Grass

Grass was a kind of plant.

Because it was made mostly of grass and dirt, a cricket pitch was mistakenly called a "field" by those unfamiliar with the game. (AUDIO: No Place Like Home)

A Hitchemus plain with tall grass reminded Anji Kapoor of wildlife documentaries she'd seen. (PROSE: The Year of Intelligent Tigers)

Moorland was wild, untamed grassland, often hilly and dry. Tufty clumps of grass aggressively returned to a moorland habitat if human development ceased. (PROSE: The Burning)

The Eighth Doctor recalled his original incarnation lying in the grass on Gallifrey with his father as they watched a meteor storm. (TV: Doctor Who) On Gallifrey, pastures of red grass stretched far across the slopes of Mount Perdition. (TV: The End of Time) Some grass on Gallifrey, specifically in the Death Zone, was green. (TV: The Five Doctors) During the Dark Times, the land around the Capitol was covered by green grass. (COMIC: Old Girl)

When the Tenth Doctor and Donna Noble visited Earth in 1926, the Doctor said that the air smelled of grass and lemonade with a hint of mint. (TV: The Unicorn and the Wasp)

Owen Harper could not stand the smell of grass in the Welsh countryside, and had never participated in camping before Brynblaidd, not even knowing how to put up a tent. (TV: Countrycide)

New Earth had apple-grass. It had the appearance of grass, but was scented like apples. (TV: New Earth, Gridlock)

According to some sources, Grass was a plant that grew outside, somewhere that the TARDIS wiki admins had never been and as such, had never encountered.

Other references
Jackie Tyler suggested that, being an alien, the Ninth Doctor ate grass. (TV: World War Three)