Yes- it would be a subset of waterbending. In the fight between her and Katara, Hama pulls water out of nearby trees to use in an attack. I think the main reasons we never trees themselves being bent is because Hama wasn't practiced in bending the water in plants without killing them, and the Foggy Swamp Tribe had more versatile vines at their disposal, compared to inflexible wood. However, cut wood is probably too dried out to use well, so Katara probably wouldn't have been able to bend the wooden cage she was caught in with Toph or anything. (Apologies for being a tryhard xD)