Why You Should Never Use Epsom Salt For Plants

People have been using Epsom salt for plants for a long time. All over the internet, you see it recommended in glowing terms that really appeal to the emotions.

People claim that spritzing their plants with Epsom salt dissolved in water will kill pests, make their tomatoes huge, kill off fungal diseases on the leaves of their roses, and that it’s an effective weed killer. Epsom salt is purported to cure blossom end rot, and it’s supposed to ‘make plants stronger’.

But when you combine all of those miraculous claims together, they don’t make sense. How can one product be an effective killer of weeds without damaging your desired plants and somehow make them stronger at the same time? Can one product really kill off all garden pests?

Unfortunately, the science doesn’t back up a lot of these supposed benefits. Let’s start with what Epsom salt is, then explain some of the claims so you can decide for yourself.

What Is Epsom Salt?

In the 1600s, a small town in the county of Surrey, England called Epsom became quite the destination.

A villager named Henry Wicker had been looking after animals in the commons, but a water shortage had made things difficult. He saw water in a cow’s hoofprint in the mud and dug a shallow hole there, hoping it would fill with groundwater overnight. In the morning, it indeed had filled with groundwater, but the cattle wouldn’t touch it.

Wicker tasted the water and found it to be bitter. He also discovered that the water had a laxative effect. Since he had what he believed to be miraculous water, he began promoting his discovery as medicinal. Over time, Epsom became known for its water.

It was believed to contain magnesium sulfate, a compound of magnesium and sulfur. Later testing during the 1900s determined that most of the dissolved material in the water was actually calcium sulfate. Unfortunately, by that point, an entire industry around Epsom’s salt had been created.

The dry salt crystals we purchase as Epsom salt are magnesium sulfate (MgSO₄). They take their name from the town of Epsom in Surrey, but they may or may not actually be part of what pooled up in the cow’s hoofprint. Still, Epsom takes some civic pride in the fact that over 400 years ago, their town became popularized because of Epsom salts.

When did Epsom salt become used for gardening purposes? The answer to that is less clear.

Many Epsom salt companies began printing uses on the box in an effort to get more sales. Using Epsom salt in the garden was likely inspired by one of those boxes. Various health benefits were also printed, some of which have been disproven scientifically since.

Epsom Salt Uses in the Garden

If you skim through the internet, you’ll find many suggestions for how to use Epsom salt in the garden. Let’s explore some of the ways that people use Epsom salts and whether they’re actually effective or not!

Blossom End Rot

By and large, the most common recommendation for using Epsom salts is to treat tomatoes and peppers for blossom end rot. A tablespoon of Epsom salts in the soil before planting or a foliar spray is supposed to prevent this common problem.

But, blossom end rot is caused by a calcium deficiency in your plants. Most often, it’s directly linked to irregular watering. When the soil is dry, plants cannot absorb nutrients readily from it. This can lead to a number of deficiencies, including calcium. That’s why using timed drip irrigation is one way to prevent blossom end rot.

Epsom salt contains no calcium. It does have magnesium and sulfur. But applying that to your tomatoes to treat blossom end rot could make the condition worse.

Another common cause of calcium deficiencies is an imbalance of other nutrients in the soil. An overage of magnesium can cancel out the effects of the regular calcium in the soil, making it a dangerous additive when trying to treat blossom end rot.

Peppers and tomatoes need nitrogen for their initial plant growth and development. Once the plants are old enough to start setting fruit, phosphorous becomes more essential. Regularly fertilize with a balanced fertilizer early in the year per manufacturer’s directions, and switch to a lower-nitrogen, higher-phosphorous fertilizer for fruit development.

It’s also wise to soil test annually to see if you need to fertilize at all, as many home gardeners have rich soil to begin with.

Pest and Disease Control

Up until the 1950s and 1960s, there were sporadic claims that using Epsom salts in the garden would reduce pest populations. There have also been claims of Epsom salt foliar spray treating plant diseases.

Tests on various pests have provided no evidence that Epsom salt helped plants stay pest or disease-free. In fact, applying Epsom salt as a spray on many plants will cause leaf scorch.

But what about the sulfur in Epsom salts? Sulfur is commonly used as a disease treatment, particularly for things like rust or powdery mildew. Unfortunately, the sulfur quantity in magnesium sulfate isn’t enough to be effective.

While sulfur on its own is effective against some pests, higher magnesium levels have little benefit. Methods of pest control or disease control should be targeted to the pests or diseases identified, rather than a homebrewed solution.

Epsom Salt Fertilizer

Some people claim that Epsom salts are a viable fertilizer to prevent magnesium deficiency.

The essential nutrients most plants require are nitrogen, potassium, and phosphorous (NPK). Nitrogen promotes foliage growth. Phosphorous promotes healthy flowering and fruit production. Potassium is used for overall plant health and root and top development.

While magnesium is an essential micronutrient that plays a major role in photosynthesis, many plants only need a tiny amount. Adding Epsom salt to the garden only helps if your soil is truly magnesium deficient. In fact, garden Epsom can create an imbalance that reduces the uptake of other, more essential nutrients.

