You know, the most wonderful thing about social media and the internet is that everything you read there is true! Therefore, if someone you don't know posts a question and someone else you don't know answers it, you can rest assured that the answer will be correct. Right????

This naïve assumption is the basis for one of the most popular gardening myths making the digital rounds today. It's a "safe, natural weed-killer" made from mixing vinegar, Epsom salts, and Dawn liquid detergent. I'm not going to provide the recipe, because I pride myself in not promoting hogwash. You can Google it yourself if you want.

Proponents of this "weed-killer" gush about its immediate results. "I sprayed the weeds with it yesterday morning and they were brown and dead that afternoon!" Sounds great, but are they really dead?

See, the big advantage that "chemical" weed-killers like Roundup, Brush Killer, and Weed-B-Gon have is that when you spray them on green leaves and stems, the weed absorbs the chemical and carries it down to the roots. Thus, you truly kill it, roots and all. It's not coming back. Natural weed-killers don't do this. They kill the top growth, but if the weed is perennial or has an extensive root system (like dandelion, poison ivy, or brambles), they grow right back from the roots.

But, you say, I don't like using chemical weed-killers. They're dangerous and bad for the environment. Surely, that's not the case with the vinegar-Epsom salts-Liquid Dawn concoction! To answer that, let's examine what each of these magic ingredients actually do.

Vinegar

Household white vinegar contains 5% acetic acid. This acid draws out the moisture from stems and leaves, quickly turning them brown. Spraying it on a plant does nothing to the roots, however. It's effective only against shallow-rooted annual weeds that can't survive having their foliage torched. To kill perennial weeds with vinegar, you need to pour horticultural vinegar on them. This is 20% acetic acid.

There are four big potential problems with this. First, this vinegar is non-selective. It will damage or kill any plant that contacts it, so be careful. If your use it on the lawn, expect a lot of dead, brown grass. Second, not only will horticultural vinegar kill plants, it will also kill a lot of good things in the soil, like earthworms and beneficial microbes. Third, if your use it to kill weeds in your sidewalk or driveway, the highly acidic vinegar will eat away at the concrete. Finally, horticultural vinegar is dangerous to people. Get some on your skin and you'll blister. Get some in your eyes and you could go blind. I'd steer clear of this stuff if I were you.

Epsom Salts

Epsom salts are in this recipe for the simple fact that many people mistake them for table salt. They're two different things. Epsom salts consist of magnesium sulfate. They supply two essential plant nutrients, magnesium and sulfur, which is why people have used them for decades and decades to feed plants such as roses, tomatoes, and peppers. They don't kill plants. They make them grow better. Why would you put them in a weed-killer? To make your weeds grow faster?

OK, then, let's just replace Epsom salts with regular table salt, which is sodium chloride. That kills plants, doesn't it? Yes, unless they're salt-tolerant, like many beach plants. It also poisons the soil, so that nothing will grow back (remember what Rome did to Carthage?). Plus, it ruins soil structure, so that soil will not drain. Using table salt in the garden is just plain stupid.

Dawn Liquid Detergent

Let me begin by saying it doesn't have to be Dawn. Any brand of liquid detergent would do. The reason people recommend Dawn so often is because the same recipe keeps getting passed around on the internet.

Liquid detergent is a surfactant. It helps vinegar and the salts stick to the leaves of the weeds. By itself, it's pretty innocuous. Keep in mind, though, that it does dry foliage and can burn if applied in hot sun. That's why the label of insecticidal soap warns against doing that.

The Take-Away