Wednesday, September 29, 2010

Stone Mulching in the Desert

Some years ago in rural Michigan a forested area was cleared and all the trees except three young hemlocks were cut down. These small trees were left to manage for themselves. In the forest where they had been protected from wind and excessive heat, they also had to compete with other trees for basic nutrients. Yet they had managed to do alright.

Left on their own, however, in a bulldozed area, two of the three trees died within a year. The third tree ended up flourishing and became a beautifully shaped and canopied tree - quite different from typical forested hemlocks.

The only difference between the trees was that the surviving tree, quite by chance, happened to have a load of loose rocks dumped right next to it soon after the land was cleared. In the months following, the rocks kept vital moisture from evaporating near the tree and many worms and insects were able to enjoy the clement conditions they provided. These small creatures worked the ground into a nourishing environment for the roots. This couldn’t have happened where the ground was dry.

This is just one example (and there re many) of the value of mulching with stones. Unfortunately these discarded resources are usually considered only in the context of how to get rid of them. This is too bad. Stone mulches offer a lot of benefits. But they are particularly helpful in dry areas.

You have probably learned by experience that the soil under rocks can be moist when everything else is dry. Some people use this fact to calculate watering times for their garden. When it’s dry underneath, it’s time to water. And, in fact, the most important benefit of having stones around plants is that it keeps water from evaporating too quickly.

This can make a very big difference to a plant. In many dry areas, water usually evaporates quicker than roots can grow. Since soil dries out from the top first, plants have an incentive to grow roots as deep and as fast as possible, keeping their feet wet, so to speak. Once the soil is dry around the roots, a plant has very little time left to produce seeds before it dries out. Its entire life cycle is dependent on a few weeks of moisture every year. (This is why many desert plants have thick leaves and roots. It allows them to store more water when the rains do come, and it buys them more time to develop seed and fruits when it becomes dry again.)

But what happens if a non-succulent plant happens to be growing next to a stone in the desert? Initially the amount of rainwater may be the same as before. But because the stone slows down evaporation, it allows the plant’s roots to grow for a longer period – essentially keeping up with the rate of evaporation. Plants with deeper and better developed roots are bigger, healthier, and set more fruit. It’s no surprise that plants growing next to rocks often look so good.

Of course there are other benefits to stone mulches than just water retention. Roots often grow next to stones because it’s a place to leverage growth. It’s also a place where rainwater (or sprinkler water) seeps into the soil first. The small space left from this seepage also allows worms and insects to move easier, creating a vital microclimate for roots.

Of course the use of stone mulches is not the only way to build soil. Other mulches do too. But stone mulches have a bit more value in dry areas than other mulches do. Take, for example the experience of Dorothy Anderson.

Dorothy lived in Wisconsin a couple of generations ago. Now Wisconsin is not the driest place in the world but the summer heat can often dry things out. Dorothy was also an avid gardener and paid attention to how her neighbors did things. When she learned that mulching (with hay, weeds, etc.) was bringing bountiful harvests to others, she was determined to do the same thing herself. But then she ran in to some difficulty.

She didn’t have a lot of leftover plant material to use as mulch and when she put what little she could find in her garden, it just dried up and covered the dry ground. She didn’t get much benefit from it. It took several years and a lot of foraging to get enough weeds and other organic matter to really help her garden.

For those of us living in the dry Southwest, we certainly understand this kind of problem. In fact the problem is a lot worse for us. Not only is it hotter and drier but there’s less plant waste to go around. If we really want to mulch, we often go to the hardware store and just buy it. And if we don’t put out enough, it doesn’t do us any more good than the little Dorothy started with.

The situation changes, though, if we use stones. Place a handful or two of grass (or straw) mulch on the ground here in Fresno and it will quickly dry up and get blown away. But if you put the same handful on the ground and place a stone on top of it, things change. The most obvious thing is that the mulch stays there. It’s also shaded and small insects will crawl under the stone to get out of the sun. If moisture is added it will stay near the soil surface much longer than in surrounding areas.

A study conducted in the 70’s in the desert Southwest showed that moisture evaporates from bare soil at a fairly even rate of about an inch every three days. Under the same conditions, moisture evaporated from a stony area at a rate of about an inch every two weeks. This added moisture is as good as gold to plants in dry areas. It also creates an environment for soil-building organisms such as earthworms, arthropods and even fungi. A flat stone in the desert is a way to build soil if we know how to use it.

One of the interesting histories of the Southwest is the agricultural use of stone mulches by the Anasazi. Dale Lightfoot at Oklahoma State University has evaluated these mulches extensively in dozens of abandoned farming areas near Santa Fe, New Mexico where these mulched areas can still be identified - over 700 years after they were made. The areas show up clearly using aerial infra-red photography because they are greener than surrounding areas. These erstwhile garden sites are noted for their regular arrangements of cobbled stones with borrow pits from which the stones were taken.

Lightfoot concludes that these stone mulches not only increased (and still increase) soil moisture but that they also reduce erosion, extend the growing season and increase crop yields. This is quite a list for a dry country not known for its lush gardens.

The biggest drawback of these stone gardens is that they are not sustainable. Whatever nutrients can be found in the soil are used up by successive crops so that new areas have to be prepared every several years. In China, where stone mulching has been used (as recently as a century ago) this problem was understood to affect the children and grandchildren of farmers who would have to extend significant resources removing stones in order to work nutrients back into the soil.

This problem has not been overcome. It is one of the main reasons that stone mulching is not practiced commercially on large farms. It just isn’t practical to remove several tons of rocks from a field and then to turn around and replace them after working the soil.

But that said, stone mulches still have their place. In fact they should be more widely used in dry areas. We know a few more things about nutrient cycling today than did the Anasazi or Chinese of former times. We know, for example, that stone mulches can be sustainable if organic material (such as cut grasses, straw, fallen leaves, etc.) is placed under stones each year.

Modern gardeners who do this use larger stones than the Anasazi did - since it’s easier to move them. Flat stones are also preferred to round cobbles. Various kinds of composts are placed in a garden spot with rows of flat stones (roughly the size of salad plates) covering the compost. Plants are then allowed to grow between the rows of stones.

For trees, a thick layer of compost with stones placed around the trunk does the same thing. It only takes a little effort to remove the stones once or twice a year and add more compost, and then replace the stones. The total effort is less than that required for weeding - which, of course, is no longer required. And the results have been impressive. Difficult soils are improved with the arrival of worms and insects, moisture is retained; and, most importantly, plants are much happier (if we can use that word) and more productive.

Stone mulching may not be a realistic possibility for farmers whose livelihood depends on their harvest (although creative orchardists could likely make it work). But on a smaller scale, and for those of us who care about sustainability, it makes a lot of sense - especially out here in the desert.


Lightfoot, D.R. 1994. The Agricultural Utility of Lithic-Mulch Gardens: Past and Present. GeoJournal 34(4): 425-437.

Lightfoot, D.R. and F.W. Eddy. 1995. The Construction and Configuration of Anasazi Pebble-Mulch Gardens in the Northern Rio Grande. American Antiquity 60(3): 459-470.

Rodale, J.I. 1949. Stone Mulching in the Garden. Rodale Press, Emmaus, PA.

Rodale, R. et al. 1972. The Organic Way to Mulching. Rodale Press, Emmaus, PA.

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