How about we keep 'em Medium, but cluster the attacks like a medusa's "snakes" attack, and perhaps give it extended reach? Maybe increase Dex a tad and give it Combat Reflexes for more AoOs?
Whenever the snake grass suffers 5 points of damage, it could gain a second "snakes" attack?
Sounds good.
Modifying my earlier rough draft:
Snake Grass
Medium Plant
Hit Dice: 2d8+6 (15 hp)
Initiative: +0
Speed: 0 ft. (immobile)
Armor Class: 10, touch 10, flat-footed 10
Base Attack/Grapple: +1/+1
Attack: Snakes +1 melee (1 plus poison)
Full Attack: Snakes +1 melee (1 plus poison)
Space/Reach: 5 ft./10 ft.
Special Attacks: Poison, pruned growth
Special Defenses: All-around vision, darkvision 60 ft., low-light vision, plant traits, scent
Saves: Fort +6, Ref +0, Will -1
Abilities: Str 10, Dex 10, Con 16, Int --, Wis 8, Cha 3?
Feats: Combat Reflexes (B), Weapon Finesse (B) ?
Environment: ?
Organization: Solitary or bed (?)
Challenge Rating: x
Treasure: Incidental or none?
Alignment: Always neutral
Advancement: x
Level Adjustment: —
Pruned Growth (Ex): Whenever a patch of snake grass has taken 5 hp of damage, its damaged reeds sprout new snake heads, which increases the damage of its snakes attack by an additional die (maximum additional damage dice equals the HD of the patch of snake grass, so a standard patch can grow up to 4d4 damage). Extra heads gained from pruned growth wither and die within a day, returning the damage of the snakes attack to its original amount.