A Greensman or Greensperson or Greenskeeper is any production personnel on a film set which is responsible for obtaining and taking care of anything "green" – plants, grass, trees, flowers, and other various landscaping

Landscaping

Landscaping refers to any activity that modifies the visible features of an area of land, including:# living elements, such as flora or fauna; or what is commonly referred to as gardening, the art and craft of growing plants with a goal of creating a beautiful environment within the landscape.#...

 materials like rocks, gravel, sand, etc. Considered part of the art department

Art department

Art department in movie terms means the section of a production's crew concerned with visual artistry. Working under the supervision of the production designer and/or art director, the art department is responsible for arranging the overall look of the film as desired by the film director...

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This includes artificial forms of these materials up to and including those representing greens, such as camouflage netting. The greensman is essentially a set dresser with a different specialization and can work as a prop person or set dresser within union guidelines.

Greens work is generally considered to be one of the dirtiest, most thankless, and physically demanding jobs in set construction. In addition to set construction greensmen often work "on set" assisting the camera and other departments in adjusting the set to the changing demands of different camera positions and lighting set ups.