How to Prevent Yellowing of Gardenia Leaves

How to Prevent Yellowing of Gardenia Leaves

If you have spotted a few yellow leaves on your gardenia plant, quit worrying. This Gardenerdy article will tell you why yellowing occurs in gardenia leaves, and how you can stop it.
Gardenerdy Staff
Did You Know?
Gardenia plants are members of the coffee family.
Gardenias are evergreen plants that are popular for their large, white blossoms. These plants flower during the warm months of November to May, and in many cases their excellent fragrance is associated with summer. These sun-loving plants can either be grown outside or as houseplants. They can be grown in the USDA hardiness zones 8 - 10, and require dry, acidic soils. These plants are used as hedges, ground covers, mass plantings, or are simply grown as single specimens in gardens.

Growing them in some cold states can be a challenge, and sometimes, even experienced gardeners can do little to stop the plant from dying out. While this plant is difficult to grow and care for, the most common problem faced is the yellowing of its leaves, also called chlorosis.

Chlorosis is a problem in which the amount of chlorophyll in the leaves gets reduced. Chlorophyll is a pigment present in the leaves and young stems of plants which gives them their characteristic green color. When the content of this pigment goes down, the leaves appear yellow, brownish, or even white in some cases. While there can be a variety of causes as to why gardenia show yellow leaves, the most common are over-watering, mineral deficiencies, and insufficient sunlight. Given below are the causes of yellow gardenia leaves, and some remedies to stop this yellowing.

Causes and Prevention

Magnesium Deficiency

Soils that are low in magnesium are a common cause for the yellowing of gardenia leaves. This is commonly manifested by the yellowing of the bottom leaves of branches, while the leaves at the top remain green. Also, a small, green triangle shaped like the plant's leaf will be seen at the base of yellow leaves.

Remedy
Add a fertilizer rich in magnesium to the soil, or add Epsom (magnesium sulfate) salts. Dilute one teaspoon of Epsom in one gallon of water and apply this mixture once every three to four weeks. However, some experts criticize this method, saying Epsom has no effect on plants. Some advise applying Epsom to soil not more than once every year.

Manganese and Iron Deficiency

Iron deficiency can cause the yellowing of gardenia leaves, though in a different pattern than magnesium deficiency. In this case, the leaves at the bottom of the branches and the veins stay green, while those at the top turn yellow. Manganese deficiency shows symptoms similar to iron deficiency, making a soil test all the more necessary to identify the real culprit.

Remedy
Using a balanced fertilizer of ratios 6-6-6 or 10-10-10 will reduce both iron and manganese deficiency. Applying chelated iron is a good way to reduce iron deficiency, and an added advantage is that these salts are taken up slowly. Apply these formulations once every two to four weeks.

Fertilizers

Using excess fertilizers will damage the plant roots, in addition to causing yellowing of the leaves. This root damage is generally irreversible and the fertilizer may even burn the plant. The root damage will restrict the uptake of nutrients by the plant, causing the leaves to turn yellow.

Remedy
When using a fertilizer, always select one that is designed for flowering and acid-loving plants like gardenia. Follow the manufacturer's directions too. Apply fertilizers at a distance of 1 foot away from the plant root to prevent them from getting absorbed at their full strength.

Pests

Pests like aphids, mealybugs, scales, and white flies too are commonly responsible. The common symptoms of these pests is damage to the stem of the plant, to the edges of the leaves, and thin, cottony threads found all over the plant. These pests suck the plant sap and leave behind a sticky secretion called honeydew, which causes the growth of the fungus sooty mold. The fungus blocks the leaves from sunlight, causing leaf yellowing.

Remedy
Take 2½ teaspoons of horticultural oil and dilute it with 1 gallon of water. Spray this solution over the gardenia until the leaves start dripping. Alternately, you can release a blast of water from a hose to remove the pests. One can also pick the pests from the plant using the hands or a cloth.

Water Quality

Watering the gardenia using normal tap water may cause yellowing of its leaves, since tap water is mostly hard, containing calcium present as limestone. Also, water runoffs from concrete, being alkaline, can affect nutrient uptake, causing chlorosis and yellow leaves. On the other hand, using soft water is not advisable since it contains salts.

