traveled north with the harvest seasons, mostly in the great plains. They still do, but now there are fewer since the advent of the combines. National Geographic did an issue on these migrant workers about 15-20 years ago, but I can't find the exact article now.
These are some quotes from a different source; Oregon State University:
Here, where the North American wheat belt extends from West Texas to the Canadian prairies, as many as a quarter of a million migrant and seasonal workers traversed the plains, harvesting and threshing wheat, until the advent of the combine in the second half of the 1920s. But there are also other significant subregions of the West where farmers developed extensive and intensive agriculture. Eastern Washington's Palouse and the fruit-growing areas of the Yakima and Wenatchee Valleys were some of the earliest western agricultural zones to require tens of thousands of seasonal workers every harvest and packing season during the early decades of the twentieth century. And as early as the 1870s, the farms of California's central river valleys required seasonal laborers. California's agribusiness would eventually become the largest user of migrant farm labor in the American West
(snip)
During the first few decades of the twentieth century, the vast majority of migrant and seasonal laborers working in western agriculture were white, native-born men. Among these predominantly unmarried workers, traveling by rail from job to job and living in "jungles," harvest Wobblies developed a distinctive culture of work and of life on the road, which I have termed their "worklife culture." They shared much of this culture with other migrant and seasonal agricultural laborers of the day. Yet the Wobblies embodied a unique camaraderie in the jungles, worksites, union halls, skid rows, jails, and freight cars of the American West. Their common experiences forged a socilcultural bond that was further strengthened by aggressive opposition to employers, law enforcement officers, and "hi-jacks," the robbers and confidence men who preyed on migrant harvest workers. Harvest Wobblies expressed themselves through an impressive outpouring of writing, impassioned speeches, union organizing, humor, and song. Sustained through several decades by a common sense of identity and purpose, they created a worklife culture that appealed to Great Plains migrant and seasonal workers, bringing tens of thousands of them into their union.
http://oregonstate.edu/dept/press/g-h/HarvestWobIntro.html