other hand, care must also be taken that hay is never exposed to any possible source of heat or flame, as dry hay and the dust it produces are highly flammable. Early methods Columella in his De re rustica describes the usual haying process of the early Roman Empire. Timothy grass and clover were the most common plants used for hay in the early 20th century in the United States, though both plants are native to Europe. Much hay was originally cut by scythe by teams of workers, dried in the field and gathered loose on wagons. Later, haying was accomplished with horse-drawn implements such as mowers. With the invention of agricultural machinery such as the tractor and the baler, most hay production became mechanized by the 1930s. After hay was cut and dried, it was raked or rowed up by raking it into a linear heap by hand or with a horse-drawn implement. Turning hay, when needed, originally was done by hand with a fork or rake. Once the dried hay was rowed up, pitchforks were used to pile it loose, originally onto a horse-drawn cart or wagon, later onto a truck or tractor-drawn trailer, for which a sweep could be used instead of pitch forks. Loose hay was taken to an area designated for storage—usually a slightly raised area for drainage—and built into a haystack. The stack was made waterproof as it was built (a skilled task) and the hay would compress under its own weight and cure by the release of heat from the residual moisture in the hay and from the compression forces. The stack was fenced from the rest of the paddock in a rick yard, and often thatched or sheeted to keep it dry. When needed, slices of hay would be cut using a hay knife and fed to animals each day. On some farms, the loose hay was stored in a barrack, shed, or barn, normally in such a way that it would compress down and cure. Hay could be stored in a specially designed barn with little internal structure to allow more room for the hay loft. Alternatively, an upper storey of a cow-shed or stable was used, with hatches in the floor to allow hay to be thrown down into hay-racks below. Depending on the region, the term "hay rick" could refer to the machine for cutting hay, the haystack or