Rather than an attack, this seems like improved grab triggered by stumbling into it. I think I'd probably go with this as a hazard, and pretty similar to the last one.
Giant Butterwort (CR X)
A giant butterwort is a carnivorous plant that can be found growing in many environments, it favors waterlogged ground with acidic or alkaline soil. The plant is a rosette or long, flat leaves spread across a circle from 10 to 15 feet in diameter. The leaves blend into their surroundings, it requires a DC Y Spot check to notice a giant butterwort.
The leaves are coated in a powerful adhesive, any creature that steps on the leaves will find itself stuck fast unless it can make a DC 20 Strength check to break free. If a giant butterwort plant feels the pressure of a creature between Tiny and Medium-sized standing upon it, the plant contracts its leaves into a tight ball around its prey; the victim is entitled to attempt a Strength check to escape the adhesive before becoming trapped. A trapped victim is constricted for 1d4 bludgeoning damage plus 2 points of acid damage per round. Trapped victims can be freed by using light slashing or piercing weapons to sever the leaves (AC 13, hp 20) or by killing the plant by severing its stem (AC13, hp 40). Creatures trapped inside the plant's leaf-ball can not reach its stem.