The angel trumpet or Brugmansia plant gets its name from its trumpet-shaped yellow, pink or white flowers. They can also be grown in pots as house plants. While the flowers may be yellow, yellowing leaves are not normal; they can be a sign of either watering problems or insect infestation. Both should be dealt with as quickly as possible.
Improper watering can lead the leaves of the angel trumpet plant to turn yellow. If watering is your trouble, the yellowing leaves will be accompanied by overall wilting or drooping in the plant; if it gets bad enough, the leaves may even fall off. Angel trumpets use a large amount of water during the growing season to promote their vibrant blooms, and if they don't receive enough water, you will notice the tell-tale signs of drought.
Pay close attention to your angel trumpets in spring and summer, as this is when they use the most water. Depending on the temperature and humidity of your area, you may need to water the plants every single day. If you are unsure if they need to be watered, stick your finger in the soil. If the top inch of soil is dry to the touch, water them until the soil is uniformly moist. These plants need less water in the winter when they are not growing.
Spider mites are attracted to angel trumpets when they are suffering from water drought or when the air around the flowers is particularly dry. The little red bugs infest the underside of the leaves of the angel trumpet and feed on the leaves, causing them to fade from green to yellow and to fall from the plant. If not treated, the spider mites can cause complete defoliation of the plant and potentially lead to its death.
Treating Spider Mites
Inspect the plant and remove any stems or leaves on which you see white webbing; this is a sign of the spider mites. You must remove all webs, or the mites spread to other areas of the plant or nearby healthy plants. Make sure you follow watering recommendations, as dry plants are more susceptible to the mites. If the air is particularly dry, run a humidifier near your trumpet house plant, or mist your outdoor plant every morning with fine spray from a hose. This increases the humidity around the plant. If necessary, treat stubborn infestations with pesticides approved for spider mites. Follow all product instructions for safe and effective application.