• Andie Marsh

Let the Weeds Speak

Updated: May 23

Weeds as Indicator Plants

You might be familiar with the concept of weeds being indicator plants. The gist being, nearly every weed can be viewed as a representative for what’s missing - from a nutrient and biology standpoint - in the rootzone.

For example, when I see Dandelion I want to know more about the calcium content of the soil as Dandelion is particularly adept at not only growing in calcium-poor soils but also uptaking calcium from its taproot and complexing it into its plant cells. Then, when it dies it decomposes and deposits plant-available calcium into the soil. If that’s the case it would seem Dandelion is working itself out of a job and place to stay! Nature is funny that way and humans are funny in a different way…

Our Perspective vs. Nature’s Perspective

We seem to see aggressive behaviors and competitive intentions threatening our crops, when really there is a complex ecosystem at work. Nature works through a matter of succession - an endless process of building upon itself. We tend to work through a matter of control - an endless chase, constantly bringing us back to where we started.

Reconsidering our Relationship with Weeds

I have a few cases to make to help us better understand weeds…

  • Fungi:Bacteria Indicator: weeds reflect to us the type of ecology operating belowground. In general, weeds and a bacteria-dominant (low F:B) rootzone work in concert together. The Fungi:Bacteria ratio increases with the maturation of the landscape. For instance, the flip side to weeds is a forest which thrives with and supports a fungal-dominated rootzone. This correlation can support us in reading our landscapes - if our soil system is supporting weeds and we want it to support a higher-successional plant like most vegetables, bushes, or trees, we know we could benefit from adding more fungi and fungal foods. Microscopy is a tool we can use to ensure the inoculum or amendment actually has beneficial fungi in it.

  • Chemistry Indicator: weeds can also indicate nutrient availability or lack thereof. One useful source is USDA’s Dr. Duke’s Phytochemical and Ethnobotanical Database - you can look up nearly any plant and learn its phytochemical makeup which gives us insight into what minerals these plants take up and subsequently make available to the topsoil. It’d be interesting to look at this data alongside a standard soil nutrient analysis.

  • Protection: in an otherwise vegetated landscape, weeds tend to colonize bare ground or areas suffering from erosion. Their canopy, roots, and root exudates all provide protection to the soil to prevent it from baking in the sun, blowing in the wind, or running off. By letting the weed fill its role, we know we’re at least allowing ecology to move forward. Alternatively, when we pull or spray the weed, we can know we’re keeping the aboveground and belowground ecosystem at the current ecological stage (i.e. we’re preventing progress and repair), guaranteeing weeds season after season, year after year.

  • Water Retention: on a similar note, weeds can actually contribute to water absorption and retention in the soil. As we often think defensively, we might be tempted to say “hey! If that weed is taking up water and nutrients, that means there’s less to go around to my crop!” But think about it: roots of any plant draw rainwater down into the soil, the canopy mitigates evaporation of soil moisture to the atmosphere, their exudates are contributing to aggregate formation which acts like a sponge belowground… The vegetation of a rainforest essentially creates its own weather to an extent, the evapotranspiration from all of those leaves adds moisture to the air which eventually condenses and returns as rainfall. The more leaf surfaces, the better the water cycle works. Ideally we’d want all those leaf surfaces to be desired plants, right? But if we’re growing them in such a way that bare ground is exposed at any time, we can consider the weeds to be providing a service.

  • Decompaction: many weedy species have tap roots which are capable of penetrating soil that is compacted, by doing so the root effectively creates a channel for air and water to move through the soil profile. And again, the roots are also supporting soil microbes which are necessary for improving soil structure through aggregate formation.

dandelion doin' their thing on a recently disrupted soil

Landscape Nurturing > Weed Management

Now, even if we can start appreciating weeds a bit more, it’s likely we still want to work towards a weed-free future for our crops. We just want to be thoughtful in how we choose to do so. If we choose to till we are guaranteed to set our ecological succession clock back every single time we do it. Same if we choose to use herbicides as this also harms the ecology belowground, not through mechanical disturbance like the tilling, but through chemical interactions.

Even still we may find an herbicide or tilling may be the best course of action in some very specific instances. The important thing is to make such a decision from an informed and long-term perspective, not from a place of impulse or fear. When in doubt, in the early stages of transition to a weed-free landscape, we want to find ways to outcompete weeds (think short-growing perennial cover crops), and leave the weeds that do pop up, perhaps thinning them manually if they crowd out sunlight to a crop.

We can let the indicator plant itself be the informant of how to respond to it. If we find a certain prolific weed to be associated with a particular soil chemistry or nutrient deficiency, we can investigate that further with some conventional soil testing and an amendment that may nudge the soil system in the direction we want it to go. Always keeping in mind that we don’t want the amendment itself to harm the soil life we so depend on for long-term fertility and plant health (no salts!). We’re not aiming to overcorrect, we’re just aiming to tip the scale in favor of the desired chemistry and let the plants and microbes work with us to take it the rest of the way.

If a target crop / plant is considered a higher successional plant and we’re seeing many weedy species, we can help the soil system along by inoculating with and providing food for fungi (think mulch!). If we left the landscape to its own devices it would naturally become more and more fungal over time. When we inoculate we’re again tipping the scale in favor of the plants we desire - introducing new partners underground and on leaf surfaces.

With all this scale tipping, the weedy species become less and less supported belowground and they’ll become the plant that’s susceptible to pests and pathogens, while the vigor of desired plants increases under their preferred conditions. That is nature’s way of responding to good stewardship :)

Are you curious about the compost or biological amendments you’re using? Are you ready to get a read on the state of your soil biology? Reach out to me here or at andie@rhizos.science to discuss if microscopy testing could help you move closer to your stewardship goals.

For more food for thought, you may enjoy some of my recent newsletters all about disturbance and weeds.




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