Dirt as Mulch

By Carolyn Dunmire

Growing in the Weatherhill: An occasional column from the southwest corner of the state where we grow with the dryland traditions of the Colorado Plateau. The Weatherhill is known for its vivid red color, consistent ability to support dryland crops, and as a foundation material for adobe brick.

Above: Sleeping Ute Mountain seen from Cortez   PHOTOS: Carolyn Dunmire

When I started my garden in the Weatherhill loam more than 25 years ago, one of the first things I did was a soil test. The results showed a well-balanced loam with average pH and good mineral content. What it didn’t include is how to enliven this red clay dirt to support a thriving garden and orchard during the coming years of drought, increasing temperatures, and Wizard-of-Oz worthy dust storms. Over the years, I’ve had some spectacular successes such as dryland rhubarb and asparagus, and dismal failures like the year when grasshoppers ate my entire garden including the spectacular rhubarb. Through it all, I have adhered to an organic ethic and experimental spirit that keeps me growing as well.

My garden is surrounded by dryland bean fields that miraculously emerge each June without the benefit of any surface moisture. The secret lies not in the bean seed that is planted, although the Cahone bean is adapted to our local growing conditions, but in how it is planted.

This year my new growth is focused on transitioning my garden to no-till. This is a formidable challenge because my first gardening experience was in the now famous Boulder Community garden in the late 1980’s where I dutifully employed the trendy French double-digging technique. The deep spade work yielded encouraging results and a sore back, even then. Now I have to reverse my thinking to employ a much less invasive method for managing the soil that saves my back and the critters and fungi that I have so lovingly curated here. The hurdle I have yet to clear is the type of mulch to use in a place where dryland traditions dictate that bare dirt is best.

My garden is surrounded by dryland bean fields that miraculously emerge each June without the benefit of any surface moisture. The secret lies not in the bean seed that is planted, although the Cahone bean is adapted to our local growing conditions, but in how it is planted. The farmers will “drill” to a depth of up to a foot to get the bean seed started in the damp soil beneath the dry red dirt. Hopi farmers, the native dryland farmers of the Colorado Plateau, developed this planting strategy for their corn fields centuries ago, planting a handful of blue corn seeds at the depth of soil moisture. The resulting cornfields have bunches of corn plants widely spaced across a sandy wash looking nothing like the columns of corn soldiers found in the Midwest. Both the bean and corn farmers are diligent about eliminating any competition in their fields by tilling frequently during the growing season. Plant growth of any kind is not tolerated outside the clumps or rows.

On the surface, this looks like the opposite of no-till. But after several failed attempts at using other types of mulch, I believe it is an ingenious adaption of the “when life gives you lemons” approach to mulching. My first attempt employed weed cloth to protect tender shrublings as part of a reclamation project to convert a dryland alfalfa field into wildlife habitat. We were required to install weed cloth to discourage weed and alfalfa regrowth. After days of back breaking work and frustrating hours with a cloth-burying contraption attached to a tractor, we had 10 rows of seedlings tucked under a few acres of black ground cloth. In no time at all, weeds grew up in the openings for the shrubs and the first windstorm untucked much of the cloth so it was soon flapping in the wind. Within a year, the intense ultra-violet sunlight at 7000-feet elevation degraded what remained into tatters. Mercifully, the remnants of the ground cloth have since been buried in several inches of red dirt.

Above: Part of the Dunmires' garden where they grow a wide assortment of vegetables. They also tend an orchard with 200 fruit trees.

In the garden, I attempted to use abandoned alfalfa hay as mulch, in part to create a walkway between the garden rows during our infamous mud season and as potential weed deterrent. This mulch experiment proved to be a failure. The hay mostly blew away in the spring winds and what remained was mashed into the red clay soil creating adobe bricks. Weatherhill loam has the characteristics of many fine-grained clay soils in that it “seals off” causing rain and irrigation to “run off” rather than percolate into the soil. Introducing woody compost is one solution, however it also is the same recipe used for making adobe brick. Most recently, I tried a cover crop thinking that mulch that anchors itself might be the solution. However, planting anything late in the growing season at this elevation can prove challenging from both an irrigation and hardiness point-of-view. My first cover crop was a partial success in that I had good germination and growth. Perhaps too good, because the experiment turned into a persistent weed problem. I obviously need to learn the art of solarizing.

The truth is that dryland farmers on the Colorado Plateau have successfully exploited the difference between dirt and soil for centuries. Dirt is mulch because the living soil is buried beneath it in the ground moisture. Implementing no-till practices with dirt mulch presents the challenge of managing this mulch when surface moisture arrives, and weeds inevitably sprout. Dryland practices demand that all competition be eliminated with light tilling. In practice, this tilling does probably not violate the principals of no-till since it only affects the dirt mulch.

However, as our climate becomes hotter and drier, reliable ground moisture is largely unreachable, putting our dryland farming heritage in peril. For the first time in memory, local farmers did not plant dryland pinto beans in 2018 because there wasn’t enough ground moisture. As predicted, with the advent of irrigation in the dryland fields the time has come for new farming and gardening practices that manage and protect our living soil. That means finding a mulch that works with the red dirt while suppressing weeds, preserving the living soil, and minimizing soil disturbance. Thicker flakes of hay might work in the garden, or a cover crop may yet prove to be a viable mulching strategy with the right seed and irrigation plan. Some dryland farmers are now leaving wheat stubble in the fields.

The shift from blown-in dirt as mulch to a no-till environment has a ways to go here in the dryland-based fields and gardens of the Colorado Plateau. We understand the need for healthy soil. We just need some ingenious growers to discover how to protect that soil with a mulch that is cheap and easy to manage while discouraging weeds and preserving every drop of moisture. If dirt were dollars…

Carolyn Dunmire has been a Colorado gardener for more than 30 years, most of that time in the Weatherhill at 7000 feet. She and her husband have 60 acres where they grow a wide variety of fruits and vegetables year-round in a large outdoor garden, a greenhouse, a hoop house, and an orchard with 200 fruit trees.