centre) / width
+				if edge > 1.0:
+					continue
+				var shade := (0.7 + 0.3 * (1.0 - edge * edge)) * (0.75 + 0.25 * t)
+				var color := Color(0.46 + hue_shift, 0.66 + hue_shift * 0.5, 0.3, 1.0) * shade
+				color.a = 1.0
+				image.set_pixel(x, y, color)
+	return image
+
+
+## A sea fan: branching net inside a half-ellipse rising from a short stem. Ridged
+## noise above a threshold gives the branches, denser near the stem.
+static func _fan_image(seed: int) -> Image:
+	var noise := FastNoiseLite.new()
+	noise.noise_type = FastNoiseLite.TYPE_SIMPLEX_SMOOTH
+	noise.seed = seed
+	noise.frequency = 0.045
+	noise.fractal_type = FastNoiseLite.FRACTAL_RIDGED
+	noise.fractal_octaves = 3
+	var image := Image.create(FAN_TEXTURE_SIZE, FAN_TEXTURE_SIZE, false, Image.FORMAT_RGBA8)
+	image.fill(Color(0.0, 0.0, 0.0, 0.0))
+	var centre_x := float(FAN_TEXTURE_SIZE) * 0.5
+	for y in FAN_TEXTURE_SIZE:
+		var height := 1.0 - float(y) / float(FAN_TEXTURE_SIZE - 1)  # 0 at the base
+		for x in FAN_TEXTURE_SIZE:
+			var dx := (float(x) - centre_x) / (float(FAN_TEXTURE_SIZE) * 0.48)
+			var dy := (height - 0.1) / 0.9
+			var inside := dx * dx + dy * dy
+			var stem := absf(dx) < 0.03 and height < 0.25
+			if inside > 1.0 and not stem:
+				continue
+			var ridge := noise.get_noise_2d(float(x), float(y)) * 0.5 + 0.5
+			var density_threshold := 0.5 + 0.22 * inside
+			if not stem and ridge < density_threshold:
+				continue
+			var shade := 0.7 + 0.3 * ridge
+			image.set_pixel(x, y, Color(shade, shade * 0.92, shade * 0.85, 1.0))
+	return image
+
+
+static func _texture_from(image: Image) -> ImageTexture:
+	image.generate_mipmaps()
+	return ImageTexture.create_from_image(image)
diff --git a/game/world/sites/seabed_terrain.gdshader b/game/world/sites/seabed_terrain.gdshader
index 69ddf31..4e8628f 100644
--- a/game/world/sites/seabed_terrain.gdshader
+++ b/game/world/sites/seabed_terrain.gdshader
@@ -15,8 +15,20 @@ render_mode depth_draw_opaque;
 // projectors are not attenuated, so they stay useful on the drop-off. Everything is
 // normalised at reference_depth_m, where the look is the authored palette.
 
+#include "res://game/world/sites/water_light.gdshaderinc"
+
 uniform vec3 sand_color : source_color = vec3(0.56, 0.52, 0.42);
 uniform vec3 rock_color : source_color = vec3(0.22, 0.21, 0.19);
+// Encrusting growth on rock: turf algae on sunlit faces, coralline crusts in patches,
+// encrusting sponges in the shade. Patches come from the grain map read at large
+// scales; all of it thins out with depth as the light goes.
+uniform vec3 turf_color : source_color = vec3(0.3, 0.38, 0.2);
+uniform vec3 coralline_color : source_color = vec3(0.6, 0.4, 0.46);
+uniform vec3 sponge_color : source_color = vec3(0.82, 0.4, 0.18);
+uniform float growth_tile_m = 4.2;
+uniform float growth_strength = 1.0;
+uniform float growth_fade_start_m = 40.0;
+uniform float growth_fade_end_m = 75.0;
 uniform float sand_slope_cosine = 0.9397; // cos(20 deg)
 uniform float rock_slope_cosine = 0.7660; // cos(40 deg)
 // Per-channel absorption of clear tropical water, per metre, scaled for readability.
@@ -123,19 +135,29 @@ void fragment() {
 	// Albedo: mottled rock, lightly grained sand.
 	vec3 rock_albedo = rock_color * (1.0 + (grain - 0.5) * 2.0 * grain_albedo_contrast);
 	vec3 sand_albedo = sand_color * (1.0 + (grain - 0.5) * 0.3);
+	float depth_m = -world_position.y;
+
+	// Encrusting growth on the rock.
+	float growth_light = (1.0 - smoothstep(growth_fade_start_m, growth_fade_end_m, depth_m)) * growth_strength;
+	float sunlit = clamp(n.y * 0.5 + 0.5, 0.0, 1.0);
+	float patch_a = triplanar_grain(rock_grain_map, world_position / growth_tile_m + vec3(0.21, 0.73, 0.37), n);
+	float patch_b = triplanar_grain(rock_grain_map, world_position / (growth_tile_m * 0.53) + vec3(0.61, 0.11, 0.9), n);
+	float turf_mask = smoothstep(0.4, 0.58, patch_a) * sunlit * growth_light;
+	float coralline_mask = smoothstep(0.6, 0.74, patch_b) * growth_light;
+	float sponge_mask = smoothstep(0.62, 0.74, patch_a * 0.5 + patch_b * 0.5) * (1.0 - 0.6 * sunlit) * growth_light;
+	float grain_mottle = 1.0 + (grain - 0.5) * 0.6;
+	rock_albedo = mix(rock_albedo, turf_color * grain_mottle, turf_mask);
+	rock_albedo = mix(rock_albedo, coralline_color * grain_mottle, coralline_mask);
+	rock_albedo = mix(rock_albedo, sponge_color * grain_mottle, sponge_mask);
+	float rock_roughness = 0.78 + 0.12 * grain;
+	rock_roughness = mix(rock_roughness, 0.6, coralline_mask);
+	rock_roughness = mix(rock_roughness, 0.9, sponge_mask);
 	vec3 albedo = mix(sand_albedo, rock_albedo, rockness);
 
