The forest was dry, and most of it devastated prior to Bane's arrival:

It would be several more weeks before the rains abated and gave way to the sweltering heat of Ruusan's long summer. Until then Kaan's followers would continue to suffer from the damp and cold.
...
The tide of battle was ever changing on Ruusan, and with the end of the rainy season the pendulum had swung back in favor of the Sith.

---

Three great fleet battles had taken place high above the world in the past year, scattering debris from the losing side across the sparsely populated world each time. Scorched and twisted hulls that had once been ships had crashed into the lush forests, igniting wildfires that had reduced much of the small world's surface to ash and barren soil.
...
Kopecz shook his head as he studied the battle plan Kaan had laid out on a makeshift table in the middle of his tent. The holomap of Ruusan's terrain showed the positions of the Sith forces as glowing red triangles floating above the map. The Jedi positions were represented by green squares. Despite this high-tech advancement, the rest of the map was a simple two-dimensional representation of the surrounding area's topography. It did nothing to convey the grim devastation that had left Ruusan a virtual wasteland, ravaged by war.

- Darth Bane: Path of Destruction


Vegetation remained and it was only directed at the forest(s) Bane overlooked from plateau:

Kaan, Githany, and the rest of the Dark Lords had gathered atop a barren plateau overlooking the vast forests where Hoth and his army were hiding. They had come on their fliers: short-range, single-person, airborne vehicles front-mounted with heavy blaster guns. The fliers were parked at the edge of the plateau, fifty meters away from where the Sith sat in a loose circle. The ritual had begun.

---

""The war against the Jedi is almost won," he declared. "They are on the verge of collapse. They have retreated into the forests, but they are running out of places to hide.""

"Our spies indicate Hoth's main camp is located here," he said, jabbing a finger at a heavily wooded section of the map. "If we can flush them out of the forest we might be able to-"

"As I was saying, the Jedi are hiding in the forests. We can flush them out if we split our numbers. If we deploy our fliers, we can flank their southern lines-"

"Now look at this map and think like a Sith. Don't just fight in the forest ... destroy the forest!"

"But victory here is not assured. We may have Hoth's army surrounded and pinned down, but there's a Jedi fleet with hundreds of reinforcements lurking on the edges of this system."

"Your plan worked," Kaan replied curtly. "The forest is destroyed, the Jedi have fled to open ground. They are exposed, vulnerable. Now we go to finish them off."

She remembered what Laa had said just before the entire forest had exploded in flames: Bad dreams, Rain. Death dreams. The fires had killed most of the other bouncers.

"I don't understand," she said, looking around at the bushes surrounding the clearing where she had lain down to sleep. She didn't see anything out of the ordinary. Not yet, anyway.
Before she could ask, there was a sound from the bushes.

- Darth Bane: Path of Destruction

A few meters beyond the edge of the tents a line of trees marked the start of the thick Ruusan forests. The concealing branches taunted them with offers of safety, giving even greater haste to their terror-filled flight.

Two of the mercenaries vanished into the forest, crashing through the underbrush. 

As a result, even in the regions of the planet where the forests still stood, snow-a rarity in generations gone past-became a regular yearly occurrence. The unprecedented winters typically lasted only a few months, but they were particularly brutal on an ecosystem that had evolved in a much warmer clime. Some of the flora and fauna of Ruusan, like the humans who still inhabited the world, had learned to adapt. Other species simply died off.

- Darth Bane: Rule of Two

The Jedi were able to outrun the firestorm:

As the others regained their senses, Kaan was shouting out orders and battle plans. "The fire flushed the Jedi out into the open. We can mow them down from the sky. Hurry!"
...
The fire drove General Hoth and his army from the sheltering confines of the forest. Leaving most of their supplies and equipment behind, his troops ran through the trees, a mad scramble to escape the searing heat and flames. Those who stumbled or fell were instantly swallowed up by the conflagration. Somehow most managed to stay ahead of the deadly fires, eventually bursting out of the woods and into the rocky plains where so many battles had already been fought.

- Darth Bane: Path of Destruction