Is Epsom Salt Good or Bad For Hydrangeas? (2024)

Of all the gardening hacks on the internet, Epsom salt has to be one of the most pervasive. It was entrenched in the gardening industry long before “going viral” was a thing – an established part of gardening folklore. But can you apply Epsom salt to your hydrangeas, or will it hurt them?

This compound, magnesium sulfate, is recommended for so many issues in the garden that it’s hard to keep track. It is claimed to improve growth and flowering, act as a fertilizer, deter pests and diseases, and resolve various growth problems.

However, when you look for the science to back up these claims, you’ll find evidence suspiciously absent. If you’re wondering whether a scoop of Epsom salt can solve all your hydrangea problems, you’ve come to the right place.

The Long Answer

Epsom salt, or magnesium sulfate, is a chemical compound containing magnesium, sulfur, and oxygen (MgSO₄). It’s named after a town in Surrey, England – Epsom – where it was originally discovered in the natural mineral waters (which also explains why the ‘Epsom’ is always capitalized).

Magnesium sulfate has several interesting uses across various industries. Its high magnesium content makes it a popular choice for relieving sore muscles.

It is often added to bath salts with soothing fragrance oils. Some people use it as a natural remedy for stress, insomnia, and even sunburn.

In the gardening world, Epsom salt is often used as a supplement with the goal of improving plant growth. In fact, it is one of the most widely recommended “alternative” soil amendments that I get asked about. This ubiquity has made the substance a common addition to the tool kit for many gardeners.

Why Do Gardeners Use Epsom Salt?

Epsom salt contains magnesium and sulfur, two vital secondary nutrients for plants. Secondary nutrients are not required in high amounts like macronutrients (nitrogen, phosphorus, and potassium). However, they are still essential to overall plant health.

Magnesium plays a crucial part in the formation and functioning of chlorophyll. This is the component responsible for plants’ green color and their ability to survive. It aids in photosynthesis, helping plants convert sunlight into energy or “food” for growth. That also means that the lack of it may result in stunted growth, along with other issues like yellowing leaves.

Sulfur is also a secondary nutrient vital for overall plant growth. It helps produce essential plant proteins and amino acids and also helps with chlorophyll formation as magnesium does.

Given that Epsom salt contains two nutrients essential in plant growth and survival, it became a quick and easy way for gardeners to supplement these nutrients when deficiencies are detected.

But that’s not where the uses stopped. As gardeners continued to recommend Epsom salt, claims that it aids germination, boosts flowering, and even makes flowers more vibrant began to emerge. Despite the lack of scientific evidence to back up many of these claims, it was touted as a panacea for many gardening ills.

Epsom Salt and Hydrangeas

Although Epsom salt’s supposed uses are quite general, a few uses are specific to hydrangeas, from flowering to pest control. Unfortunately, as with the other benefits mentioned, the science doesn’t quite match up with the beliefs.

Changing Flower Color

Hydrangeas are widely known to change color based on the soil’s pH level. Not all species have this ability, but some Hydrangea macrophylla and Hydrangea serrata varieties do, making them widely sought-after. While this seemingly magical change is fascinating, the use of Epsom salt does little to influence it.

Hydrangea flower colors (in species that change color) depend on the availability of aluminum ions in the soil. The soil pH directly influences aluminum availability. In acidic soils, aluminum ions are readily available, causing the flowers to be blue. In alkaline soils, aluminum becomes less available to the plant, resulting in pink flowers.

Magnesium sulfate adds magnesium and sulfur to the soil. However, neither of these elements has a large enough impact on the soil’s pH level or influences the availability of aluminum in small amounts. Therefore, using Epsom salt won’t have any impact on flower color.

In fact, adding too much magnesium (which hydrangeas don’t really require in high amounts) could potentially lead to an imbalance in soil nutrients, making it even harder for the hydrangeas to access the aluminum they need to produce blue flowers.

The verdict: Epsom salt does not change hydrangea flower color.

Improving Flowering

If Epsom salt can provide key nutrients in growth and flower production, it makes sense to assume that adding Epsom salt to the soil around your hydrangeas can boost flowering. But in practice, it’s not that simple.

Hydrangea flowering is not primarily dependent on magnesium or sulfur, the two main components of Epsom salt. In other words, adding more than necessary of either of these nutrients will not encourage the plant to push out more flowers.

However, magnesium and sulfur do play an important role in overall plant health, as mentioned. Your hydrangea will flower to the best of its ability if they are present in sufficient amounts to keep the plant happy and all other conditions are perfect. But if sulfur or magnesium, in particular, are deficient, replenishing these nutrients will improve hydrangea health, subsequently improving flowering.

But there’s a catch. Garden soil usually contains enough of these secondary nutrients to keep hydrangeas satisfied. Even if there is enough magnesium in the soil, an excess of phosphorus can limit the amount plants can absorb, leading to an assumption that magnesium is the problem when it may not be. The only way to know for sure is to conduct a soil test.

If a soil test reveals your soil is deficient in magnesium, applying Epsom salts may help temporarily improve conditions to boost flowering. However, if there is no deficiency, Epsom salt will have no effect on flowering. Even if there is a deficiency, it’s far better to improve the overall soil health with organic matter like compost than continually adding salts for short-term results.

In fact, using Epsom salts on hydrangeas long-term may be counterproductive, leading to disappointing flowers rather than prolific ones.

The verdict: Epsom salt may improve flowering if soil is deficient in magnesium, but there are better ways to improve deficiencies that contribute to overall soil health.

Deterring Pests And Diseases

In the world of pest and disease myth and legend, it seems there is nothing Epsom salt can’t do. Some believe it’s a deterrent for slugs and beetles, while others think it can ward off fungal diseases. A few pests and diseases can plague hydrangeas, so you may be considering Epsom salt as a defense.

Unfortunately, like many unfounded claims, the idea that Epsom salt deters garden pests is largely anecdotal. The theory was likely passed down from gardener to gardener without much scientific evidence to back it up.

Slugs supposedly despise magnesium sulfate, but they don’t typically have an aversion to either magnesium or sulfur. Maybe the ‘salt’ in the name led to this belief, spreading widely despite a lack of proof. In reality, most pests are more influenced by environmental conditions like temperature, moisture, and the availability of their preferred foods than by Epsom salt.

Regarding diseases, it’s true that sulfur is used in some fungicides. However, these products typically contain sulfur in a different form and in much higher concentrations than you’d find in Epsom salt. To combat a fungus problem, commercially available fungicides or organic alternatives are your best options.

Applying Epsom salt in large amounts can also lead to a potential overabundance of magnesium, inhibiting the uptake of other essential nutrients. This can leave plants more susceptible to diseases rather than protecting them.

The verdict: Epsom salt as a barrier against pests and diseases isn’t scientifically substantiated and could be potentially detrimental.