When the terminal bud is removed from tobacco by topping, a number of changes are triggered in the plant: increased root growth, nicotine synthesis, improved drought tolerance, and leaf expansion and increased thickness. These changes are affected by topping time; generally there will be less crop response to topping as topping is delayed. The most important changes, from a producer's perspective, are continued leaf expansion and thickening, with a commensurate improvement in quality and increase in yield. Topping at the proper time of plant development is often difficult on a field scale since uneven crop growth, particularly when tobacco is set early using bare-root plants, causes delay or irregular blooming. Two major factors that can cause irregular plant growth are soil compaction, usually due to tilling soil that is too wet in the spring, and black root rot. Many producers wait until 50% or more of the plants are in bloom before topping. Research was conducted from 1989 through 1991 to determine the effects of topping time on dark tobacco yield and quality.
Maksymowicz, Bill, "Effect of Topping Time on Dark Tobacco Yield" (1993). Agronomy Notes. 43.