I’ve just been taught a first-hand lesson in why hardware stores sell 30% acetic acid (vinegar) as weed killer. That’s because regular household-strength (5%) may or may not work.
Here you see a section of my driveway, sprayed yesterday with vinegar (4%). It’s now a mix of dead and clearly undead weeds.
Perhaps this explains the mixed reviews that vinegar gets, as an herbicide. Above, it clearly left most of the grasses untouched, but did seem to kill some of the crabgrass.
This might also plausibly explain some of the mixed reviews for other weed killers. Whether or not a particular approach works for you is going to depend, in part, on what your weeds are.
In any case, given that vinegar isn’t going to hack it, even for killing just the tops of the weeds (let alone the roots), I decided to do a more complete search for off-the-shelf organic weed killers.
I looked on-line for any weed killers or herbicides described as organic, alternative, or OMRI-certified. My goal is to find something that will kill the weed completely, right down to the roots.
Results in brief
The good news is that there are many such organic weed killers.
The bad news is that, with one or two exceptions, they all work exactly the same way that vinegar works. They damage the leaves of the plant so that those leaves dehydrate and die. But they don’t kill the roots of the plant, which then re-sprout. (This link takes you to the blog of an actual expert, who says the same thing.) You can kill the tops of the weeds in all manner of different ways. But the weeds will re-sprout from the roots.
There are various terms-of-art used for this type of herbicide, but I think the most common one is “burn-down” herbicide. It burns the weeds down to the ground, but that’s as far as it goes. These are all burn-down herbicides.
In short, they are chemical weed-whackers. They remove the tops of the weeds by desiccating them, rather than cutting them off. But the effect is the same.
Only one — Natria — makes any promises about keeping the roots from re-sprouting. And that one uses a sprouting inhibitor that’s questionable.
In addition, a new class of chelated-iron weed killers will kill the roots, but those are ineffective on weed grasses, which is my main problem.
Organic burn-down weed killers.
These will kill plants to the ground, but they don’t kill the roots.
I’m not putting a list of the herbicides here because the list of brand names that I looked at is nowhere near complete. (In other words, if I listed brands, those would be examples, not a complete listing.) For each of the chemicals listed below, there are probably dozens of branded products sold.
Among the substances used to injure weed leaves so that they dehydrate and die are:
- Acetic acid (vinegar).
- Salt water.
- Caprilytic/capric acid (from coconuts).
- Oil extract (clove oil, cinnamon oil).
- Citrus oil.
- Citric acid.
- Strong soaps (ammoniated soap of fatty acids, anything “octanoate”).
Plausibly, these different substances work to varying degrees on various weeds. But the point is, the end result is no different from having simply used a weed whacker. They don’t kill the roots.
(One herbicide that used essential oils said it killed “down to the roots”, but I think that was just deliberately ambiguous marketing hype.)
Finally, one herbicide (GreenMatch X) used lemongrass oil. This inhibits photosynthesis in a wide range of plants. It’s not clear that this is available through retail channels. And in any case, it’s just another burndown herbicide. It doesn’t kill the roots.
Maybe-organic, maybe-more-than-burndown weed killers.
This is a much shorter list of organic weed killers that will kill the root of the weed in addition to the top of the weed.
One herbicide (Natria) consists of a sprouting inhibitor added to a strong soap. The claim is that this will not just kill the weeds, but will also keep them from returning.
The sprouting inhibitor is maleic hydrazide (Fazor). It prevents plant cells from dividing and reproducing, so no new growth occurs once this has been applied to a plant.
I’m not sure how “organic” this is, but I haven’t managed to dig up anything really bad about it. It’s definitely mutagenic if eaten in quantity (unsurprising, given that’s how it prevents plant cells from dividing). In the U.K. they spray this stuff on onions and potatoes a week or two before harvest. It gets absorbed into the plant, and then keeps the resulting harvested crops from sprouting. (Reference). Apparently we do the same in the U.S. So, at some level, that has to be reasonably non-toxic to humans. I hope. Because I’ve been eating this all my life, apparently.
The U.S. EPA seems to give it a fairly clean bill of health (Google reference). The U.K. reviewed this chemical for continued use on potatoes in 2019 and gave it the OK (reference).
