Whether it’s pine or problems, sometimes the word knotty can apply to both. Knotty can describe something that has a lot of knots, either literally or figuratively.
The adjective knotty is a way to describe something that’s covered in knots. It can be used in a literal sense to refer to wood that is covered in hard, irregular knots — you’ve probably seen these circular, knobby imperfections that sometime appear on wood. The word can also be used figuratively to describe something that is intricate and difficult to solve, such as a knotty political problem involving two warring nations or a knotty legal issue involving complicated constitutional issues.