Looking for Hope With Stiltgrass

Often, we think of good as struggling against evil, but a lot of problems are caused by too much of a good thing. Too much carbon dioxide in the air is an obvious though invisible example--vital in small amounts, destabilizing if allowed to radically increase. Plastic sure is handy, but a problem as it builds up in the environment. Many species considered invasive or weedy would be welcomed in the garden, forest and field if they didn't have a proclivity for taking over. What to do, then, about Japanese stiltgrass, the uber-invasive that rises like a wave in late August, lifting itself up and over other plants, preparing to carpet the ground with a deluge of seeds that will surely expand its dominion the following year? If only animals ate it, to keep it in check, but the plant seems of value only to itself. Its one use to people--historically as "packing grass" to protect porcelain for shipment--was also its means of spreading across the planet. 

Keeping some areas stiltgrass-free involves a lot of effort in August--the month of opportunity before it goes to seed. If I were a biochemist, working in a lab rather than out in the field, I would dream of someday discovering a compound that would magically release the landscape from stiltgrass's smothering grasp.

The vexing irony of stiltgrass (Microstegium vimineum) is that it is both frail and indomitable. Weak roots make it easy to pull, yet the dense congregation of billions of plants will quickly overwhelm and exhaust the spirit. Nor have any herbivores found its wiry, flimsy frame worth eating.

One of stiltgrass's strengths is also one of its potential vulnerabilities The species uses something called C4 photosynthesis. There are two main biochemical pathways plants use to harvest energy from the sun, called C3 and C4. It has to do with how the process of photosynthesis is organized in the cells. The "C" stands for carbon, which is ever so conveniently supplied to the plants in the form of airborne carbon dioxide, so they can turn it into sugars and carbohydrates--the building blocks of growth. Plants using the C3 pathway tend to prosper in cooler months and languish in hot weather, while C4 plants grow more efficiently in the heat of the summer. (Note: Follow the link above to a short video that explains why.)

If you've ever wondered why crabgrass becomes a problem in yards in late summer, its because it is a "warm season" grass that uses C4 photosynthesis to outcompete the C3 "cool season" turfgrasses. Corn is another example of a warm season C4 plant that gets going later in the spring, then grows robustly through the hot summer months. What makes stiltgrass invasive in nature preserves, while crabgrass is not, is that stiltgrass is one of the few C4 plants that grows really well in the shade, allowing it to leapfrog over the C3 plants all around it during hot weather in late August. 

If someone could develop a compound that selectively kills C4 plants while not harming others, it would be really handy for spraying stiltgrass in the shady forest understory where it may be the only C4 species. So, every now and then I do an internet search, using a phrase like: herbicide specific to plants that use the c4 carbon pathway. AI actually comes up with some interesting results, which you can access by clicking on "see more" at the bottom of this post.

In the meantime, there's still reason to harbor a little bit of optimism about stiltgrass. I remember the first time I saw it, a graceful little plant growing on a streambank in Durham, NC. It seemed harmless until it later exploded across the landscape in the wake of Hurricane Fran's mass disturbance in 1996. A similar story can be told in New Jersey, but even now, thirty years later, it's hold on the landscape is hardly uniform, giving some reason for hope. In my August perambulations, I find it clogging floodplains and proliferating across moist slopes, but many other areas remain surprisingly stiltgrass-free. Your yard may be clogged with it, but many yards have yet to be invaded, or have only a few that can easily be pulled before it goes to seed. If you're one of the lucky ones, now is the time to inspect your yard and pull out every last one, even the annoyingly tiny ones.

Princeton University is a good example of a campus that has very little stiltgrass and could remain so with vigilance. I recently found a patch just up from their New South building. This could easily be sprayed using a highly targeted, minimalist approach.  Stiltgrass's weak roots mean the spray can be very dilute. Otherwise, seeds from this patch will spread downhill, with the problem growing year after year. I alerted the grounds staff at PU, and hope they are proactive.

Meanwhile, in the yard in this photo, the infestation was small enough that I was able to pull up every last stiltgrass before it could form seed, tossing them on the driveway to dry. Since they were already flowering, they needed to go in the trashcan, not the compost. Sometimes multiple followups several days apart are necessary, because one always misses a few. The payoff comes as the job gets easier each year. 

For an annual grass like stiltgrass, no seed means no future.

Related post: Stop Stiltgrass From Taking Over Your Yard

Here is AI's response when I recently typed into google: herbicide specific to plants that use the c4 carbon pathway

There is currently no commercially available, broad-use chemical herbicide explicitly labeled as selective only for C4 plants. While traditional selectives like atrazine or mesotrione target specific weeds in C4 crops like corn, they are not designed to universally target the C4 carbon pathway itself. However, significant scientific progress published in journals like Molecular Plant has identified highly targeted, experimental small-molecule inhibitors (SMIs) and plant-derived compounds that specifically block the C4 photosynthetic machinery without harming C3 crops. [1, 2, 3]

Target Enzymes of the C4 Pathway

The unique anatomy and carbon-concentrating mechanisms of C4 plants rely on enzymes that either do not exist or function differently in C3 plants. Experimental herbicides are designed to target these three primary enzymes: [1, 2]

Phosphoenolpyruvate Carboxylase (PEPC): This enzyme is responsible for the initial carbon fixation in the mesophyll cells of C4 plants.
Pyruvate Phosphate Dikinase (PPDK): This enzyme regenerates phosphoenolpyruvate (PEP), which allows the C4 carbon-trap loop to continue running.
NADP-Malic Enzyme (NADP-ME): This enzyme releases CO2 inside the bundle-sheath cells for use by Rubisco. [1, 2, 3]

Experimental and Natural C4 Inhibitors

Researchers have validated several synthetic and plant-derived compounds that successfully stunt C4 weeds while leaving C3 crops completely unaffected:

Okanin: A plant-derived flavonoid that acts as a potent, natural inhibitor of the PEPC enzyme. [1, 2]
Bisindolylmaleimide IV: A specialized small-molecule compound proven to selectively inhibit PPDK activity in vivo. [1]
Phloretin: A natural dihydrochalcone that severely disrupts C4 photosynthesis, causing a massive reduction in net carbon assimilation and bundle-sheath leakiness. [1]
3-Cyanobenzoic Acid: A synthetic compound that heavily suppresses C4 plant growth by lowering stomatal conductance and disrupting electron transport rates. [1]

Why This Matters for Weed Management

Many of the world's most aggressive agricultural weeds (such as barnyard grass, crabgrass, and pigweed) are C4 species, while key global food crops (such as wheat, rice, and soybeans) are C3 plants. Developing these C4-specific pathway inhibitors into consumer products provides a brand-new mode of action to bypass the widespread chemical resistance modern weeds have built up against traditional options like glyphosate. [1, 2, 3, 4, 5]