To sow a second crop after a first one (the cover crop) has become established, such that they both develop at the same time.

What does undersow mean?

: to sow (a crop) with or after a main crop to grow on after the main crop has been harvested barley was undersown with lespedeza.

What is under sowing?

​What is undersowing? ​Pastures are typically sown either directly or in combination with a cover-crop (also called undersowing; the practice of sowing pasture seed simultaneously with a crop that is intended for grain production in the first year)