Many herbicides are safe on a crop only when applied at a specific growth stage.
Similarly, weeds are controlled only when they are at certain growth stages. For most post-emergence products, growth stages are described by number of leaves. Cereals and Annual Grass Weeds Leaves are counted, starting at one for the first leaf, and progressing up the primary shoot. Tillers are important but not counted as leaves. A leaf should be counted as soon as it emerges, but may be labeled as early, mid or full leaf.