Both ruellia and oleander plants are an option for an ornamental garden. They are both shrubs that prefer warm regions and are drought tolerant. They do not handle frosts well, so they are more suited to conservatory planting in cooler climates such as in Britain.
Oleander and ruellia plants are part of different genus groups in plant taxonomy. According to the Royal Botanic Gardens at Kew, oleander is in the genus "Nerium", and therefore has a full scientific name of Nerium oleander. Ruellia is itself a genus, but according to the University of Arizona in the U.S., the species Ruellia brittoniana is common ruellia.
Oleander can grow to be about six metres tall, towering over the ruellia, which has a maximum height of about 90 centimetres. Dwarf ruellia hybrids are also available from garden shops, which can only reach to about 30 cm in height. Oleander is tall enough to act as part of a hedge as well as an ornamental shrub in its own right. Ruellia tends to be more weed-like than Oleander, and spreads quickly. The leaves of ruellia are spear-shaped, with a purple vein underneath and serrated sides, compared to the much narrower spear-shaped leaves of Oleander. Oleander also produces a sticky gum from damaged stems.
The flowers of oleander are tubular in shape and have five lobes. In the wild, the flowers are red or pink, but human culture has also created purple, white, cream and yellow flower subtypes. Ruellia flowers are either blue or purple, and are shaped like a funnel. Some ruellia hybrids also produce white or pink flowers.
Oleander is toxic to humans, as the plant contains molecules which adversely affect heart function. For this reason, the shrub forms part of traditional medical remedies for heart problems, although possible side effects include vomiting, irregular heartbeats, low blood pressure and even death. Leprosy and cancer are other examples of illnesses traditional medical practitioners used oleander extract to treat in the past. Ruellia has no history of medical use.
Both species prefer warm climates, but ruellia is native to the southwest of the United States and Mexico, which is why it is also known as Mexican petunia. However, the plant is not closely related to petunias. Oleander originally grew around the mediterranean coast, in India, China and Iran.