Does Epsom Salt Really Help Hydrangeas Bloom?

Closeup of blue hydrangea flowers surrounded by green foliage — Credit:
aassemany / Getty Images

Key Points

Understanding what really helps hydrangeas bloom can save you time, money, and protect your garden from harm.
Epsom salt won’t help hydrangeas bloom or change color—it doesn't affect the soil in a way that generally boosts flowers.
However, you can add Epsom salt if a soil test indicates a magnesium deficiency.
To turn hydrangeas blue, lower your soil’s pH using sulfur—not Epsom salt.

Adding Epsom salts to your plants is a common practice that benefits flowers like roses. However, there are some misconceptions about using Epsom salts to grow hydrangeas. To learn more about what using Epsom salts can do for your hydrangeas and the precautions to take, we spoke to gardening experts. Ahead, we debunk the myth about adding this ingredient to your hydrangeas and offer an alternative approach for achieving bigger, brighter blooms.

Tony Avent, proprietor at Plant Delights Nursery, Inc., at Juniper Level Botanic Garden
Barbara Fair, associate professor and landscape extension specialist at North Carolina State University and certified arborist

What Is Epsom Salt?

Epsom salt is magnesium sulfate, a natural mineral compound that is made of magnesium, sulfur, and oxygen. In gardening, Epsom salt can be used to treat magnesium deficiency, which can cause yellow leaves and poor plant growth.

Is Epsom Salt Good for Hydrangeas?

Before adding any nutrient elements to your soil, it is highly recommended that you have a professional-grade soil test done to confirm any deficiencies in your garden. At-home test kits are not recommended for accuracy; they typically only show pH and don't provide the reasons behind it, says Tony Avent, proprietor at Plant Delights Nursery, Inc. "If the soil test shows that soils are low in magnesium, but the calcium levels and pH are already correct, then it could be good for the soil, overall," says Avent. When using a foliar application, it is important to follow the instructions to prevent burning the petals of the flower.

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Can Epsom Salt Turn Hydrangeas Blue?

"Epsom salts have absolutely no effect on the color of any hydrangea species," says Avent. The primary factor influencing the color of hydrangeas is the availability of aluminum, which depends on the soil's pH. "A soil pH below 5.5 will produce a blue inflorescence on Hydrangea macrophylla, while a pH above 7.0 will produce a pink inflorescence. Any pH between 5.5 and 7.0 will produce plants with a purple inflorescence," according to Avent.

Epsom salts are composed of magnesium, which raises the pH, and sulfur, which lowers the pH of the soil. Since Epsom salts have both of these compounds, they counteract each other and, as a result, do not influence the flower color. "My recommendation would be to add a sulfate-type product that lowers the pH to get the blue flower colors, but does not necessarily add any nutrient elements. You can do this by adding elemental sulfur," says Barbara Fair, associate professor and landscape extension specialist at North Carolina State University.

Risks of Using Epsom Salt

If your soil isn't deficient in magnesium, adding Epsom salt can cause more harm than good to your garden, so using a professional soil test is crucial. Adjusting the pH for a hydrangea can have disastrous effects on all of your other nearby plants, says Advent. Misusing this nutrient element can shift the overall soil pH. "You could also disrupt the balance between calcium and magnesium in the soil," says Fair.