They are always complimentary. The normal, healthy, functional, durable existence of everything in Nature depends on the mutual enhancement and beneficial interaction of opposite forces. On one level, when there is an antagonistic relationship between opposites this leads to destruction. However, from the larger perspective, the balance of Nature is always maintained. Yin nourishes and sustains and controls Yang and vice versa.
Opposites also hold each other in balance and exert mutual control.
If there is insufficient care and attention by the workers to the crops there may be less crops and more weeds. Everything in the physical world has an opposite. There are no absolutes in the manifest universe. Yin attracts Yang and Yang attracts Yin. The greater the difference, the greater the attraction.