Why the north side of your house turns green

Green algae, black mildew and mold, and moss all want the same three things: moisture, shade, and something to hold on to. Siding, fences, deck boards and concrete all give them the third. Where the other two are present, they arrive.

That is why the pattern on a house is so predictable. The north wall, which never sees direct sun, goes first. Then the walls behind shrubs, under big trees, beside the downspouts and anywhere the sprinkler reaches.

The Northwoods is close to ideal for it. Cabins and lake homes sit under pines and hardwoods, the ground holds moisture, and on a lake the air itself stays humid through the summer. A seasonal home that is closed up for months — no one brushing past the siding, no one sweeping the deck — gives it an uninterrupted run.

A house on an open lot in town and a cabin under the trees on a lake can be the same age and the same siding and need cleaning on completely different schedules.

High pressure takes off what you can see. It does not reliably kill what is left in the texture of the siding, the grain of the wood or the pores of the concrete, and those survivors regrow — which is why a house that was blasted in May can be going green again by the following spring.

Soft washing works the other way around. A cleaning solution applied at low pressure kills the growth, is given time to work, and is then rinsed off. The result is clean and it stays clean longer, without the pressure that forces water behind siding panels or furs up wood.

Hard, black specks about the size of a pinhead, stuck to the lower siding or to a car parked beside a flower bed, are not dirt and not mildew. They are artillery fungus. It grows in wood mulch and fires its sticky spore capsules at light-colored surfaces nearby, and they bond like tar.

A wash does not remove them. They usually have to be scraped off one at a time, and often leave a faint mark even then. They come from the mulch beds rather than from the siding, so the beds along the foundation are where to deal with them.

Lichen is the crusty grey-green or orange growth on stone, fences and old wood. Moss is the soft green cushion in shaded corners of patios and steps. Both are killed by cleaning, but lichen in particular holds on after it is dead and releases over the following weeks. It is normal to still see faint marks for a while after the job is done.

In late spring, a yellow-green dust over every flat surface — decks, cars, the windowsills, the lake — is usually pine pollen rather than growth. It rinses off with a hose. It is also a good reason not to book a wash in the middle of pollen season if you can help it.

Anything that lets a surface dry out faster helps. Trim shrubs back from the siding so air moves behind them. Keep gutters clear so they don't spill onto the walls. Turn sprinklers away from the house. On a shaded lot, those three things can make a real difference to how soon the green comes back.