A rock garden is typically one of two things; In landscaping or gardening a rock garden is an ornamental garden comprised mostly of of rocks and stones. In mountain biking it refers to something very different and far less pleasant; a section of path from which rocks sprout up as if they grew right out of the ground, or where they are generally strewn loosely around the path.
Ornamental rock gardens usually include not only rocks, but also such plants as flourish in a rocky environment, principally those from alpine regions. "Zen gardens" (Japanese rock gardens]) typically contain very few plants.
Hazardous rock gardens exist in the middle of many mountain biking singletrack trails. The term is used somewhat interchangeably (and confusingly) to refer both to a section of trail strewn with loose rocks (see also death cookies) and a section of trail where the rocks are planted. In general the latter is the result of either erosion or simple wear on a trail due to repeated use, which over time compacts and/or wears away the soil and causes the rocks to be uncovered; the former often accompanies the latter, and can also include pieces broken off of rocks by unsuspecting bicycle frames as well as pieces broken off of bicycle frames by unsuspecting rocks.