 	// Water column: spectral transmittance relative to the reference depth.
-	float depth_m = -world_position.y;
-	vec3 transmittance = exp(-water_absorption_per_m * (depth_m - reference_depth_m));
-	float brightness = dot(transmittance, vec3(0.2126, 0.7152, 0.0722));
-	vec3 chroma = transmittance / max(brightness, 0.001);
-	daylight = max(brightness, minimum_daylight);
-	// Normalise so the authored palette holds at the reference depth once the
-	// reference transmittance (red already filtered by 24 m of water) is applied.
-	vec3 reference_chroma = exp(-water_absorption_per_m * reference_depth_m);
-	reference_chroma /= dot(reference_chroma, vec3(0.2126, 0.7152, 0.0722));
-	ALBEDO = albedo * chroma * reference_chroma;
+	vec3 transmittance = water_transmittance(depth_m, water_absorption_per_m, reference_depth_m);
+	daylight = water_daylight(transmittance, minimum_daylight);
+	ALBEDO = albedo * water_chroma(transmittance, water_absorption_per_m, reference_depth_m);
 
 	// Detail shading: pits of the grain map and ripple troughs read as occlusion;
 	// the daylight factor dims the sky ambient with depth.
@@ -143,14 +165,14 @@ void fragment() {
 	float sand_ao = mix(1.0, 0.8 + 0.2 * clamp(sand_tangent.z, 0.0, 1.0), grain_ao_strength * 0.6);
 	AO = mix(sand_ao, rock_ao, rockness) * daylight;
 	AO_LIGHT_AFFECT = 0.35;
-	ROUGHNESS = mix(0.95, 0.78 + 0.12 * grain, rockness);
+	ROUGHNESS = mix(0.95, rock_roughness, rockness);
 	METALLIC = 0.0;
 }
 
