Epsom salt, chemically known as magnesium sulfate, is a common household item used as a garden supplement to improve the health of hydrangeas. This compound provides two secondary nutrients—magnesium and sulfur—that plants need for lush foliage and robust growth. Applying Epsom salt addresses specific nutrient deficiencies in the soil, enhancing the plant’s overall appearance. Understanding proper application is key to supporting your hydrangeas without causing unintended harm.
Why Hydrangeas Need Magnesium and Sulfur
Hydrangeas, like all plants, require magnesium and sulfur for basic biological functions. Magnesium is the central atom in the chlorophyll molecule, the pigment responsible for capturing sunlight during photosynthesis. If a hydrangea is deficient in this nutrient, its older leaves will often begin to display interveinal chlorosis, which is a yellowing of the leaf tissue while the veins remain green.
Sulfur plays a significant role in the plant’s metabolism. It is necessary for the formation of certain amino acids and proteins, which are the building blocks for healthy plant structure. A lack of sulfur can cause a general yellowing of the younger, newer leaves. Applying magnesium sulfate is a highly soluble way to quickly replenish both nutrients if a deficiency is present.
Step-by-Step Mixing and Application
The most common way to apply Epsom salt is by dissolving it in water to create a liquid soil drench. For an established hydrangea shrub, mix about one tablespoon of Epsom salt into one gallon of water. Stir the solution thoroughly until the crystals are completely dissolved, ensuring the roots can immediately absorb the nutrients. Pour this liquid directly onto the soil around the base of the plant, extending out to the drip line.
Avoid splashing the mixture onto the leaves or stems, as the concentrated solution can sometimes cause leaf scorch. For smaller or newly planted hydrangeas, reduce the amount of solution applied proportionally to avoid over-fertilization.
If you prefer a dry application method, sprinkle about one tablespoon of Epsom salt for every foot of the shrub’s height around the drip line. Once the dry salt is distributed, water the area thoroughly. This watering is essential to dissolve the magnesium sulfate crystals and carry the nutrients down into the root zone.
Optimal Timing for Treatment
The best time to apply Epsom salt is during the active growing season, generally starting in early spring as new growth emerges. Applying it then ensures the plant has the necessary nutrients to support new foliage and the development of flower buds. The frequency of application should be limited to prevent nutrient build-up in the soil.
Gardeners often reapply the solution every four to six weeks throughout late spring and summer, especially during the blooming season. However, it is safer to limit applications to once a month or only when signs of magnesium deficiency, such as yellowing older leaves, are visible. Application is unnecessary in the fall and winter when the plant is preparing for dormancy, as nutrient uptake is significantly reduced.
When to Avoid Using Epsom Salt
Applying Epsom salt without a confirmed deficiency is not recommended, as it can disrupt the balance of other nutrients in the soil. An excess of magnesium can interfere with a hydrangea’s ability to absorb other macronutrients, specifically calcium and potassium. This imbalance can lead to other deficiency symptoms, potentially causing more harm than good.
It is advisable to perform a soil test before regularly adding magnesium sulfate to your garden. A soil test confirms if the magnesium level is low and provides a specific recommendation for supplementation. Epsom salt does not change the soil’s pH, meaning it will not influence the bloom color of Hydrangea macrophylla (bigleaf hydrangea), which is a common misconception.