Penta plants (Pentas lanceolata) are herbaceous perennials or annuals prized for their brightly colored clusters of star-shaped flowers. Hardy in US Department of Agriculture growing zones 8 through 11, and perennials in zones 10 and 11, they are known for their tough nature and resistance to drought conditions and diseases. Still, they can suffer from a few minor problems.
Pentas grow best in warm conditions. In the fall and winter, freezing temperatures can kill them to the ground. If your plant is showing blackened leaves and stems, it has probably succumbed to frost damage. In some cases, the dead foliage can be cut away from the plant, and the rest of the plant protected from further freezes with a thick layer of mulch. Exposure to full sun is also important. Along with the warmth, the steady rays of the sun will encourage blooming. Locate your plants where they will get a full day's worth of sun in order to stimulate flower production. Poor blooming is often caused by too much shade.
Insect Pest Problems
Pentas can be bothered by aphids and spider mites, both of which are sap-sucking insects. Heavy infestations can cause the leaves to wilt and blooms to drop. Caterpillars can also nibble on the broad leaves of the plant. Spray your pentas with an insecticide when you first notice insect pest activity to control for these pesky bugs.
Soil acidity can also cause a problem for pentas. They need soil with a pH level of 7.0 to produce blooms. Test the pH with a soil testing kit, and if the level is too low, amend the soil with some lime. Use the amount of lime and till to the depth recommended by the results of the test. In addition, make sure the soil is well-draining. Overly saturated soil will cause root rot to develop, a fungal disease that can destroy the roots of penta plants and other ornamental perennials.
Due to their love of warmth, pentas are sometimes grown indoors. Indoor penta plants also need well-draining soil and containers with drainage holes, as well as exposure to as much sunlight as possible. In fact, artificial growing lights might be needed to stimulate blooming. Although the plants probably won't be attacked by aphids, they might suffer from whiteflies, according to the Missouri Botanical Garden. Knock them off with a strong stream of water, or spray the plant with an insecticide, but make sure you take the plant outside first before spraying it.