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think like a weed

What is a weed? The easy definition is a plant out of place, but hidden in that definition is an entire plant lifestyle. After all, you don’t find yourself hoeing roses and redwoods out of your lettuce patch. Some plants just do not fit the description of “weedy”.

The key is disturbance. A mature forest or grassland is not beset by pigweed, crabgrass, or annual bluegrass. The existing vegetation is strongly competitive, maybe even allelopathic, and quick-growing annuals fail at the seedling stage if they are foolish enough to germinate. But if there is a disturbance that removes vegetation, there appears an ecological vacuum, and Nature abhors a vacuum.

One of the most dramatic disturbances in living memory was the explosion of Mount St. Helens in southern Washington state. In 1980 the north face of this volcano, along with 800 feet from its peak, slid into the valley below. The suddenly exposed magma chamber exploded with a barrage of boulders. The surrounding forest was either flattened or buried. The area around the mountain looked like a moonscape.

Ecologist Peter Kareiva arrived on the scene to document what happened next. The first seeds to arrive were of the appropriately named fireweed, a plant in the genus Epilobium, sister to the weed known as panicled willow-herb when it sprouts on a farm. The seeds of plants in this genus are lighter than dandelion seeds, and the fast-growing plants are adept at snatching up nutrients from cleared sites. Kareiva observed that each fireweed plant was an island of life amid the desolation. Aphids, also drifting on air currents, had landed on fireweed plants, and ladybugs and ants followed. Naturally the fireweed roots were hard at work depositing organic matter and enabling microbes that happened to be carried in on the wind to survive and multiply. With the ground being prepared for plants with higher soil resource requirements, the restoration of the ecosystem was under way.

A less dramatic but far larger disturbance takes place across inhabited continents on a regular basis – plowing on farms. With the vegetation gone, the soil is prone to washing or blowing away. Soluble nutrients can wash out of the soil, and microbial life has to survive by eating up the existing organic matter or going dormant. Into this vacuum come the weeds, ready to do their job. The seeds are already there from the seed rain and from dormant seeds deposited years earlier. Perennials with their rhizomes and bulblets buried deep in the soil start strong while the tiny annual seedlings catch up. Without any help from humans, the ecosystem begins to restore itself.

The story of how all those seeds get there is somewhat involved. Everyone knows that some seeds have structures to ride to new sites on the wind. Other means of transportation include hitching a ride in animal fur, surviving in the gut of animals and getting deposited in a plop of natural fertilizer, or even being flung yards away by a spring-loaded fruiting structure. To these pre-agricultural mechanisms add an agriculture-specific one – moving around with soil, whether it be in pots of transplants or stuck on tires and farm implements. Most annual weed seeds are tiny, on the scale of poppy seeds. Any little spot of mud could be the vehicle for a weed to relocate to a new site.

There is a reason weed seeds are small. Finding a suitable spot is hit-or-miss. If a weed spent all its resources producing large seeds, it would be limited to a smaller quantity, the chance of them all failing would be unacceptably high. The lineage would go extinct sooner or later. However, if a weed produces 100,000 tiny seeds, even if 99.9% of them fail to survive, that’s 100 that are available for a future generation.

I almost wrote “the next season” instead of “a future generation”, but that would be wrong. Weeds have another strategy for making sure they have future generations, and that is a state of variable dormancy. There is always a chance that next season there will be no disturbance, no opportunity for tiny seedlings to survive, or that the disturbance will happen after germination, killing them all. If all the seeds germinate that season? Extinction. But if a portion of the seeds are programmed to stay dormant and germinate two seasons later, those seeds will escape the bad season.

Now you’re probably thinking, what if there are two bad seasons in a row? And you’ve probably already figured out that there is another cohort of seeds that will stay dormant for both of those seasons. And maybe you’ve also figured out what annual weeds figured out long ago through trial and error – the species that are not extinct are the ones that produce seeds that are mostly dormant. Every year a small percentage of the seeds produced by a single plant will be ready to germinate, while the majority will remain in an enforced state of dormancy.

And this dormancy can be taken to extremes. In World War I, Flanders Field saw intense bombing. The following spring, as if in mourning for the dead men, the field was carpeted with red poppies that had not grown there in living memory. The seeds had been dormant for a very long time and were activated by the explosions.

