Interference (Semiosis Duology, 2) [Burke, Sue]

Over two hundred years after the first colonists landed on Pax, a new set of explorers arrives from Earth on what they claim is a temporary scientific mission.

The Earthlings think the humans are in charge of the settlement, dismiss the Glassmakers as “pets”, and fail to understand that the sentient plant, Stevland, is in charge. Even as Stevland attempts to protect his humans from the intrusion, a more insidious enemy than the Earthlings makes itself known.

The humans and the Glassmakers weren't the only colonists on Pax, and this new arrival has no desire for peace—only conquest.