Rehoboth Beach is two blocks from the Atlantic, and salt is the whole story. Salt is hygroscopic — it pulls moisture out of the air and holds it against your siding, so surfaces here stay damp long after the sun is out, even on days it never rains. An inland home gets wet and then dries; a Rehoboth home gets wet and then stays humid at the surface, because the salt deposited on it keeps drawing water back out of the ocean air. That’s the difference between here and Georgetown, twenty minutes inland, and it’s why homes in this town grow algae at a pace that genuinely surprises people who move from anywhere else. It isn’t the humidity number on the weather app — Georgetown’s is similar. It’s the salt holding that humidity against the surface around the clock.

Then there’s what the census says about who’s actually home to notice: 74 percent of Rehoboth housing units sit vacant — second homes and vacation rentals that stand closed up between stays. A house with no airflow, no windows opening, no one running the HVAC, and salt-damp siding grows faster than an occupied one, and most of this town is that house. Owners arrive in May to a green north wall and assume something happened over the winter. Nothing happened — the house just did what unattended coastal houses do, which is why the smart rental owners here run exterior care on a calendar instead of on a surprise.

The third factor is the housing itself. The median Rehoboth home was built in 1975, and nearly a fifth of the stock predates 1940 — South Rehoboth’s Victorians, Henlopen Acres’ cedar shake, painted wood trim everywhere you look between the Avenue and Silver Lake. Those surfaces cannot take high pressure: the grain raises, the paint strips, the shake splits, and water gets driven into wood that has stayed sound for a century precisely because nobody ever forced water into it. In a town where the median property value hit $1.49 million in 2024 — up 20 percent in a single year — wand marks on hundred-year-old trim are a five-figure mistake, and we’ve been called to look at more than one.

What the growth actually is matters too, because three different organisms get blamed on “dirt” here. The green film on shaded siding is algae, feeding on moisture and whatever organic material settles into the surface texture. The black streaks running down roofs are Gloeocapsa magma, a blue-green algae that eats the limestone filler in asphalt shingles and thrives in exactly this climate — humid, salt-damp, pine-shaded. And the fuzzy growth in deck boards and fence posts is true mildew, living in the grain rather than on it. All three are alive, which is the whole point: knock the visible layer off with pressure and the organism underneath grows back, usually within a season, because nothing about the moisture or the food supply changed.

Geography closes the loop. Silver Lake and Lake Comegys put spring-fed freshwater against the south end of town, the Lewes-Rehoboth Canal runs the northwest boundary, and Rehoboth Bay sits behind it all — so even the blocks that don’t face the ocean face water. The Pines adds tree cover to the equation: pine shade means siding that never fully dries and needle drop that feeds roof algae year-round. Put it together and Rehoboth is ringed and roofed by moisture on every side, with a housing stock too delicate for shortcuts and a majority of owners not present to watch the green arrive.

There’s also a rhythm to it that inland towns don’t have. Rehoboth’s growth calendar runs on the rental season: exteriors get inspected in April when owners and property managers arrive, photographed in June when listings refresh, and ignored from October to March while the salt keeps working. That’s backwards from the biology — the growth compounds exactly during the months nobody’s looking — which is why the houses that stay clean here are the ones on a standing fall-and-spring rhythm rather than a panic-in-May one.

And the stakes are simply higher per square foot than anywhere else in the county. When the median property is worth $1.49 million, exterior condition isn’t cosmetic — it’s listing photos, rental bookings, and appraisal value. A green north wall on a Millsboro rancher is an eyesore; on a South Rehoboth Victorian it’s a line item. So the Rehoboth pattern, plainly: salt-held moisture on everything, most homes empty and growing unwatched, delicate high-value surfaces that demand soft washing over blasting — and a fix that follows the diagnosis: low pressure and a solution that kills the growth at the root on anything delicate, real pressure only on the concrete and pavers that can take it, and a schedule set by your block’s exposure rather than a one-size calendar. That’s not upsell logic; it’s what the map and the census already say about this town.
One more thing the brochures skip: storms. Rehoboth sits exposed to nor’easters, and the town’s modern shape literally comes from one — the Ash Wednesday storm of 1962 wrecked the boardwalk and much of the oceanfront, and the beach has been rebuilt and replenished more than once since. Day to day, that exposure shows up smaller but constantly: a good coastal blow drives salt spray blocks inland, and the film it leaves on windows, siding, and railings is why an oceanside exterior can look dull a week after it was cleaned by rain alone. Wind-driven salt is also why the ocean-facing sides of homes in North Shores and Henlopen Acres age differently than their own street-facing sides — same house, two weather patterns. It’s not a reason to panic; it’s a reason the maintenance rhythm here is a rhythm, and why the houses that always look right in this town are the ones whose owners treat the exterior like the roof — on a schedule, not on a whim.
And if you only remember one thing from all of this, make it the simplest: green that comes back a season after it was blasted off didn’t survive the cleaning — it survived the method. Killing the organism is the job; removing its top layer is just rearranging it. In Rehoboth’s conditions, that difference is the whole difference between an exterior that holds for years and one that’s green again by the next changeover.

All three are living things. High pressure knocks the top layer off and leaves the root, so it grows back — and on the way it forces water behind vinyl siding and strips protective granules off shingles. Soft washing uses low pressure and a solution that kills the growth at the root. It cleans deeper, it lasts, and it doesn’t wreck the surface underneath.