Crews spray green slurry across bare slopes, using wood fiber and binder to help protect soil from erosion while vegetation regrows

When you strip a hillside down to dirt, you create a big problem. The problem gets worse when the first big rainstorm comes. No roots are left to hold the soil in place. No grass is there to slow down the runoff. You are left with earth that will slide downhill as soon as water hits it. That is what often happens after a wildfire. Weeks later, crews arrive with trucks full of a thick green spray called hydromulch, a mixture of water, wood fiber, and a special kind of glue called tackifier, according to the USDA Natural Resources Conservation Service. Sometimes crews also mix seeds into the hydromulch. The goal isn't cosmetic; it's to buy the ground time to let real plants grow back and hold the soil in place while they do. If you live near areas where wildfires happen, like the hills outside Denver or the canyons near Los Angeles, this work matters. It helps keep your street safe from mudslides during storms.

What's inside that green mixture

The green color is a dye, added so equipment operators can see at a glance which areas have already been treated. The trucks or trailers carry large tanks holding water, typically in the range of 1,000 to 3,000 gallons, per NRCS guidelines. Workers mix wood fiber and tackifier into the water, and a pump keeps the mixture continually agitated so it stays evenly blended before being sprayed through a hose or nozzle.


Crews typically apply 2,000 to 3,000 pounds of wood fiber per acre. They also add 75-100 pounds of a specific guar-based tackifier. As a rule, at least 70 percent of the area should be covered by the solution, and the coverage should last until plants grow back. When seeds and fertilizer are included in the slurry, the process is known as hydroseeding. Crews often apply this in a two-stage approach: first spraying the seeds and fertilizer, followed by a top layer of mulch which promotes better seed-to-soil contact than spraying a single combined mix.

Steep, high-risk slopes get priority

Hydromulching does not solve every erosion problem, and cost helps determine where it is used. This method is most common on slopes with a grade range of 20% and 60%. Priority is given to slopes near housing complexes or water bodies, or to slopes that need extra protection. Gentler slopes are usually treated with dry mulch, which is cheaper and works well on less steep ground. There is also another limitation. Ground equipment cannot operate on slopes more than 300 feet from a road or path, since hoses typically run only about 200 feet, limiting how far crews can extend their reach from the truck. Sometimes helicopters are used in areas that are inaccessible, but it is still difficult to distribute the material evenly. That is one reason aerial spraying is less common. Hydromulch is avoided on terrains where more than one-fourth of the area consists of exposed rock, as vegetation cannot easily take root on solid stone.

Does the spray actually work

The research gives a mixed answer. A Forest Service study in the Santa Ana Mountains in Southern California found hydromulch reduced erosion on slopes during light to moderate rain, but during a major storm, it protected far less effectively. Separate Forest Service research found hydromulch performs worse than straw mulch at keeping burned ground in place, and the hydromulch layer itself usually breaks down within a year, leaving the hill exposed again if rain follows. Hydromulch is still useful in some situations, but crews have to weigh cost, access, and the level of protection actually needed.

The safety concerns

Spraying a scorched hillside isn't simple. Crews can't just drive up and start spraying; ground crews must work from roads that remain open to traffic, and they have to confirm the road is safe before starting. Workers wear protective clothing to guard against the chemicals used, and mulch should never be sprayed on concrete or painted surfaces. Burned trees pose their own danger too, since fire damage can make them fall without warning.

What this means once you're downstream

Erosion on a burned slope can stay unstable for a long time. Surface soil, ash, and debris can wash into waterways, reservoirs, and the pipes that supply city water. Very steep slopes create the worst runoff problems, often during the first rainfall after a fire: runoff dangerous enough to destroy property and block road access, as has happened in both California and Colorado during past fire seasons. That's why erosion control, temporary as it is, buys the one thing burned slopes need most: time for grass and shrubs to take root again.