I am a weed watcher. When an ordinary person walks around a diversified farm, they look at the crops, ignoring the weeds as background noise to the main event. I, on the other hand, pay as much attention to the weeds, registering them as individual species, each with its own personality. In my studies of agroecology I learned that weeds are early pioneer species that can have roles in improving the soil and providing nectar and fodder for insects and other animals. I also came across statistics that herbicides are the most widely used pesticide in conventional agriculture, while in organic agriculture, weeding is the most time-consuming task. This is just the kind of dramatic tension that makes for a complex and engrossing story. I wanted to find out more.
In the 1970s Juan Carlos Chacón and Steve Gliessman carried out a study of traditional farmers’ views of weeds in polyculture systems in the Mexican state of Tabasco. There the practitioners do not have a concept of weeds, but rather of wild plants, which can be good or bad. The good ones might be medicinal or fodder for animals, or they may be valued for their soil-improving qualities; the bad ones might be sharp-edged or give the impression of heating the soil. Interestingly, there was little agreement among the respondents regarding which species were good, even though all agreed that there were wild plants that fall into this category.
Chacón & Gliessman acknowledged the insights of an earlier book, Weeds: Guardians of the Soil by Joe Cocannouer. Cocannouer was a weed watcher, and his observations coalesced into a picture of weeds as indicators of soil conditions. In his view, an infestation of field bindweed, known for both its miniature white version of morning glory flowers and its implacability in giving up its territory, indicates undecomposed organic matter under the soil surface. By this reckoning, if the soil condition were corrected, the bindweed issue would be resolved. When the soil reached a good state of tilth, it would be characterized by a community of easily controlled soil-improving weeds like pigweed.
Yeah, pigweed. The one that can produce 100,000 seeds if allowed to mature. The one that can re-root if pulled out and left on moist soil. I set Cocannouer’s musings aside as food for thought. I did, however, begin to make my list of good and bad weeds. Some bad ones are obvious. Perennial weeds like yellow nutsedge, bermudagrass, johnsongrass, and of course field bindweed are particularly problematic because they grow underground stems and bulblets that can spread when a field is plowed and quickly resprout. The worst of these, johnsongrass, has rhizomes so tough that it is reported to break the shovels of those who try to dig it out, plus it is known for allelopathy, the release of compounds into the environment that interfere with the growth of neighboring plants.
One bad weed in a category of its own is puncture vine. The name refers to its dry fruits, which are like thumbtacks that always point upward. It is reported to grow along railroad tracks, a habit that mystified me at first. How could a weed disperse by sticking its seeds to the steel exterior of a train using points made of straw? Then it hit me – bicyclists traveling along the tracks pick up these stickers with their tires. The tire goes flat, the cyclist negligently tosses aside the sticker with its seed, and the plant sprouts in a new place along the track. This to me seems like proof of the existence of evil – for a seed to disperse, a bike tire must be sacrificed. Needless to say, where one finds evil, one must act. Whenever I see those dark green feathery leaves with those yellow flowers, I dismount, uproot the monster, and send it to the landfill. I have gotten so good at latching onto those visual cues that I once braked hard just as I was about to ride over one, before I even knew why I was stopping. But I digress.
The real challenge is finding weeds that could fall into the category of “good”. Walking with my head bowed in thought, I noticed a certain weed that grows in sidewalk cracks and unpaved parking lots – places where it thrives in spite of the constant trampling. Common knotweed is not common in planted areas, and it grows low and slow, so it is less likely to interfere with crops or ornamentals. It provides greenery where plants are not supposed to be, and it persists without rain. These simple qualities suggest small ecosystem services, such as capture of carbon from the atmosphere and a long taproot that pulls up deep nutrients and loosens the soil for rain infiltration. In addition, a study by entomologist Bob Bugg (his real name!) found that the flowers of knotweed provide nectar to tiny beneficial insects that use the sugar to fuel their search for prey, which consist of similarly tiny pests of plants and their eggs. This nectar source is so important to the tiny beneficials that they only venture a few feet out from it. I also saw a report of knotweed using allelopathy to displace bermudagrass in a lawn, although that may have been a different knotweed species.
Capture of carbon and nutrients and loosening of soil are qualities of plants in general. A portion of the captured carbon makes its way to the soil, which becomes stabilized by microbial action on that carbon, improving the soil. It all sounds like the job description of a cover crop. Could weeds be the lazy person’s cover crop? Maybe in some cases, but there is more to consider.
First of all, a cover crop is not supposed to interfere with the main crop. This condition is normally fulfilled by terminating the cover crop before it goes to seed. Many weeds buck this practice by going to seed early. Then of course there are the dreaded perennials with underground structures that persist or thrive in spite of many of the usual control practices. Second, cover crops commonly are chosen because of agronomic functions they perform, such as providing nitrogen to the soil, carpeting the soil with a thick organic mulch, or suppressing weeds through allelopathy. There are weeds that can do these, but their raison d’être is to make more copies of themselves, and they are often able to accomplish their goal while skimping on the agronomic services. So perhaps they could be considered a less-than-ideal cover crop for the field manager in a rush.
