Gardenias don't do well without extra attention here in the Central Valley. The reasons for their failure to thrive provide an important lesson in how soil pH and salts levels affect crops, turf grasses, trees and landscape plantings in our area.
Gardenias grown here often show signs of iron deficiency-yellow leaves with green veins, twig dieback, stunted growth and reduced flower production. But adding extra iron to the soil doesn't return leaf color to normal or improve the plant's vigor. Why is that? The problem lies with our highly alkaline soil and water as well as the high levels of salts in our soil.
Alkaline soil (above 7.5 pH) is common in hot arid climates like ours. Acid soil (5 to 6.5 pH) is common in cooler, rainy climates and heavily forested zones like the Northern Pacific coastal regions. Most plants, including roses, grow best in neutral or slightly acidic soils with pH levels from 5.5 to 7.5. Gardenias, azaleas and blueberries thrive in acid soils with pH levels at 5 or 5.5 (and blue hydrangeas stay blue in acid soils).
The high pH level of our soils interferes with the plant roots' ability to draw up essential nutrients, especially iron. That's why adding iron to the soil is not enough to correct the problem. We need to lower the soil pH so that the roots can take up the additional iron.
Adding sulfur to the soil will lower soil pH levels over time. Sulfur granules used to be easy to find and fairly inexpensive, but fewer garden centers are stocking the granules. Ammonium sulfate, applied to the soil to keep blue hydrangeas blue, is one source of additional sulfur. Epsom salts or magnesium sulfate, is another. Fertilizers formulated for shade plants including azaleas often contain additional sulfur, usually about 5%.
Feed sickly gardenias monthly during the summer bloom period with a half cup of a shade plant fertilizer, a half cup of iron and a half cup of sulfur granules, or ammonium sulfate or magnesium sulfate and work the sulfur product into the soil. The leaves should start turning green in about a month, but it may take several months for them to turn really dark green and have a shine to them. After the sulfur has permeated the soil around the plant, a couple of applications annually of additional sulfur should keep the gardenias healthy.
Salts build up in our soil because our annual rainfall amounts are insufficient to wash them out or leach them out and because high temperatures cause moisture to evaporate quickly. The buildup of salts also interferes with plants' ability to draw up nutrients. In wet years (not this dry one), we should let the bubblers run a couple of times a year until the soil is saturated, reducing the salts content. This dry year, we need to lay down a 3 to 4-inch layer of an organic mulch to reduce evaporation and slow down the salts buildup.