In many other beds, I used cedar mulch (matches our cedar shutters and door), and those plants are very healthy.
I have checked for disease, insects, soil moisture, etc. and everything seems fine. I've had a landscape pro look at it too, and the only answer he could come up with was that maybe they didn't get enough water at some point in early summer (he admitted that the moisture in the soil was fine at that point), but I don't believe this was the problem, because I pay careful attention to how much we water and how moist the soil is, and we had a cooler wetter summer than normal, and these plants have been established for five+ years. Out of desperation, I did dramatically increase watering this particular zone back in May, and it didn't yield any change; the plants continued to die.
Most of the monkey grass in this photo is now gone (this is about a fifth of what I lost), and the only commonality that I can come up with regarding the plants in various beds dying or not dying is the use of this mulch. About 100 bags worth.
Am I crazy? Is this common?
BTW, the bag pictured below is a leftover bag that I didn't spread out. It was on a slope in the far backyard, and the mondo grass directly downhill from the bag is all dead now. The other surrounding mondo grass is healthy.
