Mandevilla plants, formerly called dipladenia plants, are flowering and vining plants that put out large flowers each growing season against a backdrop of dark green oval leaves. If the leaves of the mandevilla turn yellow, the plant is experiencing a health problem that may affect its ability to produce further flowers.
Temperature and Sunlight
Mandevillas are tropical plants that require a great deal of direct sunlight and warm temperatures to survive. A minimum of six hours of full sun each day is necessary for the successful growth of these tropical plants. Additionally, mandevillas will not survive in temperate areas of the United States where they face freezing temperatures. A lack of sunlight or overnight or seasonal temperatures that drop too low damage the mandevilla and cause it to produce yellow leaves.
As tropical plants, mandevillas need a great deal of water to sustain the health of their leaves and flowers. Long periods of drought are certain doom for these plants, although they can go brief periods in dry soil, extended lengths of time cause the leaves to turn yellow, beginning at the edges of the leaves. Eventually leaves turn brown and drop from the plant. Water a mandevilla plant at least once a week to keep the soil healthy and moist.
The condition of the soil is key to healthy, dark green leaves on mandevilla plants. A pH range of 5.5 to 6.5 is ideal; perform a soil test each spring to determine whether or not the soil is right for the mandevilla. To increase the pH of the soil, add lime. To decrease the pH of the soil, add aluminium sulphate or sulphur, according to the Clemson Cooperative Extension. Manage the soil properly, as a pH that is out of balance causes the mandevilla to turn yellow.
Spider mites, mealybugs, scale insects and aphids all feed on the sap and nutrients in the mandevilla's leaves. Although minor pest infestations may cause only a few leaves to turn yellow, which can be pinched off to maintain the appearance of the plant, a severe infestation leads to problems with stunted growth, greater exposure to disease and leaves that turn yellow and die prematurely. Apply a pesticide or insecticidal soap to the mandevilla to maintain the health of its dark green leaves.