Golden trumpet or common allamanda is a flowering plant commonly seen as a landscape ornamental in tropical locations. These beautiful plants with large, fragrant, deep golden yellow flowers are Allamanda cathartica, a member of the dogbane family (Apocynaceae). This evergreen plant, native to northern South America, is frost tender, but can be grown in a greenhouse in cool climates or as a summer annual. It is less successful as a houseplant because of its high light and humidity requirements that are difficult to provide in the average home.
The plant is a vigorous, rambling woody climber that can get up to 15 feet tall although it can be pruned to grow as a shrub with a more compact shape, especially when grown in a container. Because it does not twine or have tendrils, if grown as a vine it must be trained onto a trellis or other support structure. The wavy-margined, obovate leaves are 4-6 inches long. The whorls of 2-4 large, glossy, leathery leaves set off the flamboyant flowers in dramatic fashion. It can grow quite large, so may not be an appropriate house plant unless you have a lot of room.
Allamanda flowers primarily in the summer and fall. The large (to 5 inches across), trumpet shaped blossoms are composed of 5 large overlapping petals, that flare widely. There are both single and double forms. The clusters of flowers occur at the tip of new shoots. There can be one to several flowers in each cluster. When pollinated, green, spiny, round seed capsules may follow the flowers. Hummingbirds will visit the flowers.
This plant does best with full sun, rich, well-drained soil and plenty of moisture during the growing season. When grown indoors it should get at least 4 hours of direct sunlight daily. Being from the tropics, it needs warm conditions (night temperatures of 60-65˚F and day temperatures of 70˚F or higher) and also prefers moderate to high humidity. Water regularly and deeply, allowing the soil to dry slightly between waterings (but not completely dry, which will cause premature flower drop). Fertilize regularly (every 2-3 weeks) during the growing season, especially when blooming heavily.
Allow the plant to rest during the winter months. Reduce the frequency in late August and stop fertilization in late September. Cut back watering, too, when flowering ceases, keeping the plant on the dry side for the winter. Resume fertilization in April as the plant starts to resume growth. Plants can be cut back to 1 or 2 nodes in the spring just before growth resumes to produce a more compact and floriferous plant. Potted plants can be moved outside when the weather is consistently warm, but should be gradually acclimated to higher light levels. Plants should be moved inside when nights get down to 40ºF. It can also be planted in the ground as summer annual.
Insect pests include mealybugs, whiteflies, scales, and spider mites. This plant is susceptible to root rot when overwatered. It can easily be propagated by half-hardened cuttings taken in late spring. Provide bottom heat and enclose the cuttings to provide high humidity.
There are a few cultivars, although they may not be readily available in the Midwest.
- ‘Flore Plena’ has double flowers
- ‘Golden Butterflies’ has tubular flowers whose petals flap in the wind like butterflies.
- ‘Grandiflora’ has lots of very large flowers.
- ‘Halley’s Comet’ has both double and semi-double flowers.
- ‘Hendersonii’ has smaller flowers with white or orange markings in the throat and brown buds.
- ‘Nobilis’ sports larger flowers with a wider flare.
All parts of the plant are mildly toxic if ingested, and strong alkaloids in the milky latex sap can irritate the skin or eyes.
In addition to A. cathartica, some other species occasional available as ornamentals include bush Allamanda (A. schottii, sometimes also incorrectly listed as A. nerifolia or A. cathartica var. schottii) with golden yellow flowers streaked with light orange-red, and purple Allamanda (A. blanchetti) with large, dull magenta-violet flowers.
– Susan Mahr, University of Wisconsin – Madison