I'll take a stab at answering your question, /u/rewdea, but I want to share something before I get going. Modern single-crop agriculture is a relatively new phenomenon, mostly from the second half of the 20th century. Before that, farms -- even the big cotton plantations of the Lower South -- grew other products and raised livestock alongside their "main" crop.
Let's talk about a man named James Hamilton Couper. He's sometimes called the prototypical antebellum southern planter, but he was more modern than most. He was born in 1794 and graduated from Yale in 1814 -- not exactly typical, except for the very wealthy. He traveled to Holland and learned water control methods (irrigation and dikes) he would later use on his plantation at home in Georgia.
When his father died, he inherited his father's acres on St. Simon Island. He began scientific research and experimentation, trying to find the best way to get the most growing potential out of his land. He started out as a cotton planter but expanded into other crops, including rice and sugarcane. In 1829, he built a modern sugar mill. He became the first American to build and operate a cottonseed mill.
He had impeccable manners, offered graceful conversation, owned an extensive library, and was popular socially.
He also owned slaves. Many. According to some records, the figure approached 2,000 at its peak.
The Civil War ruined him. Two of his sons died in the fighting, his slaves were freed, and his industrial efforts were foiled. He died in 1866, but he left behind an extensive collection of journals and records. These were collected into Rice Gold, a book explaining plantation life in coastal Georgia.
Now, if we talked only about Couper's perspective, you wouldn't get a good answer. As fugitive slave John Little said in 1855: "T'isn't he who has stood and looked on, that can tell you what slavery is 'tis he who has endured." If you want to understand what plantation life was like, you NEED to read the perspective of the slaves themselves. One of the most convenient resources for an online reader to do that are the Works Progress Administration Slave Narratives. These were collected between 1936 and 1938 and include more than 2,300 first-person accounts of slavery.
These are filled with color and character, and if you want to understand slavery from the point of view of those who experienced it, you need to search and view these.
Now, with that preamble out of the way, let me give you an answer.
When it came to rice, winter was the time when the stubble from the previous year's crop was either plowed under or burned off. Irrigation ditches were cleaned and cleared, and if any soil maintenance was needed, it was done then.
For cotton, winter included the first steps in the spring planting. The stubble of the previous year's crop was plowed under. Manure -- (at Couper's plantations, this was rice straw and cane trash that had been 'seasoned' in the stock pens and mixed with marsh mud) -- was plowed into the earth with the previous crop. The land was plowed and then cross-plowed to pulverize the earth. This process usually started at the New Year, following the Christmas holidays, and lasted until it was time to seed everything in March. Plowing also meant putting drainage ditches in the fields to keep them from being flooded. Unlike rice, flooding wasn't good for cotton.
For sugar cane, Couper subdivided his fields into two-acre plots to separate his slaves into work teams. Each splace was plowed deep to pulverize the soil, then harrowed smooth. Plows would come through again, creating ridges into which the sugar cane would be planted.
Winter also tended to be the time that land was cleared of brush and timber. "On New Year's Day, if dere warn't too much snow on de ground, de Niggers burnt brush and cleared new ground," said Addie Vinson, who was a slave in Georgia on a large cotton plantation. There's an important point to be made here -- even if a planter owned a set amount of acreage, all of that usually wasn't cleared fields or orderly orchards.
Louis Hughes, who wrote Thirty Years A Slave: From Bondage to Freedom in 1897, describes the process:
"The season selected for clearing the land was winter, beginning with January. The trees, except the larger ones, were cut down, cut into lengths convenient for handling and piled into great heaps, called “log heaps,” and burned. The undergrowth was grubbed out and also piled and burned. The burning was done at night and the sight was often weird and grand. The chopping was done by the men slaves and the grubbing by women. All the trees that blew down during the summer were left as they fell till winter when they were removed. This went on, year after year, until all the trees were cleared out. The first year after the new land was cleared corn was put in, the next season cotton. As a rule corn and cotton were planted alternately, especially if the land was poor, if not, cotton would be continued year after year on the same land. Old corn stalks were always plowed under for the next year’s crop and they served as an excellent fertilizer. Cotton was seldom planted on newly cleared land, as the roots and stumps rendered it difficult to cultivate the land without injury to the growing plant."
Phillis Hicks, who was born shortly after the war's end, said winter was the time for making cloth and clothing, at least where she lived in Arkansas. That way, the wearers of that clothing would have the thickest possible fabric to stay warm in. As the summer wore on, the clothes would wear out, and they would become thinner and cooler. Paul Smith, at a plantation in Georgia, had a similar experience, as did Vinson. Vinson and Smith both said that slaves didn't wear shoes except in winter, when a traveling man would set up shop at the plantation until everyone had shoes to wear for winter.
Hughes offers a description of clothes-making as well:
"Through the winter and on rainy days in summer, the women of the field had to card the wool and spin it into yarn. They generally worked in pairs, a spinning wheel and cards being assigned to each pair, and while one carded the wool into rolls, the other spun it into yarn suitable for weaving into cloth, or a coarse, heavy thread used in making bridles and lines for the mules that were used in the fields. This work was done in the cabins, and the women working together alternated in the carding and spinning. Four cuts were considered a task or day’s work, and if any one failed to complete her task she received a whipping from the madam."
As Smith explained: "When us warn't out in de fields, us done little jobs 'round de big house, de cabins, barns, and yards. Us used to help de older slaves git out whiteoak splits, and dey larnt us to make cheer [chair] bottoms and baskets out of dem splits."
At Couper's plantations, there were the cotton and saw mills, the rice and cane mills, blacksmith shop, cooperage, cattle barns and dairies. There was work smoking and seasoning meat, preserving fruit and vegetables, making saddles and tack, and mending and making clothes. From time to time, Couper even operated an ice house, each winter packing great hunks of North-shipped ice into sawdust-filled buildings for preservation into the summer.
Slaves with specialized skills, such as carpenters and bricklayers, could be hired out to people needing those services.