The element magnesium comprises the central molecule in the chlorophyll pigment in green plants. If magnesium is lacking in the soil, a plant often displays this nutrient deficiency with the yellowing of older, mature leaves. Epsom salts (magnesium sulphate) are water-soluble crystals that enrich soils with magnesium molecules for acid-loving plants like gardenias to utilise. According to Washington State University, there is little research that precisely lists dosages for evergreen plants. Over-applying magnesium can lead to deficiencies in iron or manganese, and excessive Epsom salt applications leach through the soil and end up elsewhere in the ecosystem. A humus-rich, acidic soil treated each spring with some Epsom salt should provide a fertile area for gardenias.
Fill a gallon-sized container with water.
Measure out 1 teaspoon of Epsom salt crystals and pour it into the water. There's no need for a precise measurement; the teaspoon can be heaping or levelled for an effective amount to make the solution.
Shake or stir the water to dissolve and distribute the salts throughout the water.
Pour the Epsom salt solution into a sprinkling can or directly onto the soil surrounding the gardenia shrub. Don't pour the solution toward the trunk, but focus it in the area just under the edges of the leaves where more tender, tiny root hairs grow.
Mix additional solution as needed to wet the entire root zone of the gardenia shrub. Larger shrubs grown outdoors in the ground need more water than shrubs grown in containers as houseplants or patio container specimens.
Maintaining an acidic, humus-rich soil for gardenias diminishes the chances of plants developing magnesium deficiencies. Neutral and alkaline soil bind magnesium molecules, which can result in yellowed leaves. Apply the Epsom salt solution in spring just before new growth emerges. You're acting to prevent the deficiency in leaves. If you treat the gardenia in summer, fall or winter, many weeks will pass before the roots absorb and transport the magnesium into all the leaves across the plant.
The effects of watering with Epsom salt aren't immediate. It takes weeks to months for the yellowing of leaves to diminish, depending on how much the gardenia plant is growing and metabolising. Don't over-fertilise with Epsom salt solely to fix the problem. Remember, Epsom salt only supplies magnesium and sulphur to the soil, and doesn't add nitrogen, phosphorus, potassium or other trace nutrients needed for plant health. Replenish soil organic matter content and maintain an acidic soil. Coffee grounds, pine bark nuggets and acid-forming fertilisers help gardenia roots take up magnesium molecules.