Sheviis 1:1

Until when may one plow an orchard in the sixth year of the shemittah (sabbatical) cycle? Beis Shammai say so long as doing so is good for the fruit of the sixth year; Beis Hillel say until Shavuos. These two times are pretty close.

Sheviis 1:2

The definition of an orchard for our purposes is when there are three trees in a beis seah (a plot of land measuring 2,500 square cubits, a cubit being in the vicinity of 18 inches). If the fruit of these trees can yield a cake of pressed figs weighing as much as 60 Italian maneh (about 53 lbs.), the entire plot of land may be plowed for them. Less than this, then only the space required for a person to gather with his basket may be plowed.