Remedy
When possible, use rainwater or distilled water to water the plant. The water should always be near room temperature. Dissolve 100 ml of vinegar in one liter of water to acidify the water, and water the plant with this mixture once every month. You can also dump used tea leaves or coffee grounds in the soil to create slightly acidic conditions. Add mulch to the soil to lower its pH.

Poor Drainage

Soil with poor drainage can cause leaf yellowing. Drainage problems can be caused due to over-watering the plant. Gardenias need most soil, which should not be too wet. Over-watering and poor drainage are the most common causes of yellowing of gardenia leaves in pots. Too much watering will prevent the gardenia from absorbing nutrients from the soil, leading to yellow leaves.

Remedy
Add compost to the soil to set up proper drainage. For potted gardenias, place the pot on a tray filled with water and pebbles. This will provide a humid environment without causing any ill-effects of over-watering. Make sure that the pots have holes at the bottom for drainage. Provide the plants with an inch of water once a week, without over-watering them.

Soil pH

Soils with a pH of 7.0 or above may cause yellowing of the leaves, as the plant prefers a pH range of 5.0 to 6.5. This may happen if the soil is rich in salts, or if it has been watered with hard water. Alkaline soils prevent plants from absorbing minerals like iron and manganese from the soil, causing yellowing of the leaves.

Remedy
Check the soil pH. Regularly leach the soil using rainwater to drain away the salts. Add manure or compost to improve the soil properties. Using an acid-forming fertilizer will prevent the ill-effects of alkaline soil, such as yellowing of the leaves. Add aluminum sulfate or water-soluble sulfur to the soil to improve its iron availability.

Lack of Ventilation or Light

Lack of ventilation may be one of the reasons too. If the gardenia is placed too close to a wall or another plant, the lack of airflow may cause conditions suitable for fungal growth. The fungus then reduces the rate of photosynthesis, causing yellowing. Gardenias need bright sunlight for good growth. If kept indoors in poor light, the leaves may start yellowing. Too much exposure to sunlight, on the other hand, may cause excessive darkening of the leaves.

Remedy
Trim the surrounding plants or the gardenia itself, or move the plant to a more ventilated place. Provide the plant with plenty of sunlight. For a potted gardenia, exposure to bright sunlight is important, but preferably keep it out of the hot afternoon sun. It is advisable to expose the plant to six hours of direct sunlight for optimum growth. However, exposing it to very hot sunlight can burn the leaves.

Cold Temperatures

Exposure to cold conditions can cause both, yellowing of the leaves and growing points. The plant prefers warm temperatures. Temperatures that go below 70°F are harmful and cause yellowing. The plant thrives at temperatures in the range of 65°F to 70°F in the daytime, and 60°F to 62°F at night. A wide fluctuation in temperature is likely to add to this problem.

Remedy
Place potted gardenias in a heated area. If placed outdoors, shift them indoors during winter. Expose them to a few hours of sunlight every day. A sun lamp can be used to provide warmth. Gardenias can be grown between two shrubs such as roses and others, which reduces the effects of cold weather.

Plant Diseases

Some plant diseases are the chief causes of yellowing leaves. Root rot, a fungal disease that affects the roots, is one such disease. The older leaves first turn yellow and start falling off. Canker is another disease caused by the fungus Phomopsis gardeniae. A swollen region in the stem with the area above the swollen region having turned yellow, is a classic symptom of this disease. Powdery mildew is another disease, in which a powdery substance is visible on the leaves before they start yellowing.

Remedy
Grow the gardenia in loose, well-drained soil. Before buying the plant, it's preferable to check if its roots are white. If the soil is soggy, transfer the plant to a raised bed. There is no cure for canker. The plant has to be uprooted, and ensure that no other plant is grown at the same site. In case of powdery mildew, snip off the infected branches with bleach-sterilized equipment and ensure that the soil around the plant is without any plant debris.

Natural yellowing of leaves is nothing to be afraid of, for it is a part of the plant's life cycle. It is the yellowing due to external causes, such as the ones listed above, that you need to beware of. Just provide your gardenia with humid, well-drained soil and good sunlight, and leave them alone. In case of gardenias, too much love can be dangerous.