Wet-Season Erosion Control in Portland, OR

Wet-season erosion control in Portland is about the months when it hardly stops raining. From November through January the airport averages more than five inches of precipitation a month, and rain often falls as days of steady drizzle, contributing to about 157 days a year with measurable precipitation. A slope that looked stable in September can lose its surface by Thanksgiving. As erosion control contractors we work for hillside homes, builders and public projects: hydromulch, bonded fiber matrix, erosion control blankets, wattles and seed, chosen for the grade and the time of year.

Matching the cover to the slope

On gentle ground, a standard wood or paper fiber hydromulch with tackifier, sprayed with seed, gives enough protection while grass establishes. As slopes steepen, the mulch has to bond tighter. A bonded fiber matrix forms a continuous crust over the soil and resists raindrop impact without needing the seed to root first. On the steepest cuts, banks along drainageways and swales that carry concentrated flow, we pin blankets over the seeded soil: straw alone, a straw and coconut fiber blend, or 100 percent coir.

Hillside lots

The West Hills, the buttes in the east county and the bluffs along the rivers are where we do most slope stabilization. Portland's own rules recognize the risk: in environmental zones within the Balch Creek and Forest Park subdistricts of the Northwest Hills plan district, ground-disturbing activity is not allowed at all from October 1 to April 30. Elsewhere, hillside work that runs into fall needs a cover plan ready before the rain.

Swales, ditches and channels

Water that concentrates in a swale or roadside ditch cuts a groove faster than sheet flow ever will. Hydromulch alone usually washes out of a flowing channel. We line swales and ditches with blankets stapled in check slots and seeded underneath, or with turf reinforcement where the design calls for it, and seed with a mix that tolerates both standing water in winter and drought in August.

Wattles and slope length

Long uninterrupted slopes build up runoff speed. Straw wattles, trenched and staked across the slope on contour, break that length into shorter sections and trap the sediment that does move. We usually combine them with hydromulch or blankets rather than relying on wattles alone.

Timing matters as much as material

The ideal is to seed a slope in late summer so roots are established before November. Grass seeded in late August or early September has weeks of warm soil ahead of it, while grass sprayed in December may sit dormant until spring. When work finishes late, we use heavier hydro mulching or blankets for the winter and come back to overseed thin spots in spring.

Pricing

Erosion control is priced by area and material. Mulch and blanket rows in the table come from a DOT unit-rate sheet (2025), and the slope surcharge comes from a published residential hydroseeding guide. Once you sign off on our written figure, that number holds and nothing gets tacked on later. Get a free estimate