Wood mulch is out. Colorado, get over your obsession with wood mulch.

Here on the Front Range, we receive only 12–18 inches of precipitation each year. Even more important, most of our rain events are tiny. Many produce only 0.02–0.10 inches, and very few exceed 0.25 inches. We don’t receive long, soaking rains like much of the eastern United States. We get short, localized thunderstorms followed by intense sunshine, low humidity, and wind.

This is where wood mulch becomes interesting. A layer of organic mulch has to absorb moisture before that water can move into the soil below. Research has shown that mulch can intercept rainfall, especially during smaller precipitation events. Depending on mulch type, depth, and conditions, it may take roughly 0.25–0.50 inches of precipitation before water meaningfully moves through a 1-inch mulch layer.

That matters in Colorado.

A 0.05-inch storm does not behave the same way under an inch of wood mulch as it does over exposed mineral soil. Many of our smallest storms may wet the mulch layer without significantly recharging the soil. When our entire annual precipitation budget is only 12–18 inches, those small events matter.

So why are we landscaping like we live in Ohio? 1 inch of mulch is death.

Wood mulch has become the default recommendation because it works well in climates that receive 30–60 inches of annual precipitation. Those regions get enough consistent rainfall that wetting the mulch before the soil isn’t a major concern. Colorado is different.

When you place an inch or two of wood mulch over the soil, every rain event has to wet the mulch before it can benefit the roots below. During many of our small rain events, a significant amount of that moisture never infiltrates as deeply as people assume before our dry air and intense sun begin evaporating it away. That’s one of the biggest reasons I don’t believe wood mulch is the best choice for most Front Range landscapes.

Wood mulch makes the most sense when you’re irrigating, especially on drip irrigation, because you’re intentionally applying enough water to get through the mulch layer. If your goal is a truly low water landscape that takes advantage of our natural rainfall, wood mulch begins working against you instead of with you.

Think about where wood mulch naturally exists.

Forests.
Cool, shaded environments.
Leaf litter.
Decaying logs.
Rich organic soils.

Now think about the plants we’re trying to grow in low water Colorado.

Penstemons. Agastaches. Yuccas. Prickly pears. Lavender. Buckwheats. Blue grama. Native wildflowers.

Most of these plants didn’t evolve growing under inches of decomposing bark. They evolved growing through rock, gravel, decomposed granite, sandstone, and lean mineral soils with exceptional drainage. So why are we surrounding prairie and desert plants with a forest floor?

The other issue is weeds.

People think wood mulch prevents weeds forever. It doesn’t. Weeds are part of your life as an descendent of Eastern European travel. As it breaks down, it becomes organic matter, and organic matter is exactly what many weed seeds love to germinate in. Then every few years you’re buying more mulch because it has faded, decomposed, blown away, or washed into other parts of the landscape.

The Front Range has developed a habit of following landscaping recommendations from the eastern United States. The problem is that the science doesn’t always translate west of Kansas. Once you move into western Kansas and eastern Colorado, rainfall drops dramatically, humidity decreases, evaporation increases, and storms become much more localized. Our climate is unique, and our landscapes should reflect that.

For Denver, Fort Collins, Colorado Springs, Castle Rock, and much of the Front Range, I believe mineral mulch simply makes more sense. Gravel, crusher fines, decomposed granite, and other rock mulches allow even our smallest rain events to reach the soil instead of first wetting a layer of bark. When you’re only receiving 12–18 inches of precipitation each year, every drop matters.

Now let’s talk about rock.

One of the biggest arguments against rock mulch is weeds, and honestly, people aren’t completely wrong. But I think the problem is usually the size of the rock, not the fact that it’s rock.

Once you get above about one inch in diameter, weeds become much harder to pull because they establish between the larger stones. That’s why I generally prefer rock that’s ¾ inch or smaller. It creates a cleaner look, is easier to maintain, and weeds are much easier to remove. Some people even claim that rock smaller than ⅜ inch suppresses weeds even better. I don’t have enough experience to confidently make that claim, but I do believe ¾ inch is about the practical upper limit for most residential landscapes.

I think it’s time we rethink how we landscape in Colorado.

Instead of copying landscape practices from climates that receive two to five times more rainfall than we do, let’s start designing landscapes for the climate we actually live in. Less water. More sunshine. More wind. More evaporation. More drought.

Now before everyone comes after me, I’ll say this: wood mulch absolutely has a place.

It makes sense in shade gardens, woodland gardens, around newly planted trees, vegetable gardens, and for plants that naturally grow in rich organic soils.

But if I see weed fabric covered with wood mulch in a Front Range xeriscape, that’s usually a red flag. It tells me someone installed the same landscape they would have built in the Midwest instead of designing specifically for Colorado.

At the end of the day, I’ll choose mineral mulch almost every time for a Front Range xeric landscape. It better reflects our natural environment, lasts decades instead of years, allows our limited rainfall to reach the soil, and pairs far better with the plants that actually evolved to thrive here.

Or better yet…

Plant densely enough that one day your plants become the mulch.