Why didn't Saruman kill the Ents himself?

[William Cloud Hicklin]

Why did Saruman decide to move all his force into Rohan having lost his control over king Theoden?

Wormtongue didn't actually reach Orthanc with the news until it was too late: Saruman's army had been crushed at the Hornburg the night before, and Isengard was a steaming, flooded wreck.

OTOH, an unpublished time-scheme indicates that Saruman was notified by his bird-spies of the riding of several eoreds from Edoras westward (which, one might presume, he assumed like Ceorl did to be led by Eomer or some other marshal) .... and it was precisely in response to this that that night he launched the second assault on the Fords and the invasion of western Rohan. It seems his motivation, as I read it, was to overwhelm Elfhelm and Grimbold before they could be reinforced, and to destroy Rohan's army in detail. In this case we would have in miniature the situation with Aragorn/Palantir/Sauron: a move by the Allies provokes a hastily fatal couintermove by the Enemies.