But there’s an additional piece. The science of cover cropping has advanced in the age of microbiome analysis. The soil microbiome is responsible for nutrient cycling – the release, transformation, retention, and availability of minerals that plants need. It transforms organic matter to stabilize soil structure, and it can even suppress plant diseases. Plants, in turn, feed the soil microbiome with organic carbon from their roots. Whereas the traditional deployment of cover crops has involved one to a few species, the work of researcher Amélie Gaudin is indicating that the real microbiome synergy kicks in when you reach eight functionally different plant species. If your planted mix is made up of only a half-dozen species, perhaps those oddball weed species, maybe cheeseweed or fiddleneck, red maids or henbit, could get those extra cylinders firing and get the microbiome over the finish line. Nice work, just as long as they’re not dense enough to significantly interfere with the planted crop.
Back to the good weeds, what could be another weed that doesn’t interfere with a crop, that’s good for more than just being a plant? Hmm. When I grew a crop of pumpkins, I noticed chickweed growing in the moist soil deep in the shadow of the crop leaves. It is a low grower and clearly not harming the crop, but the main thing is, it’s delicious! The leaves and stems are crisp and succulent with a mild taste, making it an excellent salad green. Having it already established by the time the pumpkin leaves die back at the end of the season is akin to undersowing a cover crop, a practice that gives the cover a head start on providing its agronomic services. The chickweed may not be great at putting on biomass, but at least it’s there, as long as you don’t eat it all.
Now for some of the dramatic tension that I referred to earlier. In this scene, I find myself working on some greenhouse experiments, and the greenhouses have an intractable infestation of oxalis. The leaves of this plant look like shamrocks, but it is not closely related to the Irish clover. The oxalis fruits are little pillars that have ridges which from above look like a five-pointed star. The fruit has a surprise, though – when you touch it, it shoots brownish-red seeds in all directions. Adding to the scare, the seeds look like tiny insects, or maybe ticks, on first glance. The plant also spreads by rhizomes, so you can think you have pulled it out, only to have it resprout. Without sustained effort, the oxalis can take over all the pots, and when it comes time to pull up the experimental plants for measurement, the tangle of oxalis roots will makes your hand hurt from all the effort cutting through it. Oxalis has earned its spot on my list of bad weeds.
Cut to the next scene, where I have moved into a place with a yard. The landscaping has been neglected, with the back lawn consisting of only some clumps of tough grass separated by bare spaces containing what turn out to be an accumulation of weed seeds. In the front, the lawn is fully green but has a good number of weeds, including that same species of oxalis that plagued me in the greenhouse. One of my first tasks is to pull the oxalis out, but as I begin the task, a tiny iridescent fly comes to visit one of the tiny yellow oxalis flowers. This is during the season when nectar sources are scarce, so I confront my urge to keep pulling. I have no experimental plants in the lawn, so there is little to gain by keeping it free of oxalis. Instead, the weed is performing a service of supporting tiny pollinators. In this new context, oxalis moves off the list of bad weeds and finds a tenuous spot on the list of good.
In a later scene, the opposite move happens in the backyard. To fill the gaps, I have broadcast grass and clover seed at the start of the rains, but since the existing grass is so sparse, I feel that I can temporarily let some of the weeds help vegetate the bare spots and provide some of those cover crop services. I am ruthless with the annual bluegrass, though, because this fastest-growing-of-all weed can really smother other grasses if it comes in too thick. As a weed-watcher, I have learned to distinguish annual bluegrass from lawn grasses by its lighter color, its prow-shaped leaf tips, the three-way branching from the base that it often exhibits, and at maturity, its delicate spray of seed spikelets. Meanwhile, I spare the bur clover – not an actual clover but an inaccurately named relative of alfalfa – because I consider it a good weed due to of its ability to provide nitrogen. Then it gets big, there is more of it than I can remove, and it dominates the bare patches. Then it finishes it life cycle and leaves the patches bare again. What really changes my mind about its benevolence, though, is the innumerable burs that get tracked into the house. This leads me in the following year to pull it up wherever I see it, having learned to distinguish the blunt leaflets with the chevron of veins from the leaflets of the clover that I sowed, which are darker and pointed. Bur clover has lost its spot on my list of the good.
