This post is about killing a whole lot of weeds growing up through a long stretch of poorly-maintained asphalt driveway. Options considered include:
- Commercial weed killers (Round-Up, Spectracide).
- Vinegar
- Salt
- Mechanical (weed whacker)
- Heat.
- Propane weed flamer
- Electric heat gun
- Boiling water
- Steam
- Solar, up to and including solar oven
This post is just about gathering the facts, trying to make my mind up. I haven’t really tested any of these yet.
Arguably the biggest revelation (to me) is that several of the alternatives (e.g., vinegar, heat) are really no different from using a weed whacker. It took me a while to figure that out, but all they do is take the tops off the weeds, leaving the live roots intact. That’s just a roundabout way of doing what a weed wacker does directly. In which case, it’s hard to argue in favor of some D-I-Y approach, when there’s a tool actually designed for the job.
After a false start with vinegar, I’ve decided to go with heat. Primarily a solar approach, which seems to fit my situation well. It’s an approach that isn’t typically used so there isn’t a lot of information out there on how well it works. It’s plausible that I can amp it up enough to kill the roots. Plus, I already have the materials on hand to try it.
The idea is not just to get the driveway surface hot enough to kill the top of the weed (and have it re-sprout), but to bake the driveway so hard that it kills the entire weed, root and all. We’ll see if I can achieve that, or whether that’s just so much wishful thinking.
Partly Weeds
My house has a long, old, poorly-made asphalt driveway. At one point, the back half was in such bad shape that I figured I’d just let it go and see what happened. But I’ve changed my mind about that. And now I have to deal with it.
I’m not showing a photo because it is frankly embarrassing that I’ve let my driveway go to seed. Literally.
Following National Weather Service terminology above, the front portion of the driveway, from street to house, is “asphalt”. But further back, it ranges anywhere from “mostly asphalt” down to “mostly weeds”.
This is America, so to deal with this, I’m supposed to pick up a few gallons of death-in-a-jug and have at it. I’m sure I could make quick work of this with a few gallons of Round-Up (glyphosphate) or Spectracide (diquat bromide and friends). At just $14 and $8.50a gallon, respectively.
But where’s the fun in that. I mean, just because something’s a suspected carcinogen, banned in several European countries, banned in some U.S. cities, banned for use near daycare centers and schools in some U.S. states, and so on (one reference) … Just because a bunch of snowaflakes shy away from it, is that really a reason to avoid using it right next to my garden?
Anyway, I’ve decided to do this the hard-but-low-risk way. I tried solarizing the weeds earlier in the year, just covering them with a sheet of black plastic. That knocks them back pretty well, but they came right back as soon as the plastic came up.
So now I’m amping it up.
Starting down the vinegar path: Vinegar is just a weed whacker in a bottle.
What set off this round of weed killing is not so much the desire for a nice driveway, as the need to dispose of a dangerous substance. In this case, vinegar.
Somehow, I managed to pick up a gallon of 4% distilled vinegar. I didn’t even know they made such a thing. Which is probably how I managed to come home with it and not notice. But they do.
Home canners will immediately recognize the danger of having 4% vinegar in the kitchen. The plastic bottle looks just like standard (5% acetic acid) vinegar. But if I were to use this in canning, there’s (small) some risk I’d end up killing myself. That 4% stuff is not sufficiently acidic ensure that botulism won’t grow in canned food. That’s why the USDA Guide to Home Canning says 5%, and nothing but 5%, will do.
So I need to dump it. And rather than waste the $2.50, I put it in my sprayer and I sprayed it on the copious weeds growing through my driveway pavement. I sprayed a gallon of it and I’m nowhere near done yet.
Because, hey, we all know that vinegar kills weeds. It says so right on the internet. I have weeds to kill, I need to dump the vinegar, seems like a perfect match.
But as those pickled weeds are now baking in the sun, I figured I might just look into this a bit deeper. Does vinegar kill weeds? How well does vinegar kill weeds? What’s the best way to kill weeds with vinegar? And so on.
Just so I know that I’m not just blindly following some hazily-recalled bit of folklore.
All the earmarks of folklore.
Let me distill the conventional wisdom on vinegar as weed killer in a few simple bullet points. This is my summary of plowing through maybe 30 sets of internet instructions on using vinegar to kill weeds.
There’s absolutely no doubt that it works to a greater or lesser degree. It’ll kill the tops of weeds in just a few hours. But not the roots. You need to re-apply it a few times as the roots re-sprout.
- It works better if the weeds are in the sun, it’s hot, and there’s no rain. Works worse in the shade, cold, and wet.
- It works better if the weeds are growing in pavement (very little soil), rather than in a larger patch of soil.
