The Note to Rule 13-4 clarifies;
At any time, including at address or in the backward movement for the stroke, the player may touch, with a club or otherwise, any obstruction, any construction declared by the Committee to be an integral part of the course or any grass, bush, tree or other growing thing.Of course, the player must not do anything to improve their lie, area of intended stance or swing, or line of play by moving, bending or breaking anything growing, because this would be a breach of Rule 13-2.
In a playoff in the Verizon Heritage PGA event in 2010 between Jim Furyk and Brian Davis’. Brian Davis hit his ball near the 18th green, rolled down a slope and into a hazard adjacent to the putting surface. The hazard had twigs, grass and other debris in it.
During his backswing, Davis believed that he had touched blades of grass or reeds that were growing in the hazard and called a penalty on himself, forfeiting the victory.