I really enjoyed a recent Nature episode: What Plants Talk About. It is probably information you would already know, but had a very alternative thesis to modern scientific thinking: Plants behaving like animals.
Cool! It's a good intro-level presentation, but they also get very specific about certain plant interactions that are regionally pretty important.
I especially liked the part about lupine being a "protector" plant for other natives when knapweed threatens. They said the oxalic acid that lupines can release seems to counteract the knapweed's allelopathic poisons.
I wonder if mulching with plants containing oxalic acid would also help. For example, we have a lot of lamb's quarters that grow like weeds in the garden, and though they're edible and tasty in season, I've heard from friends that over-eating them isn't good because they contain oxalic acid. I might try chop-and-dropping them on top of any knapweed that shows its virulent little head around here.
Seeding out lupine in response sounds good too, of course.