- Regular (5%) household vinegar works, but high-strength vinegar (below) works better, or faster, or requires fewer re-sprays, or some combination of those.
- Some people add salt (one cup per gallon, maybe more), soap (a bit per gallon), or both.
Typical for folklore, you will find different people asserting different formulations. Everybody says that their is the one that works. Some people assert that none of them work. Nobody actually tests one against another in a controlled way. And there is almost zero discussion — other than bland assertions — as to how it actually works.
If you look into this, you’ll find suggestions to use:
- Regular household (5%) vinegar.
- That, plus salt (one cup per gallon, the strength used in making pickles).
- That, plus dish soap (“to make the vinegar stick to the leaves”)
- That, plus both salt and dish soap.
- Higher strength vinegar, up to a reasonably dangerous 30% acetic acid solution.
- That, plus salt, dish soap, or both.
The only explanation I’ve seen as to how this works is that it “dries out the leaves”. That seems a bit dubious for something that’s 95% water. But upon further reading, the explanation is that the acid disrupts cell membranes (which I interpret as the cell walls). (Reference). When the cell walls are broken down, the leaf dessicates in the heat. So it doesn’t directly “dry out” the leaves, it disrupts the cell walls and all and so allows the cells in the leaf to dessicate.
That’s the theory.
But everybody agrees that, unlike commercial weed killers, this doesn’t kill the roots. So unlike the stuff you can buy at the hardware store, this isn’t a one-and-done. You have to reapply (at least) a few times to kill the weeds entirely.
When I boil all that down, vinegar alone appears to be merely a chemical version of a weed-whacker. It removes the top of the weed, but does not damage the roots. The weeds then continues to re-grow until you’ve done that enough times to weaken the weed so that it finally dies a natural death. On pavement, at least, it appears to be a weed killer that does nothing that you couldn’t do with a weed whacker.
A quick detour around 30% acetic acid solution
Some people recommend using highly concentrated (e.g., 30%) acetic acid solution, as opposed to household (5%) acetic acid solution, a.k.a., vinegar.
I’m supposed to make a note here of how dangerous 30% acetic acid is. The bottom line is that it is a hazardous chemical. But nowhere near as hazardous as, say, battery acid.
Sure, you can buy this off the shelf at your local Home Depot. But I also note that you can buy straight-up battery acid there too. If nothing else, that demonstrates that just because it’s sold retail doesn’t mean it’s safe for casual use.
For sure, caution is advisable. But I don’t want to err too far in the other direction either. You want to give this all due respect.
When I type 30% acetic acid solution into a couple of different on-line calculators, it comes out as pH 2.0 or so. That makes it about 2.5 times as acidic as household (5%) vinegar (pH 2.4). By contrast, retail battery acid has a pH of about 0.8, so 30% acetic acid is about 1/16th as acidic as battery acid.
Based on the acidity (pH), 30% acetic acid solution is definitely more dangerous than household vinegar. But not nearly as dangerous as battery acid.
I’m not sure that pH alone tells the whole story. Another dimension is the speed of the reaction. My vague recollection is that battery acid reacts instantaneously with (e.g.) skin. (Ask me how I know.) If you get a drop on you, you get a burn, no matter how fast you rinse that off. Same for putting holes in clothing. And a drop in your eye would mean instant impairment of vision. I’m not sure how much time you have, if any, to rinse 30% acetic acid off your skin before it makes it through your epidermis and into living tissue.
Based on the Home Depot comments, people seem to use that 30% stuff pretty cavalierly. For example, to clean their bathrooms, and so on. While I wouldn’t recommend it, that suggests that it’s not quite in the same liquid-death category as battery acid.
The best user comment I came across at Home Depot said: “Wear face protection. Splatters sting.” That has the sound of experience behind it, and likely sums up the precautions due 30% acetic acid. Likely, any stray drops of it will eat a hole in clothing or skin if left in place long enough. Likely any material amount in your eyes risks permanent damage to your vision.
I wouldn’t fear using it, but if it comes to that, I’d wear goggles, gloves, and clothing that I don’t care about. And generally handle it to avoid splashes and drifting mist.
Near as I can tell, the only reason to buy it in this concentrated form is that it is reputed to a more effective herbicide than standard (5%) household vinegar.
That said, if you have to resort to 30% acetic acid, this is a fairly expensive way to kill weeds. Per unit of acetic acid, a gallon of the 30% solution for $20 is about the same price as regular grocery-store 5% distilled vinegar, typically around $3 per gallon.
In my case, it’s going to take several gallons of it, likely applied several times, to kill everything growing in my driveway. Whereas a gallon or two of store-bought weed killer would likely do the trick, at lower money cost, and with less labor to boot. In my case, the 30%-acetic-acid method will have a money cost and time cost several times higher than just spraying with standard industrial herbicides.
