It can be difficult to determine a date based on the clothing of farmers in an historic image. Their clothing was functional, not fashionable, and changed little over the years. However, a few small details can be observed in the evolution of the farmer's attire that can give a relatively rough date. Below are several representative images of farmers from the mid-19th century through the mid-20th century.
During the 19th century, sack coats (worn by the man on the left), button down shirts, and soft, felt hats, usually wide-brimmed, were frequently worn. The men in the image below, which dates from the 1860s, wear overalls, but they were not as commonly worn until around the 1890s.
What stands out in the late 19th century images below are the soft, felt hats and sack coats worn by most of the men.
The images below date from the early 20th century. Note the overalls and the pants held up by suspenders. The men all appear to wear button down shirts and soft, felt hats.
What is most notable about the 1920s and 1930s era of farmer's clothing is the preference toward the flat caps worn by most of the men in the images below. The soft, felt hats of the previous decades were largely replaced. Otherwise, the clothing hasn't changed remarkably. The men in the images below wear a variety of outfits, including overalls, button down shirts, vests, and sweaters.
During the 1940s, the soft cap is still worn quite frequently, in addition to more modern wide-brimmed hats (worn by the man in the image directly below). The farming attire appears to become more casual, with jeans and short-sleeve shirt.
The 1950s ushered in the ball cap and the short jacket, both of which were worn by many farmers. Although many of the traditional outfits were still worn, such as overalls and button down shirts, blue jeans and short-sleeve t-shirts were frequently worn in warmer months.