Gardenias are popular flowering plants in the southern United States. Gardenias grow best between 68 and 74 degrees. However, they also grow in temperatures above this ideal range. Aphids are small soft-bodied pear-shaped insects. They are less than 1/8-inch long and begin to thrive in temperatures above 75 degrees. As temperatures rise, aphids can sometimes be a problem for gardenias.
Aphids are primarily a spring and summer pest. As spring temperatures increase to temperatures ideal for the insects, aphids that have wintered over will begin to hatch. Seven to 10 days after they've hatched, adult aphids begin to lay eggs. Unless treated, aphids will continue living and reproducing until autumn temperatures drop below the temperatures at which they can live. Aphids won't survive as adults below 45 degrees but can survive with limited activity between 45 degrees and 75 degrees.
Aphids feed on leaves. Although a few aphids may not do visible damage to the leaves, they excrete a sticky substance called honeydew that can become a host for mildew. The most common mildew that grows in this honeydew is sooty mold fungus. Sooty mold fungus is a black mold that can, in addition to being unsightly, interfere with photosynthesis in your gardenias.
Although a number of types of aphids can infest your gardenias, cotton aphids are a common variety that plagues gardenia growers. This aphid is between 1/16- and 1/8-inch long and is yellowish to dark green in color. It sometimes has a black head and thorax. This type of aphid overwinters in greenhouses, so treat any infestations thoroughly to prevent these bugs from coming back in subsequent years.
Aphids are a problem in most places where gardenias can grow outside. Gardenia grows in USDA Hardiness Zones 8 through 10. It grows in coastal parts of the U.S. from the Carolinas south around through Texas then up the west coast as far north as northern coastal Washington State. Because all of these areas regularly experience warmer temperatures, these climates are also ideal for aphids.
According to Cornell University, light colored or reflective mulch may deter aphids from moving on to your gardenia. Use natural aphid predators, such as ladybugs, to control aphid populations that are already established. Releasing a container of ladybugs on your gardenia plants will reduce or eliminate the infestation. Avoid using broad-spectrum insecticides as these can, in addition to killing the aphids, kill beneficial insects and aphid predators.