Recently the drama moved to another location. I had the opportunity to volunteer on an organic farm over the course of a year. The main weed management strategy was tillage, which has the downside of degrading soil structure and organic matter. The no-till method is unfortunately mostly associated with conventional agriculture, with its heavy use of herbicides, which has led to the development of herbicide resistance in various weed species, not to mention contamination of drinking water sources. On the organic farm plenty of weeds escaped tillage, so the secondary weed management strategy was a guy with sturdy knees and intact vertebral disks who burned through jaw-dropping quantities of physical energy as he chopped out the thickest weeds. A third strategy, applied to some of the beds, was soil solarization, where beds are covered with clear plastic mulch during the height of summer to cook the weed seeds in the top layer. Bare fallow, as is found under the plastic, has its own problems, such as eliminating entire groups of organisms in the microbiome, that secret ingredient for nutrient cycling on an organic farm, a key factor that allows the organic system to succeed without chemicals. The most obvious endangered microbe group is mycorrhizal fungi, which help the plant take up phosphate and some micronutrients and give it some protection from disease. Mycorrhizal fungi can only survive while connected to a living root. I’m guessing, though, that between the potatoes that escaped harvest and the bindweed, there may have been some deep roots still present during the solarization.
That farm had an infestation of nutsedge poking up from its underground “nuts”, which was alarming to my weed-watcher eyes, but I appreciated the carpet of knotweed on the dirt roads between the fields. When the fields were wet, I always preferred walking in furrows with weeds rather than in mud. Some crops, like lettuce and carrots, were completely removed at harvest, leaving the bed without the main vegetation. Under perfectly clean cultivation, there would remain a dearth of roots to support the mycorrhizal fungi. Here, however, there were always some weeds that escaped the energetic guy. Many weeds belong to families that don’t support mycorrhizal fungi, but, one supportive species that could almost always be counted on was that object of my own ruthlessness, annual bluegrass. Here it was not at all dense on the beds, thanks to the various management practices, but it was found here and there, growing taller than normal in the amended soil without interference from other vegetation, and presenting its roots to the fungi. It could also be found in beds with crops from unsupportive families, like broccoli, where it performed the same service. I can’t say I welcomed it onto my list of good weeds, but in those particular circumstances I was glad to see it. It would go on to drop some seeds before being incorporated or overtopped by a cover crop, and as long as the farm staff kept up their management chores, it would remain at that sweet spot, not absent, but not dense.
In the latest plot twist, the mainstream agronomic journal Weed Science came out with an article presenting the concept of “neutral weed communities”. A collaboration of weed ecologists in Italy and the US, led by doctoral researcher Marco Esposito, has reviewed documented cases where certain assemblages of weed species have caused no loss of crop yield at all. These are typically diverse assemblages lacking the strongly dominating species that farmers dread, and in some cases this absence of crop loss has been found where there is more total weed growth in the plots with the diverse weeds than in plots with the harmful species. That’s right – more weeds, less crop loss, as long as they are the right weeds. The authors turn to agroecological theory for explanations. It could be resource partitioning, such as found in successful intercrop combinations, that allows weeds to grow among crop plants without competing against them. It could be a reduction in pest pressure or an augmentation of the mycorrhizal fungi diversity that results from greater plant diversity, either of which could balance out any direct interference from the weeds. It could also be that there is a diversity of soil resources, such as might result from cover cropping, crop rotation, and different organic amendments, that allows crops and weeds to avoid competition by drawing on different resource pools. If that were the case, though, then perhaps the weed diversity is not the cause of the favorable yield, but rather it is Cocannouer’s tilth that keeps the crops happy and at the same time clears up the problem weeds.
Nowhere do Esposito et al. mention allelopathy, which is implicated in making certain weeds particularly dominant and particularly harmful to crops. Nor do they reference Chacón & Gliessman’s work presenting the non-weed concept. And there is a paucity of actual experimental data, where the weed community would have been manipulated through planting or removal to tease apart the effect of weeds on crops versus the effect of agronomic practices on crops but also affecting weed communities. One manipulation study they do mention is the deliberate planting of false strawberry in an orchard, a non-crop plant which was itself strongly dominant, but which caused no yield loss and kept out other weeds that would have been more harmful. Another study was of the planting of a leguminous weed that actually improved crop yield by providing nitrogen. The only other example of a deliberately planted weed was that of pigweed in corn, but that was for a forage crop, and the measure of success was the combined yield of the two species relative to corn alone, a result that is not enough to diminish my wariness of pigweed. I was very intrigued, however, by a couple of the weed species that kept turning up in the benign communities – chickweed and various species of knotweed, the ones from my own list. Meanwhile, one species implicated in harming one of the crops was that disgraced former good-lister of mine, bur clover.
Like a Netflix series that leaves you hanging, this drama is not over. Esposito’s group throws in caveats about different weed communities having differential effects on the various crops, modulated by the different environmental conditions in different places, and even the hazard of neutral weeds in one crop persisting and being detrimental to the next crop in the rotation. They recommend a diversity of weed management practices in order to help achieve that environmental heterogeneity that is associated with neutral weed communities. They also propose a strategy of selectively removing the aggressive dominant weeds, envisioning scenarios of herbicide-wielding drones or interfering RNA. More research must be done before particular recommendations can be made. One hopeful consideration, though, is that if selective removal is effective – and it does seem promising – the benefits will accrue preferentially to the smallest growers, like those traditional farmers in Mexico, who have been doing selective weeding for generations. And that’s a season finale that I can find satisfaction in.