Which brings up another trick that weed seeds have for tracking favorable environments – they can detect disturbance. If tiny seeds germinate deep in the soil, they are too frail to reach the light at the surface before using up their tiny packet of energy. They have to get their little solar collectors into the light before they can move beyond the size of a snippet of thread. The cold darkness of deep soil maintains them in a dormant state, while the fluctuating temperatures and light within the thin layer of soil at the surface signal them to break dormancy. Also, the wearing down of the seed coat can accelerate the emergence from dormancy. A bombing campaign accomplishes both the exposure of the seeds and the disruption of the seed coat. As does plowing.

Weed scientists Charles Mohler, John Teasdale, and Antonio DiTommaso, in their 2021 book, called simply Manage Weeds On Your Farm, have noted the common traits of weeds, along with those of individual weed species, and performed agroecological jiu jitsu on them, turning their strengths against them. And although weeds are the heroes of this blog post, I concede that even a hero can go crazy and become oppressive. When the weeds are not needed in a field, they are welcome to go to a different area and restore the soil there.

The first thing to remember in encouraging weeds to leave your plot alone is that seeing bare soil in the ecosystem is like seeing blood. Humans carry around quarts of blood inside us, and it performs important functions, but when it becomes visible on the outside, that’s cause for alarm – the body is compromised and susceptible to potential breakdown of integrity; the systems for recovery are signaled to activate. When bare soil comes into view, Nature’s system of recovery – weeds – gets activated. If you can substitute your own steps for recovery, one that is more in line with your goals for the land, the default system will be disfavored and possibly greatly attenuated.

Plants are key for ecosystem recovery, so crop plants can serve as an important component of the overall plan. Annual crops, however, share a preference for disturbance with weeds, meaning that without other measures, weeds and crops will grow together. One way to avoid ripping open the soil’s cover to grow annuals is to underseed them in a maturing crop. Another way is to plant into mulch, but in order for a mulch to be effective at discouraging weeds it has to be very thick. Large-seeded crop plants would have an advantage sprouting up through a thick mulch, but small-seeded crops would do better if transplanted into thick mulch as starts. Some mulches will release allelochemicals into the soil that inhibit small-seeded plants, so the recommendation for planting large seeds or starts also applies.

More plants means faster recovery, so another strategy is to plant densely and then thin when the seedlings get bigger. A greater variety of plants is even better for recovery, so an intercrop is potentially better, with species having complementary traits such as differential shade tolerance or development time allowing for greater contributions to soil health while also giving adequate yield from each species. A living mulch would also fill the spaces between crop plants with additional plants. I am reminded of the farmer with a plot cleared from the eastern forest of Nicaragua who grew his corn in a patch of sweet potato. The tangle of sweet potato vines lowered the corn yield, but all the other farmers had their plots overrun with cogon grass and ended up with nothing. Perhaps in other situations a more docile living mulch such as subterranean clover or native clump grasses would allow the corn to better thrive.

If there will be a gap between the harvest of one crop and the planting of another, a species-rich cover crop will do wonders to heal the wounded soil. Over the long term, the ecosystem’s routine for completing the healing process is to undergo succession, where the fireweed or other pioneer species (weeds) give way to perennials, shrubs, and maybe trees. A farmer can create a succession analog, planting a perennial pasture, an orchard, or variations of these. Ecologist John Ewel and his team did a radical succession mimic in Costa Rica. In matched plots, the existing rainforest was cut and burned. One plot was left to recover naturally. Another plot was maintained bare. In the succession mimic plot, the team replaced every herb, shrub, tree, and vine with a similar plant, the only stipulation being that the replacements be plants that would ordinarily not be able to get to that site. By the end of the experiment, the bare plot had experienced severe degradation, such that the job of removing plants became quite easy – not much would grow there. The succession mimic had maintained soil health and nutrient cycling comparable to the naturally recovered plot.

There are ways to use weeds’ dormancy characteristics favor crop plants. Weeds locked in dormancy will not grow in a crop, and weeds that come out of dormancy at an inopportune time will die. The way to keep them locked in dormancy is through no-till agriculture, leaving the soil mostly undisturbed during planting. On the other hand, the seed rain will deposit additional seeds on the surface, so no-till will present its own weed issues. The most inopportune time to come out of dormancy would be right before another disturbance. Farmer emeritus Jim Leap has long used this insight to devastating effect for weed management by preparing the beds, watering without planting the crop, and then smacking any weeds that come up using an implement that cuts the roots without turning the soil. Flaming would also work, and researchers have developed a weed electrocution machine. Once the cohort of weed seeds that have broken dormancy is eliminated, Leap plants the crop with minimal additional disturbance, and the later flushes of weeds are diminished, more often than not. Mohler, Teasdale, and DiTommaso have a name for this technique: the stale seedbed.

