Large yellow to mauve flowers that look like fat trumpets adorn the stem tips on allamandas from the spring through the fall. Native to tropical parts of Central and South America, 12 species of allamanda exist. Among the most ornamental and widely grown in American gardens is species Allamanda cathartica, also called the golden trumpet. Yellowing leaves suggest improper culture, often the result of the change in seasons, or a bout with an insect.
Allamanda plants are evergreen and must not be exposed to subfreezing temperatures. In the winter, keep well-drained garden soil slightly moist to slightly dry. Water only when the soil becomes dry to the touch at a depth of 1 inch. Once spring warms and new leafy growth appears on the allamanda's branches, increase watering to maintain an evenly moist soil. Soil moisture during the summer is important, as the intense heat and sun's rays increase the need for water. Do not over-water, and do not plant allamandas in mucky or flooding soil. Both bone-dry and waterlogged soils cause unhealthy yellowing leaves.
Grow allamanda in fertile soil. Avoid dry, nutrient-poor sandy soils or subpar fill soil that is often placed around the foundations of newly constructed buildings. A nonalkaline soil that is enriched with organic matter is ideal. Infertile and alkaline soils lead to leaf yellowing and stunted growth. Allamandas respond favorably to fertilizers when they are actively growing from the spring through the fall. In conjunction with the increased watering during the warm months, apply a liquid fertilizer as part of the watering process once every two to three weeks. Use a well-balanced formulation, such as 10-10-10. Stop fertilizing in the fall when you also diminish the watering amount.
Two types of insect pests often plague allamandas. When plants are stressed and weakened by improper watering or infertile soils, infestations by spider mites or whiteflies are more likely or are exacerbated. Spider mites occur in dry conditions, especially in windless, arid air. Thin webs net leaf undersides, and reveal the tiny red bodies of spider mites. Wipe off the webs, increase soil watering and spray the foliage with a water nozzle to dislodge the mites. Whiteflies can occur any time of year. Touch an infested allamanda, and a cloud of tiny white bugs will take to the air. Use a pesticide to rid an allamanda of these fast-multiplying pests.
As summer turns to fall and early winter, allamanda plants naturally drop some of their oldest leaves. The leaves first take on yellowy hues. Temperatures below 45 degrees Fahrenheit often promote more leaf drop, and a light frost may fully yellow a plant, followed by leaf drop a day or two later. Cool nighttime temperatures coupled with wet soil can make leaf yellowing more pronounced. During the cooler days and lower sunlight intensity of winter, keep the soil slightly dry.