What are PMJ bushes? You cannot just hose it off?

Murphy's Soap, it's vegetable based and cleans well. Just mop with it, then rinse off with a hose.
Most groceries carry it, as do big box stores in their cleaning supplies departments.

What is your front porch made of? I'm assuming wood. You could use a power washer or even just a garden hose with an attachment that raises the pressure at the nozzle.

I don't know what PJM bushes are but I've used some pretty strong cleaners on porches without damage to the bushes nearby. I spray the bushes with water then try to keep the cleaning mixture off them. If they are touching the porch, you can cover them with plastic sheeting from Home Depot.

It is difficult to remove mold from a porch without using something that kills it, such as bleach, so if gentler cleaners fail, you may have to resort to covering the plants with plastic during the cleaning.

PJM's are rhododendrons.
:)

Here is a link that might be useful: PJM series rhododendrons