Excess phosphorous can make soils appear to be magnesium-deficient. Since home gardeners often don’t soil test annually, it’s easy to end up with an excess of NPK nutrients, especially when fertilizing heavily. This can mimic a magnesium deficiency where one doesn’t actually exist.

In commercial croplands, particularly those with acidic sandy soil, magnesium deficiency is possible. This is because of the intensive planting of those areas (and typically, a magnesium deficiency is not the only deficiency present). Commercial cropland is often deficient in NPK, magnesium, occasionally iron or calcium, and sometimes other micronutrients.

This is why nutrient levels overall are boosted by high-potency fertilizers in commercial agriculture. A soil test is always done before amending the magnesium levels or any other nutrients to make sure the right mix of nutrient additives is applied.

Germination Aid

Some claim the magnesium and sulfur in Epsom salt act as a seed germination aid or speed the germination of seeds. Unfortunately, this too is a common gardening myth.

A fully-developed seed is a complete package. Inside, it contains the plant embryo as well as a store of food, all neatly surrounded by the seed coat. Food isn’t a concern, which is why you can germinate seeds on a paper towel or grow edible sprouts in a jar.

What is needed for seed germination from the outside world is light (sometimes), moisture, and the right temperature for germination. The seed will also need oxygen, or else the embryo inside the seed will die off.

Bigger Fruit and Flowers or More Foliage

Claims that the use of Epsom salts improves the size of tomatoes or makes plants bushier are misleading.

What tends to improve the size of tomatoes, peppers, or other fruit is usually phosphorous. As for making plants produce more foliage, that’s usually good nitrogen uptake and a healthy pruning regimen, not the addition of magnesium sulfate.

Another common myth is that it makes an excellent fertilizer for roses. The claims for that originated from rose society members from 1930-1960, all of whom swore it made their roses bloom more. Sadly, studies did not show any evidence of that, and it’s been dismissed by most growers of roses today.

Improves Nutrient Uptake

One of the claims for adding Epsom salt into the garden is that it will improve the nutrient uptake of the standard NPK nutrients.

As described in the fertilizing section above, that’s inaccurate. It can actually reduce the uptake of nutrients, particularly if it’s applied in large quantities.

If anything, the best one could hope for is that adding more magnesium would potentially aid in photosynthesis. It’s an essential part of the chlorophyll molecules found in the cell walls of your leaves.

But if your plant is getting fertilized regularly, it already has plenty of magnesium. The magnesium levels don’t need to be sky-high as they would be if you add Epsom yourself.

Weed Killer

There is no science whatsoever to suggest that Epsom salt on its own is a weed killer. Yes, it can cause damage to plants, but only in large doses in the soil or with foliar application.

The only recipe I’ve been able to find recommends mixing one cup of Epsom salt with two gallons of vinegar and then spraying that concoction onto weeds.

While normal household vinegar does have some effect against weeds, it’s limited at best. Agricultural vinegar used to combat weeds is usually 15-20% acid, while household vinegar is usually between 3-5% acid. In fact, agricultural vinegar is such a strong acid that it can burn your skin. If it can burn you, it’s quite likely it will be effective against weeds.

In contrast, household vinegar is so mild that you can put your whole hand into a bowl of it with no damage. It’s safe enough that we eat it, so it’s not likely to be effective against persistent weeds. Boiling water may actually be more effective and less acidic in your soil afterward.

Is Epsom Salt Bad For Plants?

There are two interlinked claims that suggest Epsom salt does not linger in the soil, and that it is harmless for plants.

Unfortunately, these are erroneous. While it is water-soluble, large quantities of undiluted Epsom salt will linger in the soil. Recommendations for sprinkling as much of a cup of Epsom salts into the garden exist, and that may cause some serious damage to your plants.

Epsom salts have been linked to certain root diseases as well as fruit flavor. Even one tablespoon of Epsom salts per gallon of water has risks. It can also cause leaf scorch, especially if used in hot weather.

Further, dissolved Epsom salts can pollute groundwater. This poses risks to the ecosystem, not just your own garden. There is evidence of rising magnesium levels in areas where this practice is regularly done. That’s on top of other pollutants such as excess NPK fertilizers.

Too much of a good thing is a bad thing, particularly when it relates to your soil.

So is it a Good Idea?

Unfortunately, the only viable use for Epsom salt in gardening is if your property is on farmland that’s been worked to death. Even then, it should only be used if you’ve had a soil test done to see if your soil is deficient in magnesium.

It doesn’t help promote good plant growth in peppers or tomatoes, It doesn’t make bigger flowers on your roses. And it doesn’t help other plants grow.

The appeal of using garden Epsom is that it’s cheap. You can buy a huge box of it at the local pharmacy. But how often do you buy your gardening supplies from a pharmacy?

We recommend you pick up a good-quality organic fertilizer or some compost to supply the vital nutrients your plants need. Skip sprinkling Epsom salts around the base of plants, don’t apply it to your trees or in the planting hole for your newest seedling, and don’t mix it into a gallon of water to spray onto your plants.

There are still many gardeners who will swear it works for them, but if it isn’t able to be replicated in your garden and there are known risks, there’s just no reason to use it. After a century of study, it’s been proven unnecessary for the garden.