Mychorrhizal fungi

It probably depends on the product. I have some ericaceous feed that includes MF, and the instructions say to apply it once or twice a year as a top dressing. However AFAIK "straight" MF is generally only used once, when planting.

I think that is due to regular MF does not work with ericaceous plants? So it's not used at planting time.

Rootgrow state that one application at planting time with support the plant for its lifetime.