I visited an onion field where I saw this interesting emergence pattern with annual bluegrass (Poa annua L.). Onion beds are listed in east-west orientation (as seen in pictures). We can see annual bluegrass emergence only on the south facing slopes and none on bed-tops or north facing slopes. These pictures were taken around noon, the exact time was 12:44 PM. I assume that cultural and irrigation practices are same throughout the field. The only difference I observed was north facing slopes were shaded, but the bed tops and south facing slopes had ample light. With this situation, I am wondering what could be the possible reason for annual bluegrass to emerge on the south facing slopes but not on the bed-tops and north facing slopes.

What I have understood is there are usually fewer weeds on the north-south oriented beds compared to the east-west oriented beds. It is also common to get fewer weed at bed-tops compared to the bed slopes. But the situation is different in this onion field where annual bluegrass is only on south slope.

If anyone could add some information/clarification on this topic that would be great.