Reading Your Weeds: What Do Weeds Really Tell Us About Soil Conditions

Broadleaf Plantain Credit: USDA

Years ago, when I was a news reporter at a weekly paper, an editor yelled in my face that I wasn’t fit to be a journalist because I was too easily “spun.” It wasn’t the first time this guy had wigged out at my inability to see the world in the same black and white way that he did. But it was the last. Maybe I am easily spun. I didn’t deny it. I prefer to think, that I can usually tell the difference between someone who is selling something and someone who is offering their informed opinion—whether I agree with them or not.

Sure, it does mess with a well-defined story idea when research and sources don’t take you in the direction you thought you were going to go. But that happens sometimes. In fact it happened with this story I’m posting here, which is a longer version of a recent article I wrote for Norther Gardener magazine about using weeds as soil indicators.

I have read and heard for years that weeds can be good soil indicators, and gardeners who understand what their weeds are saying can remedy soil problems accordingly. “Read your weeds,” people often say. I never paid the idea much mind. But last summer, after hearing that advice for what felt like the zillionth time, I decided to look into it.

Creeping Charlie (looks a lot like early garlic mustard)

I found many articles connecting certain weeds with particular soil types and conditions. Everything was fairly straightforward: Got lots of dandelions? If you do, that indicates this soil type. To remedy the situation do this or that. Great! I thought, imagining myself writing an informative how-to article on the topic.

Oh boy, was I wrong. As I did more and more research, I ran into several issues that forced me to say goodbye to plans for a straightforward story. (1) Many articles were decades old and cited few if any sources. (2) Current stories repeated much of the same information without citing sources either. (3) Charts supporting the association between certain weeds and soils lacked sources, too, and had clearly been used over and over with minor adjustments. (4) New charts, which I did come across sometimes, often contradicted previous charts, so where one associated spotted spurge with poorly drained soil another would connect the weed to acidic soil.

Hmm. Obviously publications had recycled the same information, but it was hard to say how they knew it was true. Folklore, scientific research, anecdotal reports; I just wanted to know how the meat got on the bones of those stories so I could frame my piece correctly. Time to call the scientists.

Not Much Science There

Nobody I interviewed disagreed with the basic premise that, just by growing where they do, weeds can tell us a lot about soil type and conditions, such as drainage, nutrient deficiency and pH level. What’s in question, says Robert Becker, a weed scientist and professor of agronomy at the University of Minnesota, is what to do with the information gleaned from the presence of certain weeds. “Weeds are wild and resilient,” he says, “so it’s hard to know if the weeds you’re seeing are there because of the soil or for some other reason, like birds pooping as they flew over.”

Lamb's Quarters Credit: weedscience.org

There is, however, a good deal of anecdotal evidence to support the belief that some weeds prefer certain types of soil. Moss, as we all know, can be a good indication of poor drainage. But it can also be a sign of compacted soil or low fertility. Pigweed, Becker offers, prefers soil high in nitrogen where clover prefers nitrogen to be lower. Dandelions like heavy, acidic soil. But should you really read your weeds and add lime, sulfur or other things to remediate soil problems as many of the charts suggest?

Becker advocates managing overall soil health rather than honing in on particular deficiencies. He advises getting a soil test to see if your weeds are telling you an accurate story. If you need to amend a small area to grow something specific, like blueberries, that’s fine, he says. Otherwise, Becker thinks people should try harder to work with the type of soil they have and “stop letting weeds drive them crazy.”

Like Becker, University of Minnesota agronomy professor Don Wyse is wary of advice suggesting gardeners should add specific nutrients in response to the presence of certain weeds. “I’ve looked online and there is a lot of misleading information out there on this subject,” he says. “But it’s not bogus that certain plants really do well in a given environment and their presence tells us something.”

Charts, Disclaimers and Common Sense

I can’t say when people first started making connections between weeds and soil types, but Bud Markhart, a professor of organic horticulture at the University of Minnesota, helped me track down the chart from which all other weed/soil charts may have sprung. Created by Ehrenfried Pfeiffer, a German soil scientist and biodynamic farmer who studied under Rudolf Steiner, the chart first appeared in Pfeiffer’s 1951 book Weeds and What They Tell.

As a follower of biodynamics Pfeiffer, who died in 1961, took a holistic approach to soil management. In the chart he included in his book, which was reprinted in the latest issue of Northern Gardener, Pfeiffer noted the relationships between certain weeds and soil types and then offered up possible responses. For example, sorrel, docks, fingerleaf weeds, lady’s thumb and horsetail all indicate high pH, he noted. Suggested actions gardens could take included: adding non-acidic organic matter (no conifer chips), planting cover crops, improving drainage and/or adding lime.

Unlike many of the sources for this type of information that I found during my research, Pfeiffer was careful to add a disclaimer to his chart. “This chart will help you learn to recognize and respond to what the weeds are trying to tell you. These recommendations are not absolute; they’re just representative of common patterns. Don’t rely on this or any book to guide you, but let these clues help lead you to the appropriate action for your specific situation.”

While Markhart agrees with many of Pfeiffer’s associations between weeds and soil types, he strongly cautions against following any “add this and you’ll cure the problem” approach to gardening. “That’s the way commercial agriculture handles soil problems, and from an organic perspective I would suggest a more long-term approach to creating healthy soil.”

He advises, sticking to tried-and-true strategies for soil management, such as adding compost and other organic matter while disturbing the soil as little as possible when cultivating. If you really want to find out what’s going on with your soil, get a soil test, he says.

Living with Weeds

So what’s the upshot here? Got a site with a whole mess of dandelions on it, indicating acidic soil? Seize the opportunity to pluck out those unwanted invaders and replace them with acid-loving plants like blueberries, azaleas or hydrangeas. Troubled by red clover or lamb’s quarters? Fill their spots with vegetables that thrive in very fertile soil, such as broccoli and squash.

Sure those weeds might be giving you a bum steer, their seeds having been dropped off by a passing bird. But if they’re telling the truth about the soil they live in, you’ll do no harm by following their advice.