If you'd like to bring a touch of the tropics to your garden, a plumeria tree (Plumeria spp.) could be the perfect choice. Also called frangipani, they're renowned for its clusters of waxy, often fragrant flowers that appear in summer. All plumerias grow outdoors year-round in U.S. Department of Agriculture plant hardiness zones 9 through 12, or indoors as large potted plants. If your plumeria's leaves begin to turn yellow, this could be a normal process or it might signal a problem.
Not to Worry
Most plumerias have large, leathery leaves that can be 1 foot or more long. If the oldest, lower leaves on a plumeria gradually turn yellow and eventually fall off, but other leaves stay green and look healthy, this is a natural process that occurs as the plant grows actively, generally from spring through fall. If you're growing one of the deciduous types, it's also natural for leaves to turn yellow, dry up and drop from the plant in fall as cool weather approaches. Deciduous types include pink frangipani (Plumeria rubra), a 25-foot-tall tree with pink or red flowers, or either of two white-flowered varieties -- one is the 40-foot tall white frangipani, West Indian jasmine or nosegay (Plumeria alba), the other, pagoda tree (Plumeria obtusa), has smaller, 8-inch-long leaves on a 25-foot-tall, shrubby tree.
Not Enough Water
Because they need regular, even amounts of water, dry spells can cause plumeria leaves to turn the leaves yellow and cause them to drop off the plant, with young, more tender leaves suffering first. Even the more drought-tolerant, evergreen bridal bouquet (Plumeria pudica) might have yellow leaves during a long dry spell. If you see yellowing leaves, check the soil under a plumeria by digging down several inches. It it's dry, give the plant supplemental water, aiming for about 1 inch of water weekly, including any rainfall. Add a thick layer of organic mulch on the ground under the tree's canopy to help conserve soil moisture, but keep the mulch back a few inches from the main stems to discourage fungal diseases.
Plumerias are susceptible to a fungal disease called plumeria rust. It first appears as many tiny, yellow, powdery spots on the undersides of the plant's leaves, giving them a mottled, yellowish appearance. Eventually, the leaves turn a uniform, bright yellow, then become brown and drop from the plant. If the infection becomes widespread, a plumeria can lose most or all of its leaves, causing severe stress and potentially killing the plant. Once present, it's not possible to cure plumeria rust, but you can help prevent it by planting where air circulation is good and by keeping the area clean -- pick up and destroy all fallen leaves, control weeds under and around the plant to help lower humidity, and remove any leaves with early signs of infection, wiping your tools between cuts with a cloth soaked in rubbing alcohol to prevent spreading the fungus.
A plumeria that's infested with pests might develop yellow leaves -- the most likely culprit is a tiny pest called the sixspotted spider mite, although other types of mites can also infest plumeria. The leaves can develop yellow stripes down their centers and you might also see filmy webs covering young leaves. As mites feed on the plant juices, the leaves turn entirely yellow. Control mites by spraying the plumeria with a light horticultural oil diluted at a rate of 2 tablespoons per 1 gallon of water, but check the product label for additional instructions. Use a garden sprayer and ensure all parts of the plant are dripping wet, repeating the application every seven to 10 days. If treated early in the season, a plumeria will produce new leaves and can recover fully.