My 3 year old new vinyl siding seems especially suited to mildew or algae. I have a vinyl siding "crud remover" spray but I have plants below it. Will this standard spray stuff kill the plants (Hostas and Snow On The Mountain)? Would it matter if I waited til they went dormant (ie, will it kill the roots)? Would soaking the soil afterwards help or hurt?

I don't know the answer to your specific question, but I do know I have the same problem on the North side of my house, and on the shaded portions of the West side.

If you want to try a solution that will not harm your plants at any time, whether dormant or green; I use a dish soap with *detergent* in it, like Dawn. For my dishes, the protection of my hands, I only use Palmolive, 'because you can soak in it'. So with Dawn or other dish washing detergent, I use a bucket of very hot water, and an old corn broom or whatever they are called. And then I just rinse off with the hose sprayer, cold water.

I have the same thing here on the north side. Do you know what is in the "crud remover" spray Yardenman? It may say on the container.

What brand.
I used RMR mold killer on my garage and it got on the bush roses.
Killed some branches where the mist got on the leaves but did not hurt the roots.
It gave me a good reason to thin them out.

What I use is the garden hose and a broom. No chemicals. Spray the area with water from the garden hose, wait a minute or two, scrub with a broom (my broom has plastic bristles and an extending handle to reach the high parts), spray again with the hose and the mildew is gone.

Don't know the answer, but if you are at all worried, it would be far more advantageous to soak the soil before and after, than just after.

I have the same green "moss" problem on my north facing side of the house. The dove gray vinyl washes up nicely in early spring with Dawn dish soap in a bucket of warm water, but by season end there it is again. I bought the longest bamboo pole I could find, and tie wrapped a battery operated spinning bathroom scrub brush to the end. It reaches almost to the top of the second story siding without using a ladder. And, it is true, wetting down the siding and waiting a few minutes makes the job go faster. That process leaves a clean house.
I did buy the moss spray one year and applied it with a sopping wet paint roller. I did not wash it off, but just let it dry. I wasn't impressed. The moss always regrows. I was too afraid the overspray would kill my plants.

I have many shrubs and Hostas all around my house and have never had any adverse effects cleaning the siding with Spic and Span. I do pre-wash and rinse with a lot of water, so perhaps that is the reason.

If you want to get rid of it for more than a few months, you will have to use something that actually kills it.
Cover the plants and spray it on.
Use a mold killer as that kills it.
I sprayed it the North side of Sharon's house last summer. It did not come back and needed no rinsing or scrubbing.

I agree that it would be more advantageous to soak the soil before and after.

Note: this thread is from 2017

GingiKisi said:Be careful, some products are very corrosive and can simply damage your vinyl flooring

What vinyl flooring? This isn't about flooring.

Calif_Sue said:Note: this thread is from 2017

Thank you.

Calif_Sue said:

What vinyl flooring? This isn't about flooring.

Yeah, I wondered about that comment too. Thank you.

OK.
Update.
In mid summer I used Wet-n-Forget. Applied using its hose end sprayer with my hose. Afterward I rinsed plants with gentle water spray, that came in contact with product, but did not rinse vinyl siding.
This stuff works! It's been two years, still have moss-free vinyl siding.