Clipping the Leaves
When: whenever leaves in a seedling tray are shading their neighbors. Once I begin clipping, I usually have to repeat it weekly, give or take a few days.
Why: to allow all the seedlings to get sufficient light. Clipping is a form of leaf damage that the plant responds to, as though insects were munching the leaves. The stalk thickens, the roots grow more, and produce more nicotine, and the slower plants have an opportunity to catch up with the faster ones.
Commercially, clipping (which is performed with a lawnmower moved along a rack above the float trays) allows a tobacco farmer to hold seedlings in the float trays longer, while awaiting suitable weather for transplanting.
I use a sharp scissor, selecting and clipping individual leaves. This requires about 5 minutes per 1020 tray of 48 plants.
Bob