Mychorrhizal fungi

Well I'm no expert but I thought it worked by attaching itself to plant roots. You don't water it into a border.
So the roots of roses etc need to be covered or sprinkled in the planting hole.
If be you have plants that look like they need a pick me up, under plant with other plants and sprinkle those roots?

The box I bought was Q4 Vitax soluble and the instructions were to water in at planting with no mention of repeat treatments.  As you say, the majority of the products on sale are granular.  I suppose it's a case of 'suck it, and see'.  I only bought the box of soluble because it was the right price to complete the value of a voucher I needed to use up.