How Ash From Wildfires Will Affect Your Landscaping

As we’ve learned so intimately this year, wildfires spring up suddenly and quickly ravage the landscape, leaving destruction in their wake. While northern California communities were some of the most affecting this year, Portland was not unscathed. We were left emotional by the Eagle Creek wildfire that sparked over Labor Day weekend and has spread to over 48,000 acres in our own backyard, and it’s still only 50% contained.

While we are no longer dealing with the strange appearance of the blood-orange sun that punctured the sky and the ash that littered our landscaping, it still leaves us grieving over our favorite hikes that won’t be the same for years to come.

While the fire still rages on in some areas, it was definitely a surprise to many when even our far-off communities experienced the downfall of ash from the fires, and it left many of us wondering how it will affect our landscaping. While we’re no longer dealing with this rare threat (the last time ash rained down in Portland was after Mount Saint Helens erupted in 1980), it’s still a question that plagues many, especially with the continued deluge of fires across the west.

First, it’s important to remember that wildfires are often a source of regeneration for forests, such as our own. While the Eagle Creek fire was triggered by an unnatural incident, wildfires often help to cycle nutrients in the soil, for example.

Ash, in fact, is made up of organic matter – literally the basic nutrients that plants require. More specifically, it’s made up of nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur. So yes, the plants do absorb the ash, but it’s actually a source of fertilizer when distributed in small quantities. For that reason, in the Portland area, the wildfire ash that fell – while diminishing the air quality for us – was actually beneficial to our trees and gardens.

Similarly, the smog likely protected our plants from the last of the intense summer rays, even warding off drought. While the particles and gas burn our eyes and worsened chronic health conditions like asthma and heart disease, our plants, on the other hand, were actually thankful for these unexpected conditions.

However, just like we shouldn’t overeat, neither should our plants. So, areas closer to the actual fire that received higher quantities of ash likely received too much nutrition.

Similarly, the landscaping that was directly affected by the fire has a completely different relationship with the events that took place. First, the effect is related to how fire tolerant the plants in the region were, as well as the intensity of the fire.

Additionally, while the death of some plants is inevitable, the effect of the fire on the plants’ roots is a major indicator as to whether it will survive.

It’s also important to consider the effect on the soil itself. While ash and fire both alter the landscaping, an important component is the viability of the soil afterward. In fact, the worst fires are detrimental to the soil organic matter as it’s this organic matter that holds together the sand, silt, and clay giving the soil structure. And this structure is essential to plant life.

Soil structure determines how much water the land can hold, leading to soil that is too coarse and porous to sustain life. While this is a terrible effect on flat land, it’s even worse on steep terrain as it can lead to erosion. And, the immediate effect on the land is often unknown as some burned regions will bounce back within a season, while others will take decades to regenerate. Unfortunately, the outlook for the Eagle Creek fire is grim for those of us who enjoy hiking and camping in that region, as “persistent soil degradation following fire is more common in cold and/or arid climates typical in the western U.S.”

While our landscape is our main concern after a wildfire, another immediate concern is our water quality. In fact, during the Eagle Creek fire, there was once a real and immediate threat to the Bull Run Watershed. Thankfully, “less than 1 percent of the Bull Run Watershed Management Unit has burned at the northern boundary,” according to a September 15 update by the Portland Water Bureau. This is equivalent to 100 to 150 acres, they said, adding that the fire affected the Management Unit, but not the actual Bull Run Watershed.

While our drinking water was not affected by the fire to date, it is a true concern, which is why nutrient and sediment input is closely monitored following a wildfire occurrence. According to the Washington State Lake Protection Association, an increase in nutrient and sediment input is affected by soil burn severity, topography, and weather variables like wind and rain. This all affects the soil, leading to increased erosion and potential water contamination.

Within weeks to months after a fire, nearby water is vulnerable to a spike in nitrates, which then slowly declines, as nitrogen transforms into ammonium as airborne ash. The ash, as discussed, can also carry phosphorus. This is a concern as this organic carbon can negatively interact with chemicals used to treat drinking water, causing carcinogens.

While it’s been determined that our water quality is not a concern today, the fire remains a threat, even though the ash in our neighborhoods is long gone.

Wildfires are devastating when not ignited by a controlled burn. They whip mercilessly through regions, quickly destroying our landscape, homes, and even taking lives. They are extreme forces of nature that we can only slowly contain, but never truly control.

As for our landscaping, it will be just fine, maybe even better. But, the main threat we face today is the continued concern of land erosion and water quality deterioration.