That all looks pretty much right to me. We could also steal the entangle ability from what we did for the Telaxian Vine:
		
		
	 
Yes, we can steal some stuff from that. I'd prefer to leave the relevant bits of an entangle spell's mechanics in the description, just to save the poor hardworking DM from having to look it up.
I am curious why that write-up gives a caster level, since as its range and duration are no longer level-dependent, and the ability can't be dispelled, there doesn't seem any use for one.
Revising my previous proposal I think I'll go for immediate action rather than free.
Entangling Roots (Ex): Using specialized  root-tendrils, a clubthorn can entangle creatures  within its reach as an immediate action. All creatures within the area must succeed at a DC Y Reflex save or be  entangled and held fast, those that save can still move at half speed  through the area, but the clubthorn can try to entangle them again on subsequent rounds. Creatures entangled by the roots can break free and  move half their normal speed by using a full-round action to make a DC Z  Strength check or a DC Z Escape Artist check. The effect lasts until the clubthorn dies or decides to end it (a free  action). The save DC is  Strength-based, with a +4 racial bonus on the Escape Artist check. 	
I can't put in DCs because we haven't started on the stats yet!
Speaking of which...
Shall we start it as a Medium-sized plant and work up?
My original proposal for the size stages was 1-2 HD for a sapling, but I feel we'd be better off starting at 2 HD:
I'm thinking something like...
Clubthorn, Sapling
Medium Plant
Hit Dice: 2d8+4 (13 hp)
Speed: 5 ft. (1 squares)
Armor Class: 15 (+1 Dex, +4 natural) touch 11, flat-footed 14
Base Attack/Grapple: +1/+2
Attack: Slam +3 melee (1d4+1)
Full Attack: 2 slams +3 melee (1d4+1)
Space/Reach: 5 ft./5 ft.
Special Attacks: Adhesive sap, entangling roots
Special Qualities: DR 3/slashing and piercing, low-light vision, plant traits
Saves: Fort +5, Ref +1, Will +0
Abilities: Str 12, Dex 13, Con 14, Int 1, Wis 10, Cha 3
Skills: 5 - all in Listen +?
Feats: 1 - Weapon Focus (slam)?
Environment: Warm swamps and forests
Organization: Solitary or pair
Challenge Rating: ?
Treasure: Standard
Alignment: Always neutral
Advancement: 3-5 HD (Large), 6-10 HD (Huge), 11-18 HD (Gargantuan)
Level Adjustment: —
Combat Stuff
Clubthorn, Young Tree
Large Plant
 Hit Dice: 3d8+12 (25 hp)
Speed: 5 ft. (1 squares)
 Armor Class: 15 (-1 size, +6 natural) touch 9, flat-footed 15
 Base Attack/Grapple: +2/+11
 Attack: Slam +7 melee (1d6+5)
 Full Attack: 4 slams +7 melee (1d6+5)
 Space/Reach: 10 ft./10 ft.
 Special Attacks: Adhesive sap, entangling roots
 Special Qualities: DR 5/slashing and piercing, low-light vision, plant  traits
 Saves: Fort +7, Ref +1, Will +1
 Abilities: Str 20, Dex 11, Con 18, Int 1, Wis 10, Cha 3
 Skills: 6 - all in Listen +?
 Feats: 2 - Power Attack, Weapon Focus (slam) ?
Challenge Rating: ?
 
Clubthorn, Mature Tree
 Huge Plant
  Hit Dice: 6d8+36 (63 hp)
 Speed: 5 ft. (1 squares)
  Armor Class: 16 (-2 size, -1 Dex, +9 natural) touch 7, flat-footed 16
  Base Attack/Grapple: +4/+21
  Attack: Slam +12 melee (1d8+9)
  Full Attack: 6 slams +12 melee (1d8+9)
  Space/Reach: 15 ft./15 ft.
  Special Attacks: Adhesive sap, entangling roots
  Special Qualities: DR 5/slashing and piercing, low-light vision, plant  traits
  Saves: Fort +11, Ref +3, Will +2
  Abilities: Str 28, Dex 9, Con 22, Int 1, Wis 10, Cha 3
  Skills: 9 - all in Listen +?
  Feats: 3 - Lightning Reflexes, Power Attack, Weapon Focus (slam) ?
Challenge Rating: ?
 
 
Clubthorn, Old Tree
  Gargantuan Plant
   Hit Dice: 11d8+36 (63 hp)
  Speed: 5 ft. (1 squares)
   Armor Class: 18 (-4 size, -1 Dex, +13 natural) touch 5, flat-footed 18
   Base Attack/Grapple: +8/+33
   Attack: Slam +18 melee (2d6+13)
   Full Attack: 8 slams +18 melee (2d6+13)
   Space/Reach: 20 ft./20 ft.
   Special Attacks: Adhesive sap, entangling roots
   Special Qualities: DR 5/slashing and piercing, low-light vision, plant  traits
   Saves: Fort +15, Ref +4, Will +3
   Abilities: Str 36, Dex 9, Con 26, Int 1, Wis 10, Cha 3
   Skills: 14 - all in Listen +?
   Feats: 4 - Lightning Reflexes, Power Attack, Weapon Focus (slam) plus one more?
Challenge Rating: ?