 // Lambert diffuse; the sun (directional) is dimmed by the water column, other lights
 // (submarine projectors) are not. Sand and rock are rough enough to skip specular.
 void light() {
-	float sun_factor = LIGHT_IS_DIRECTIONAL ? daylight : 1.0;
+	float sun_factor = water_sun_factor(LIGHT_IS_DIRECTIONAL, daylight);
 	float lambert = clamp(dot(NORMAL, LIGHT), 0.0, 1.0);
 	DIFFUSE_LIGHT += lambert * ATTENUATION * LIGHT_COLOR * ALBEDO * sun_factor / PI;
 }
diff --git a/game/world/sites/water_light.gdshaderinc b/game/world/sites/water_light.gdshaderinc
new file mode 100644
index 0000000..c715a75
--- /dev/null
+++ b/game/world/sites/water_light.gdshaderinc
@@ -0,0 +1,27 @@
+// Shared water-column optics for the seabed and flora shaders: per-channel
+// Beer-Lambert transmittance relative to a reference depth, split into a scalar
+// daylight factor (dims the sun and the sky ambient) and a chroma (tints albedo).
+// Artificial lights are never attenuated: see water_sun_factor.
+
+const vec3 WATER_LUMINANCE_WEIGHTS = vec3(0.2126, 0.7152, 0.0722);
+
+vec3 water_transmittance(float depth_m, vec3 absorption_per_m, float reference_depth_m) {
+	return exp(-absorption_per_m * (depth_m - reference_depth_m));
+}
+
+float water_daylight(vec3 transmittance, float minimum_daylight) {
+	return max(dot(transmittance, WATER_LUMINANCE_WEIGHTS), minimum_daylight);
+}
+
+// Chroma of the transmittance, normalised so the authored palette holds at the
+// reference depth once the reference transmittance (red already filtered) is applied.
+vec3 water_chroma(vec3 transmittance, vec3 absorption_per_m, float reference_depth_m) {
+	float brightness = max(dot(transmittance, WATER_LUMINANCE_WEIGHTS), 0.001);
+	vec3 reference = exp(-absorption_per_m * reference_depth_m);
+	reference /= dot(reference, WATER_LUMINANCE_WEIGHTS);
+	return transmittance / brightness * reference;
+}
+
+float water_sun_factor(bool light_is_directional, float daylight) {
+	return light_is_directional ? daylight : 1.0;
+}
diff --git a/tests/integration/run_site_terrain_integration.gd b/tests/integration/run_site_terrain_integration.gd
index e653a95..8f3ce07 100644
--- a/tests/integration/run_site_terrain_integration.gd
+++ b/tests/integration/run_site_terrain_integration.gd
@@ -10,6 +10,7 @@ const DiveSiteDefinition = preload("res://game/domain/sites/dive_site_definition
 const SitePlacement = preload("res://game/domain/sites/site_placement.gd")
 const DiveSite = preload("res://game/world/sites/dive_site.gd")
 const SeabedDetailTextures = preload("res://game/world/sites/seabed_detail_textures.gd")
+const SeabedFloraActor = preload("res://game/world/sites/seabed_flora_actor.gd")
 const SonarCaptureRequest = preload("res://game/application/sonar/sonar_capture_request.gd")
 const ReferenceBathymetryAcquisition = preload(
 	"res://game/world/sonar/reference_bathymetry_acquisition.gd"
@@ -19,7 +20,7 @@ const DIVE_SITE_SCENE = preload("res://game/world/sites/dive_site.tscn")
 const ZONE1_SITE_PATH := "res://content/sites/zone1_vigilant.tres"
 const SHOWCASE_SITE_PATH := "res://content/sites/terrain_showcase.tres"
 const GROOVE_PROBE_OFFSET_M := 25.0
-const TEST_COUNT := 44
+const TEST_COUNT := 49
 
 var _failures: Array[String] = []
 
@@ -188,6 +189,27 @@ func _test_dive_site_scene() -> void:
 	await physics_frame
 	await physics_frame
 	_expect(site.terrain().tile_count() > 0, "site builds its terrain tiles")
+	var flora := site.flora()
+	var grass_count := flora.instance_count(SeabedFloraActor.Kind.SEAGRASS)
+	var fan_count := flora.instance_count(SeabedFloraActor.Kind.SEA_FAN)
+	_expect(grass_count > 0 and fan_count > 0, "calibration site grows seagrass and sea fans (%d, %d)" % [grass_count, fan_count])
+	var field_for_flora := definition.height_field()
+	var grass_on_sand := true
+	var grass_in_band := true
+	for transform in flora.sample_transforms(SeabedFloraActor.Kind.SEAGRASS):
+		var at := transform.origin
+		grass_on_sand = grass_on_sand and field_for_flora.material_at(at.x, at.z) == SonarMaterial.Value.SAND
+		grass_in_band = grass_in_band and -at.y < SeabedFloraActor.GRASS_DEPTH_NONE_M and absf(at.y - field_for_flora.elevation_at(at.x, at.z)) < 0.1
+	_expect(grass_on_sand, "seagrass stands on sand")
+	_expect(grass_in_band, "seagrass stays in the lit band and on the seabed")
+	var fans_on_rock := true
+	for transform in flora.sample_transforms(SeabedFloraActor.Kind.SEA_FAN):
+		var at := transform.origin
+		fans_on_rock = fans_on_rock and field_for_flora.material_at(at.x, at.z) == SonarMaterial.Value.ROCK
+	_expect(fans_on_rock, "sea fans stand on rock")
+	var first_pass := flora.sample_transforms(SeabedFloraActor.Kind.SEAGRASS)
+	flora.configure(field_for_flora, hash(String(definition.site_id)))
+	_expect(first_pass == flora.sample_transforms(SeabedFloraActor.Kind.SEAGRASS), "flora placement is deterministic for a seed")
 	var water_column := site.get_node_or_null("WaterColumn") as FogVolume
 	_expect(
 		water_column != null and water_column.material is ShaderMaterial