Cherrypicked xenophage-rewrite mechanics over.

code/modules/hydroponics/seed.dm

@@ -22,6 +22,7 @@
 	var/trash_type                 // Garbage item produced when eaten.
 	var/splat_type = /obj/effect/decal/cleanable/fruit_smudge // Graffiti decal.
 	var/has_mob_product
+	var/force_layer
 
 /datum/seed/New()
 

code/modules/hydroponics/seed_datums.dm

@@ -1106,3 +1106,22 @@
 	set_trait(TRAIT_PRODUCTION,7)
 	set_trait(TRAIT_YIELD,3)
 	set_trait(TRAIT_POTENCY,3)
+
+// Alien weeds.
+/datum/seed/xenomorph
+	name = "xenomorph"
+	seed_name = "alien weed"
+	display_name = "alien weeds"
+	force_layer = 3
+
+/datum/seed/xenomorph/New()
+	..()
+	set_trait(TRAIT_PLANT_ICON,"vine2")
+	set_trait(TRAIT_IMMUTABLE,1)
+	set_trait(TRAIT_PRODUCT_COLOUR,"#080006")
+	set_trait(TRAIT_FLESH_COLOUR,"#080006")
+	set_trait(TRAIT_PLANT_COLOUR,"#080006")
+	set_trait(TRAIT_PRODUCTION,1)
+	set_trait(TRAIT_YIELD,-1)
+	set_trait(TRAIT_SPREAD,2)
+	set_trait(TRAIT_POTENCY,50)

code/modules/hydroponics/spreading/spreading.dm

@@ -188,12 +188,12 @@
 		icon_state = "[seed.get_trait(TRAIT_PLANT_ICON)]-[growth]"
 
 	if(growth>2 && growth == max_growth)
-		layer = 5
+		layer = (seed && seed.force_layer) ? seed.force_layer : 5
 		opacity = 1
 		if(!isnull(seed.chems["woodpulp"]))
 			density = 1
 	else
-		layer = 3
+		layer = (seed && seed.force_layer) ? seed.force_layer : 5
 		density = 0
 
 /obj/effect/plant/proc/calc_dir()