Hey, what's this green stuff on my siding?

So, I go to research this piece about green stuff on my siding and Google presents to me a likely place to start -- shunshelter.com. I take the bait and give it a click. There, below the article's title is a beautiful house, one that surely doesn't have green stuff growing on its siding. But then, who is to know. I mean, the house depicted has green siding. My house has tan, vinyl siding, and on its shadiest sides, green growth … beautiful, vibrant, green growth. While the thought of color-matching this green growth with some updated siding is tempting, I decided to try to figure out what it actually is and how to combat it. Way back when, I remembered my naturalist boss calling the green tinge on siding, trees, poles and such protococcal color. I thought little of this, but I latched onto the term because it made me sound smart. At no time before today, had I ever looked it up. Turns out, it's an algae. The Free Dictionary defines it as such: of or relating to unicellular algae of the former genus Protococcus, one of which (renamed Pleurococcus vulgaris) is a green alga commonly found on trees, walls, etc. Pleurococcus vulgaris, now, there's a name to make you feel smart! And although the species name -- vulgaris -- initially seems appropriate, the fact is that this stuff does produce a very pleasant green color. This green, of course, is from chlorophyll, which makes sense, because somewhere deep within its taxonomy, algae is a plant -- a single-cell plant. Don't algae live in water? Nearly all forms of algae live in water. This one does not. In fact, it is quite tolerable of dry conditions, although being moist or damp is preferred. This alga also has a few other adaptations that allow it to live out of water. First, it has a thick cell wall that reduces water loss. Second, it can go dormant when things get dry, and then spring back to life when moisture returns. But where does it come from? As I inspected the north side of my house, I found P. vulgaris growing not just on my siding, but actually on the surface of the soil. Further, it grew, almost exclusively, on the northside of tree trunks. There was no doubt that it was common far beyond my house, and it undoubtedly was there long before my house. How the organism makes the literal jump between a tree trunk, or from nearby soil, is surprisingly easy. It floats on the wind … and when you are just a few microns in size, this is ridiculously easy to do. More than that, though, once it lands on something, it starts producing, exuding really, a sticky protein that better attaches it to its new home. Also, as single-celled organisms do, it starts cell division (mitosis), and each new cell produces more sticky protein. Exponentially, it appears as a green film to the naked eye and an eyesore to the homeowner. Now comes the question, what does it feed on? It doesn't need much, just a little bit of dust. So, over time, as a house's siding collects a minute amount of dust, the preferred habitat of P. vulgaris becomes established. But only where the sun don't shine! Here in the northern hemisphere, southern exposures are much too dry for this alga. For this reason, it is almost exclusively relegated to the shadier northern portions of houses and trees and signposts. Now, my house has a couple of exemptions. First, a decent portion of the eastern side of my home shows P. vulgaris. In my determination, this is because the large trees east of my house provide extensive shade to this side of my siding and this allows things on this side of my house to be ever so slightly damp. Also, the northern exposure of my front porch shows no signs of algae. My conclusion is that this portion of my siding doesn't ever get wet because of the porch overhang. (Now, just a few feet away, the concrete of my foundation behind my boxwoods is green as all get out!) Before venturing into some control and removal ideas, let me mention that a few other organisms might also be a part of your green siding. Most notable are forms of mold, mildew, bacteria, lichen and moss. Luckily, the treatment for all of these is similar. Few things work as well as a power washer. But be forewarned here. Power washing a house can cause damage to your home, especially if your siding is wood. Also, power washing a home that lacks a vapor barrier can allow water infiltration into the deep layers of your home's walls, a place you don't want it. Additionally, power washing can be exhausting. And, lastly, power washing works best against this sticky alga at close range. Algae growing near the peak of your house will just beg you to climb a ladder, and climbing a ladder with a power washing wand in hand is never a good idea, except for professionals. And, and yes, there are professionals that will power wash your house, and for a moment in time, they will remove all of the algae from your siding. But it will come back! So, having your house power washed on a regular basis will keep this organism at bay. There are a lot of "do-it-yourself" mixtures that will remove these algae. Included is a water/bleach mixture. I say pass on this. While it will work, it will likely taint your lawn and planting beds. There are several retail products that claim to work. A few years ago, I tried one called, Wet & Forget. I blasted the upper reaches of my house with it. I swear I heard the algae laugh at me. To this day, it is still the healthiest population of P. vulgaris on my siding. In the end, I still find it amazing how a single-celled organism reminds me that nature is always in control, and how a tiny bit of dust on my vinyl siding provides habitat for a microscopic world.