New Herbam spell

The Magus' Big Staff Re(Cr)He 30

R: Voice, D: Diam, T: Ind, Req: Cr
This spell creates a timber tree and makes it attack a target until the target is dead or the timber tree is destroyed. It has Init +0, hits automatically and causes +25 Damage. The spell have to overcome the magic resistance in order to hit the target. It is 1,5 foot in diameter and 4 paces long. Normal weapons are useless against a timber tree.

(Base 10, +2 Voice, +1 Diam, +1 Creo effect)

Is it OK?

Looks very clean to me. The only quibble is why does it stop attacking if the "wielder" (I assume this is the caster) is dead? I say let it keep beating the target until the duration runs out.

I used the text of the trecherous spear. Thanks!
I edited the description.