Weeds’ signature feature of snatching up available resources quickly can be turned into a liability by more effectively directing those resources toward the crop. The practices of planting densely or transplanting starts will close the canopy earlier, directing light preferentially to the crop. Regarding water, drip irrigation, particularly with buried drip tape, will keep most of the germinable seeds from accessing the water. By contrast, if you water the field as if it were a lawn, it will become a lawn – of weeds. Regarding nutrients, Mohler et al. present an aspect of nitrogen uptake by weeds that can be manipulated in order to direct the nitrogen to the crop. If there is plenty of nitrogen available, a crop plant will take up extra, but only up to a point, whereas a weed recognizes no limits on how much nitrogen to take up, snatching it up it as if the spigot will be turned off tomorrow. If you give the field a big slug of chemical fertilizer, the weeds are poised to take advantage of it. You can favor the crop by timing the availability of nitrogen to meet its development needs, such as applying slow-release organic fertilizer or trickling nitrogen into the irrigation water during later crop development stages.

Another important weed trait that can be manipulated to favor crop plants is the seed rain. This is actually a metaphor, as seeds can arrive at a site by various means, not just falling from the sky. Remember soil movement? If you carefully clean farm implements and boots and carefully screen purchased inputs like compost, manure, and seedling starts, you can cut the arrival of new weed seeds, including some potential headaches that you’ve been lucky not to have up to this point. If you place intentional plants, such as native or insectary plants, on field margins, rather than leaving nature to attempt to rehabilitate these areas with weeds, you reduce the seed rain. If you see a weed that has escaped other measures and is gearing up to dump 100,000 seeds, don’t be too proud to trek out and remove it – a practice known as rogueing – because that will save you time and energy in the long run. Mohler et al. have another trick, and that is to know what weeds you already have on your farm. Don’t be like the farmer who noticed that she suddenly had a johnsongrass problem where none had existed before. She blames it on sheep that had been brought in, ironically, to control weeds. If she had noticed an aggressive unfamiliar grass sprouting after the sheep’s removal, she might not now be dealing with johnsongrass’s steel cable rhizomes. Of course, to know your own weeds, you have to take an interest in them as individual species. It helps if you can see them as the ecosystem’s rescue crew rather than a the mob of thieves that conventional agriculture makes them out to be.

Mohler, Teasdale, and DiTommaso wrote a whole book on weed management, and I am not going to pack all that scholarship into a mere blog post. If you want to know more, you’ll have to get the book. Their basic message is that non-chemical weed management is like many tiny hammers that can add up to a big effect. My basic message is that weeds are good for something in the bigger scheme of things. They pre-date farming, and they will be around when farming ends, so in between they deserve respect. Thank them and let them know that their services are not required at this time, and mean it.

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smaller fleas

It was during smalltalk over a shared breakfast table on Amtrak that the man across from me began excoriating powdery mildew. He was a Colorado pot grower. His greenhouse operation held hundreds of plants, and the air circulating between his and the adjoining pot greenhouses was spreading the spores of the leaf parasitic fungus that had somehow gained a foothold inside. The greenhouse conditions provided an ideal environment for germination and growth of powdery mildew, and the high density of plants insured an endless supply of favorable landing spots for the dust-like particles that can each start a new colony. Despite the lore, pot plants are not indestructible, but rather serve as hosts to many pests and diseases, with powdery mildew being a leading factor threatening pot growers’ investment. Powdery mildew doesn’t kill a plant outright, but it weakens the plant and lowers the harvest quality.

There are about a hundred species of powdery mildew that make up this family of fungi. A powdery mildew’s thread-like growths cover the surface of a leaf and send root-like structures into the cells to feed. Its upright chains of spores give the impression of powder on the leaf. Powdery mildew is known as an “obligate biotroph”, meaning it can only survive on living host tissue. A key requirement of this infection type is that the fungus must come pre-programmed to neither kill the parasitized plant cell nor awaken the cell’s defense mechanisms, a balancing act accomplished through a tightly choreographed sequence of interactions with the cell’s machinery.

A reality of the plant world, though, is that distantly related plant species have distinct cellular machinery, meaning that a powdery mildew species that is not correctly pre-programmed, that is to say, coevolved with the plant it arrives on, will trip up and not be able to infect. Thus the powdery mildew on the sowthistle next door will not spread to your pumpkins, and the powdery mildew that emerges on the rosebush at the end of the row of grapevines and signals the grower to spray sulfur on the vines is not the species that actually infects the vines.