Apparently it degrades pretty rapidly in the environment (per this reference). That source shows a half-life in the upper soil of maybe one to three days. However, if it gets put in an anaerobic environment — say, washed into a lake and settling into the bottom sediments, the half-life increases to about half a year. You definitely don’t want to use this before a rainstorm. The manufacturer specifically recommends staying at least 20 feet away from any stream channel, body of water, or similar.
I can’t quite figure out how Natria works, because it combines the sprout inhibitor with a strong soap that should immediately kill the top of the weed. In theory, the sprout inhibitor is a systemic chemical that gets absorbed into the plant and then “translocates” to the roots. The problem is that they package it with a strong soap that destroys the leaves almost immediately. There isn’t time for it to get absorbed, let alone “translocated”. And the stuff isn’t persistent, so it should be mostly gone in just a few days. So, how does it prevent re-sprouting from the root?
I remain ambivalent about this one. It’s not persistent in the environment, and it does promise to prevent the weeds from returning. It’s used right now, on U.S. food crops. My guess is that this is about as harmless a product as I am likely to find, that has any hope of preventing the weeds from returning. In theory, any residues left on the surface should rapidly degrade. Otherwise, any residual chemical should be locked up in the weed roots, until such time as those decompose.
There is also a new class of herbicides that kill plants by overloading them with iron. (E.g., Fiesta, Bonide Weed Beater FE RVU). These are marketed for killing weeds in the lawn, as they will only kill broad-leaved weeds, but not lawn grasses. (And, crabgrass is not listed, so presumably not crabgrass.) As iron is abundant in the soils around here, I’d have to think this is about as harmless as it gets.
These won’t do much for me, as most of what’s growing in my driveway seems to be various type of weed grasses. From what I can tell, this approach won’t touch (e.g.) crabgrass, stiltgrass, and similar.
Conclusion
I sure would like to kill (ALL) the weeds in my badly-degraded driveway. Once they are gone, I can seal it, and that should (mostly) prevent them from coming back. But I can’t do that until I’ve gotten rid of what’s there. Ideally, not just the tops, but all the way down to the roots.
There aren’t a lot of options if you decide to avoid synthetic herbicides of dubious safety. Particularly if you want something that will kill the roots of the weeds so that they won’t re-sprout.
Almost all “organic” approaches work the same way: They kill off the top of the plant, leaving the roots intact. Vinegar, soap, flame, citrus oil, and so on and so forth. Functionally, they are no different from using a weed whacker.
Natria is the only seemingly-environmentally-benign product I’ve found that will kill weeds (including weed grasses) and (in theory) prevent them from coming back. And there, it takes a bit of faith to believe that the sprout-inhibiting compound is completely benign.
So, if not Natria, then I’m back to D-I-Y. Or just repeatedly using a weed whacker until the weeds give up. Or I do.
For D-I-Y approaches, my original thought was to cook the weeds with some form of a solar oven. Hoping I could cook them right down to the roots. But upon doing a little research, it’s going to be hard to warm the dirt down to where the weed roots are. It requires too much energy, over too much time, through too much insulation, for that to be likely to work.
My final though is to try to use some of the natural killers that are already in my yard, termed “allelopathic” plants. These are plants that produce substances that kill or inhibit other plants. In particular, I have a large stand of bamboo at the edge of my back yard, and I know from hard experience that the leaves are quite toxic to many food plants. (And nothing grows on the floor of a stand of bamboo, which is pretty good clue that something in the bamboo is killing off the competition.) I wonder if I were to steep a few pounds of bamboo leaves in vinegar, and spray the resulting solution, whether that would kill the weeds in my driveway. I might as well try it. It’s not as if I have any shortage of bamboo.
If nothing else, this exercise has given me new insight into why people use Roundup and other synthetic weed killers. Even if you’re concerned about potential health and environmental impacts, there really aren’t many good alternatives. If you have a problem with (say) weed grasses, and you want to kill them roots and all, it appears that you have exactly one alternative: Natria.
Or just keep using a weed whacker or burndown herbicide every few weeks. Which to me is more along the lines of weed maintenance, not weed killing.
Natria it is, I guess.
