Although it’s alarming, mature leaves on most evergreen shrubs turn yellow before falling off, typically in spring. If this occurs at other times, you should suspect that there’s a problem. Sometimes, as is the case with insect infestations, the cause is obvious. When it’s not, it may take a bit of sleuthing to determine what is ailing the Vietnamese gardenia (Gardenia augusta “Vietnam”)
Gardenias are quite particular in their requirements, and one of the most significant is soil temperature. A consistent temperature of 70 degrees Fahrenheit is important to the plant’s health. Mulching the soil helps regulate the soil temperature, but if it drops below 70 degrees, the leaves may yellow and drop. Ensure that you’re growing the shrub in U.S. Department of Agriculture plant hardiness zones 8 and higher. If not, perhaps cold soil is causing the problem. Soil pH is also an important factor in growing the Vietnamese gardenia. It does best when the pH is between 5.0 and 6.0.
Nematodes, small roundworms, live in the soil. Some are beneficial; others are root feeders and can kill the Vietnamese gardenia. If the gardenia exhibits yellow mottling on the foliage and appears to wilt during the daytime and then recover at night, suspect nematodes. Damage to the roots varies by nematode species, and a soil test is the only way to learn which one is affecting the gardenia and how to manage it. More frequent watering and fertilizing may help. If the gardenia is heavily infected, remove it and destroy it.
Improper soil pH can lead to nutrient deficiencies in many plants, including the Vietnamese gardenia. At a soil pH higher than 7.0, iron in the soil becomes unavailable to the shrub’s roots and it becomes chlorotic. Young leaves turn yellow with green veins while mature leaves may turn yellow at the edges. Manage chlorosis by applying chelated iron to the soil once a year, typically in spring. Application rates may vary according to manufacturer but a rule of thumb is to use 1 ounce of the iron chelate in 1 gallon of water. This will treat 100 square feet of soil. Drench the soil from the base of the gardenia to 1 foot beyond the drip line. You may need to make additional applications throughout the season if symptoms reappear.
When you notice yellow foliage, suspect a problem with the gardenia’s roots. Root rot, caused by a number of fungal pathogens, is evidenced by several symptoms, one of which is older leaves turning yellow and dropping from the shrub. You’ll need to inspect the Vietnamese gardenia’s roots to make a definitive diagnosis. The small, non-woody roots should be white. If they are brown, or you can easily pull away the outer layer, they are most likely rotten. Cut away those areas that are rotten and keep the soil cleared from the crown and top of the roots until they've dried completely. Use fungicide soil drenches that contain mefenoxam or phosphonates. If the Vietnamese gardenia is heavily infected, remove it and destroy it.
- National Gardening Association: Gardenia
- Clemson Cooperative Extension: Gardenia Diseases and Other Problems
- Southern Agricultural Insecticides, Inc.: Chelated Liquid Iron
- University of California Statewide Integrated Pest Management Program: Phytophthora Root and Crown Rot
- UMass Extension: Gardenia – Phytophthora Root Rot
- Ohio State University Extension: Root Problems on Plants in the Garden and Landscape