The relationship between the London merhants and the participants in the tobacco monoculture was one of outright theft. Thanks to the well-drained coast of the southern colonies, ships could go upriver and tie up at docks maintained by the plantation owners. They could get their manufactured goods delivered just as though it were Fed Ex. The London merchants, of course, could tell them they had gotten whatever price they alleged for the last tobacco crop, and they chronically lied about that. The tobacco growers would order manufactured and luxury goods by sending in a list when they shipped their crop. The London merchants then filled the order with exorbitantly priced, low quality goods, providing yet another means of robbing their "golden goose" blind. Plantation owners were chronically in debt to their factors in London. Washington, after he resigned from the command of the Virginia Militia (1758?) had realized this. He had inherited Mount Vernon from his half brother Lawrence and was by then a substantial land-owner. He had also married Martha Dandridge Custis, who, as the widow of Daniel Parke Custis, brought huge amounts of land and hundreds of slaves into the marriage. She had two children living, and George was scrupulous about managing their estates (the property was entailed on the children) as efficiently as possible. So, he decided to get out from under the debt burden and preserving the fertility of the land by diversifying his crops, and even then, years before the Stamp Act crisis, he publicly advocated buying colonial products. Therefore, he planted crops which could be sold in Virginia or the other colonies, and well as raising livestock (chiefly sheep) which would also be useful for the local production of goods.
So, he expanded the sheep prodution for the wool and the mutton, he planted grains (mostly wheat and oats) for use on the home farms and for sale locally, and he planted hemp (the kind used to make rope, not the kind one smokes) to also produce a useful crop--i believe i am correct in saying that he had a rope walk at Mount Vernon. For reference, see George Washington
, Douglas Southall Freeman, 1948. I believe this also confirmed in Thomas Flexner's biography of Washington. Washington didn't get rich from his new crop plan (he did that from his land claims, despite more than half of them being stolen by squatters), but it's profitability can be seen from the fact that he had paid off almost all of his debts to his brother Lawrence's London factor by the time the revolution rolled around.