"bed of grass" is correct and usable in written English.
You can use it to refer to a field of grass, or an area of grass in which a person or animal might lie down. For example, "The deer lay down in a peaceful bed of grass."
As I lay down on the lawn before it, my body sinks into the bed of grass.
In a bungalow in suburban Santiago a heavily pregnant woman is lying on a bed of grass.
Over the next five months, the park's complexion will change from strips of soil to a bed of grass to a field of brilliantly colored flowers.
"If he had a bed of grass under his feet and a bit of sky above his head, he was happy," said his wife, Rene.
Among the tapas, the genuine blandness of the salt-cod croquetas helped ameliorate the goutiness brought on by a rich little pot of duckling, pork, and sherry, and, from the next round, lamb dressed with hot peppers and roasted on a bed of grass from the field in which the lamb once grazed.
Even behaviors can be considered vestigial: witness the instinct in dogs to turn in a circle multiple times before lying down, presumably, though not assuredly, to tramp down a bed of grass (Irvine [2004]).
Along the left fork, underwater springs feed a rich bed of grasses, wiry bushes and small pools in which thousands of little red-spotted toad tadpoles wriggle around.
Half of the apartments face a private park, four and a half acres with beds of grasses and perennials reminiscent of the High Line and Brooklyn Bridge Park.
"You know," he said while looking quizzically at his travel companion as she paddled over a bed of sea grass.
But the morning after that, when Van pointed to a tree full of vultures, we found my zebra in a matted bed of yellow grass.
In Dodge, the Arkansas exists as a wide, flat bed of sparse grass and scrub brush, with a channel of deep sand running down the middle.