Between the hazard and the cost, and even then maybe requiring re-spray, there has to be a better way.
Salt? Well, yeah, salt kills plants.
A further common suggestion is to add salt to the vinegar. Oddly, it is added at one cup per gallon, which cooks will recognize as a standard concentration for a salt brine (e.g., for brining a turkey).
I don’t doubt that dumping enough salt on a plant will kill it. Certainly the winter snow melt typically put down on roads and sidewalks appears toxic enough to nearby plants.
But people who recommend using salt (alone) to kill plants use it at a far higher concentration than the one-cup-per-gallon recipe typically mentioned with vinegar. Again, this is comparing folklore to folklore. But in using just salt to kill weeds, I see everything from literally piling dry salt on top of the weed, to solutions as dilute as one cup salt to three cups water.
With enough salt, it would probably be possible to poison the soil around the plant so that nothing would grow back. Given that I have to treat several hundred square feet of driveway, I’m not all that enthusiastic about using that much salt.
I’m going to keep looking.
Weed whacker?
Sure, that’ll take the tops off just fine. So that’s got to be at least as good as plain household vinegar. It’s not dangerous. It’s not really any more time consuming than spraying vinegar would be. And somewhere in my garage, I have one of those, that works to some degree.
But.
But this is a form of surrender. This isn’t killing the weeds in the driveway, this is maintaining the weeds at a passable level. As with straight vinegar, this isn’t a one-and-done. This is now routine driveway maintenance.
Separately, I just plain don’t like using a weed whacker. Possibly because mine is an ancient piece of junk. Possibly for some other deep-rooted psychological reason.
There is something to be said for using a tool that’s specifically made to do that job. You want to remove weeds? Particularly, weeds sitting on a hard surface like a driveway? Well, the tool designed to do that is, in fact, a weed whacker. Among other things, it will remove the tops of the weeds, instead of leaving the dead tops in place as with vinegar (above) or surface heat (below).
At the minimum, this might make a good adjunct to some other method that promises a higher likelihood of a kill. And I may yet weed-whack a few patches and see what happens. But I don’t see this as my principal solution.
Heat, in all its forms
Well, once you open up this category, you realize that the tools range from the prosaic (boiling water) to the outright dangerously fun (weed flamer). I mean, who wouldn’t want to walk up and down their driveway pushing around a foot-long fireball at the end of a stick?
Beats using a weed whacker, if you ask me. And for sure, If I’m using the flame thrower, I’m doing my weeding at night, for maximum effect. Put on a toga and be a cosplay Prometheus.
But at the end of the day, as much fun as it is to buy new tools, I’m going to stick to solutions for which I already have the equipment and materials. In my case, that means either an electric heat gun, or something on the order of a crude, portable, bottomless solar oven.
Using a heat gun to kill weeds is a topic well-covered by internet sources. Also has to be about as much fun as chipping paint.
But the main drawback of any of the surface-heat methods (torch, heat gun) is that they probably don’t kill the roots of perennial weeds. That’s what I read from several different sources.
And so, at the end of the day, as with vinegar, surface-heat methods like a propane torch or heat gun are more-or-less no different from using a weed whacker. They’ll take the top off the weed, but the root will survive. You’ll have to re-apply as often as it takes to keep the weeds in check, or until you finally exhaust the weed to death. Given that dirt is a pretty good insulator, I doubt that either steam or boiling water is any better in that regard.
These might be satisfying approaches, they will yield good short-term results, for sure the weed flame-thrower has to look manly as hell, but the consensus of opinion is that you’ll have to redo them repeatedly to have any hope of permanently killing the weeds embedded in your driveway.
There is, however, one potential advantage of the flame weeder that might make it particularly useful here. Maybe it gets hot enough to melt asphalt. In that case, I could simultaneously weed and seal the cracks in my driveway. I need to look into that.
Otherwise, I think I’m going solar. I just plain like it. I’ve been fascinated with it since I was a kid. I have lots of bits and pieces for making up some sort of solar oven. It’s my driveway, so I can leave the equipment in place as I see fit. Maybe I can heat patches of driveway hot enough to kill even the roots of the weeds.
And there’s always this:
Source: Ali Express.
A Fresnel lens that size can, for sure, melt the driveway as it kills the weeds. Also burn you before you know it, or blind you instantaneously if you’re dumb enough to stick your face under it.
So maybe that will be the preferred approach. I need to think about it a bit before charging off into actual implementation. But as of now, I think I’m going to solarize my driveway, in some fashion, hoping not just to kill the tops of the weeds, but to kill them completely and maybe seal up the driveway surface in the bargain.
