I really don't know what this longer dark periods north of the equator references. Comparing season to season, dead of winter LA has days longer than 9h50m (
link), while in Dunedin (where Forysyth Barr is located), winter days are less than 9h long (
link). LA has longer days and shorter nights. If you want to take weather into account and throw in hours of sunshine per month, LA dominates. LA doesn't average less than 200 hours of sunshine a month, throughout the year (
link), while Dunedin never averages more than about 175 hours of sunshine a month (
link).
While people mention sunken field, that's really not what it is. The entire stadium foundation is "sunken." You don't go to the front of the field level luxury box and look down a mine shaft to watch the game. The field is lower and so are the stands. The shade/shadows are from the surrounding structure just like any other stadium.
Talking about the shadows from the stadium stands and structure is a different matter than talking about a "sunken field". Emirates and Wembley manage to grow hybrid turf in partially roofed stadiums (and crappy weather). They have tons of shadows. They also have less quality sunlight. If someone wants to say that structures at Wembley and Emirates cast less shade on the field than in Inglewood, I'm not buying it. Regardless of shade, there are shade tolerant strains of Kentucky bluegrass (shade tolerant, not
it grows in the dark).
Wembley:
Emirates:
But, bottom line is that it's all academic. They've already decided on artificial turf and that's what they're doing. That being said, I haven't seen anything from any source demonstrating that grass at Inglewood would not be feasible.