The name of the curtain "Mushroom," located on the territory of the New Garden, is associated with the shape of the gazebo that stood here at the end of the 18th — beginning of the 19th century. The "Mushroom" curtain is situated in one of the four identical squares in the New Garden of the Alexander Park. The term "curtain" in landscape gardening is defined as: "An open lawn area framed by trimmed bushes or trees." The curtain was named after a bench with a canopy above it shaped like a mushroom, which stood in this place from the 18th to the early 19th century.

Originally, in the mid-18th century, on a round artificial mound with slopes covered with turf and stairs, there was a gallery consisting of interconnected gazebos or "cabinets" arranged "like honeycombs," with gilded domes; all this was called a "scarpir."

In the 1770s, the gazebos were demolished, and instead, a "carved oak tree" with an iron tent roof painted inside "in the color of clouds" was installed. It was later replaced by a bench with a mushroom-shaped canopy, which was dismantled in the early 19th century. This peculiar gazebo gave the curtain its name. The layout of the curtain has been largely preserved: in the center, on a small elevation, there is a round platform from which eight alleys radiate like rays.

Sources:

https://spb-parki.ru/parks/aleksandrovskiy-park-tsarskoe-selo/novyy-sad/kurtina-gribok/

https://www.tzar.ru/objects/alexandrovskypark/newgarden/mushroom