What is the most effective way to remove algae forming all over the house siding so that
The treatment will last for 5 years or more? If powerwashing is first done, will it discolor
The siding so that whole house painting becomes an added project? AND when is the best
Time to wash and treat the algae seasonwise and weatherwise? Thank you!

Well nobody answered, the poster is probably long gone, but in my experience, spraying with a 50% solution of bleach and then rinsing with water is very effective at killing mold. If the algae is thick, you will have to pressure wash after bleach treatment. Then paint with a mildew resistant paint or you can get a mildewcide at Sherwin williams to add to your paint. We have a lot of algae growth here in Galveston so I have dealt with this before and I think it will work for you.

Ok, as most know I train people in the pressure washing industry.
And while some recommend bleach the commercial companies we work with would never.
It is 1. dangerous to use, 2. easy to cause damage to the home, 3. can cause damage to plants, 4 is not eviormental friendly

Part of the trouble is that the mold will grown into the siding, thus the reason after it is clean you still often see little dots or discolored areas. But by and large it is a annual cleaning that will have to be done. Normally we recommend a PH neutral Butyl cleaner

I agree with Chuck on this, do not use household bleach to kill mold. I know that 50% of the searches regarding mold will say to use bleach but it is not safe for you or your plants.

I have never seen the plants get damaged after bleaching, probably because of extensive watering done by the pressure washing or rinsing procedure.

I happily use Spic N Span liquid with a long handled brush to get the green algae off my vinyl siding. Works just great. Or if I don't have that on hand I use dish soap that has bleach alternative in it and that works just fine. I don't powerwash anything as the brush does the work. Then rinse. So far I only do my house every other year with this method. Yes, I have used the bleach and it didn't keep the algae off any longer than plain old dish soap.