Tobacco priming refers to harvesting the rows of leaves on a tobacco plant as they mature. Six rows is the average with the first priming being closest to the ground and the sixth priming near the top. The higher the priming the stronger the tobacco as it receives more sun and nutrients.
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Workers will harvest from the bottom of the plant up, usually taking two or three leaves at a time.
In this picture you can see the bottom 2-3 primes have been harvested.