What the powdery mildew species have in common is similar germination requirements. The reason for this is that the spores carry their own moisture, allowing them to germinate without the liquid water that most fungal spores require. In fact, liquid water inhibits powdery mildew spore germination and can even cause the spores to burst. The spores do need a certain amount of humidity to germinate, but it can be as low as 50% relative humidity, and germination works better within a mild temperature range and at lower light levels. Outdoors there are certain times of year when powdery mildew blooms, but greenhouses are always ideal incubators waiting for spores to arrive.

For control of powdery mildew on grapevines, great quantities of sulfur and fungicides are sprayed, the most for any pathogen. However, since weed is newly legalized and gets smoked, growers don’t have an arsenal of chemicals registered for use on it. Cannabis pathologist Zamir Punja from British Columbia has found a few treatments to be effective, including Regalia, which is an extract of giant knotweed, Milstop, which is similar to baking soda but without the sodium, and germicidal ultraviolet light for a few seconds a day. There are some biocontrols that show promise as well. These mostly produce inhibitory chemicals or destructive enzymes against the fungus, or prime the plant to fight off infection, but a recent study from Hungary led by Márk Németh working in the lab of Levente Kiss shows the potential of a biocontrol agent that acts like a creature from science fiction.

In John Carpenter’s 1982 re-visioning of the sci fi classic The Thing, there is not monster played by a man in a suit as in the original, but instead a shape-shifting menace that lives inside its victims, compelling them to act in ways that benefit this alien life form while they retain their own personality. The most riveting scene is where the Kurt Russell character uses a flamethrower to kill a crew member carrying the parasite, which had revealed itself when it had burst out of his chest to engulf the arms of a comrade. The carrier is lying on a table as the flames sear his body, but his head hanging over the edge is out of direct exposure. To the horror of the crew and the audience alike, the head grows a stalk to lower itself to the ground then sprouts legs and tries to slink away. The film proceeds with an air of paranoia as crew members try to figure out who else may be harboring the monster.

The Thing was inspired by the atmosphere of fear and suspicion associated with the cold war, as were many if not all classic sci fi movies, but it could have been inspired by Ampelomyces. This fungus lives inside the tiny threads that make up the powdery mildew. It doesn’t kill the host fungus at first, but grows inside it, finally hijacking its reproductive structures to make more Ampelomyces. The tiny powdery mildew spores become filled with the even tinier spores of the hyperparasite, so-called because it is a parasite of a parasite.

The basic life history of Ampelomyces has been known since the nineteenth century, but Németh et al. have done the definitive study by engineering this fungus to glow green under blue light, allowing them to see its diminutive threads inside its host. They accomplished this using a gene from a jellyfish and a bacterium from a plant gall. The jellyfish is the bioluminescent crystal jelly of Puget Sound, which has a gene for producing green fluorescent protein. The bacterium is the pathogen responsible crown gall in plants, and it alters the growth of its host by inserting a plant tumor gene into the host’s DNA. The scientists replaced the bacterium’s tumor gene with the green fluorescent protein gene, added the bacterium to the fungus culture, and created the green-glowing fungus they were seeking.

One of the key questions that could be answered with an easily seen fungus was, what happens to it outside its host? It turns out that it can survive on a leaf surface for weeks after germination before any host comes along. Considering how utterly tiny the hyperparasite is, and the fact that it is not eating during that time, this is quite a feat. This feature is advantageous for harnessing it as a biocontrol agent, as a grower would not have to wait until a plant is suffering from a powdery mildew infestation to apply Ampelomyces spores. With some appropriate greenhouse trials this might become an additional tool in the pot grower’s toolbox.

Németh et al. also confirmed previous work showing that, unlike powdery mildew, Ampelomyces is not specific to a single host species, but is able grow within many different species of powdery mildew. This feature might make it amenable to conservation biocontrol, where the biocontrol agent is not raised commercially and sprayed onto the crop, but rather the agroecosystem is managed in such a way that hosts are always available for the biocontrol agent to survive on site. Perhaps a grower could leave sowthistles growing around the perimeter of the system to protect the pumpkins. Perhaps the rosebushes at the ends of the grapevine rows could serve not as early warnings for spray timing, but as reservoirs of biocontrol fungus. Further study is required.

And so Jonathan Swift’s observation rings true: “…a Flea/ Hath smaller Fleas that on him prey,/ And these have smaller yet to bite ’em,/ And so proceed ad infinitum…”. Powdery mildew will always be around, but for Ampelomyces that’s a good thing.