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When a Petunia Is Not a Petunia

by Bruce Crawford, Program Leader in Home and Public Horticulture

Everyone loves using the common names for plants. It is a language that is intuitively understandable and is simply easier to remember than those cumbersome botanical names that seemingly make no sense! Yet, there are difficulties. One challenge with common names is when two different plants share the same name, making it challenging to understand which plant is being discussed. Petunias are a plant I suspect most people know and most likely have grown in a pot or hanging basket. Mexican Petunia, Ruellia simplex is of no relation to Petunias, but the common name may give you the impression that it is simply a big brother to the conventional bedding plant.

The Petunia recognized by most gardeners is botanically known as Petunia x hybrida and refers to the crosses between Petunia axillaris and Petunia integrifolia. A member of the Tobacco Family or Solanaceae, Petunia is native to regions throughout South America. The name was originally penned in 1803 by the French Botanist Antoine Laurent de Jussieu (1748–1836). It came from the word Petun, the Brazilian Tupian Indian name for this plant, which roughly translates to "worthless tobacco plant". The species are rather leggy plants with white or purple trumpet shaped flowers, consisting of five fused petals.

Starting in the late 1800's breeders in England, Germany, Japan and the US started to hybridize the species, with the Burpee Seed Co releasing the first dark purple form from seed in 1888 and the Sakata Seed Company of Japan releasing the first double petaled form from seed in the early 1900's. Today, many Petunia selections are grown from cuttings. They are relatively low growing plants, often no taller than 12–14" featuring nearly every color of the rainbow, including bicolors. Petunias are categorized into 4 general types, according to the size or number of the flowers and the plant habit. Grandifloras have large flowers to 4" in diameter while the Multifloras have a far greater number of 2" diameter flowers and the Millifloras have an abundance of even smaller flowers to around 1"!

Zoom in Petunia Crazytunia Citrus Twist.
Picture 1: Petunia Crazytunia® Citrus Twist.

The Grandifloras and Multifloras spread to 12–18" in diameter, while the Millifloras reach a more demure 6–12". The fourth category features a distinctly spreading or cascading habit and include the very popular Wave® series, which has 2" diameter flowers opening over a spreading plant of 3–4' in diameter and only 6" tall. The Grandifloras, Multifloras and spreading forms can also sport double flowers, although they often do not tolerate wet weather conditions well. In general, Petunias need a full day of sun with soil that is well drained yet moist. They prefer fertile soils and if grown in containers will need liquid fertilizing every 2 weeks while the spreading forms will benefit from a liquid feeding on a weekly basis. If flowering begins to wane or the plants become leggy, pinch them back moderately, which will initiate new growth and blooms.

Zoom in Petunia 'Queen of Hearts'.
Picture 2: Petunia Amore® Queen of Hearts.

There are an incredible number of new and very attractive selections on that market that bear investigating. Petunia Crazytunia® Citrus Twist (Picture 1) is a very vibrant yellow with orange stripes while Petunia Amore® Queen of Hearts (Picture 2) has attractive red hearts on a yellow background. Petunia Crazytunia® ‘Moonstruck' (Picture 5) offers a unique coloration of dark blue with white blotches that resemble a starry sky, seemingly inspired by Vincent Willem van Gogh!

Mexican Petunia or Ruellia simplex has a trumpet shaped flower resembling that of the true Petunia and served as the inspiration for its common name (as seen in Pictures 3 and 4). However, that is about the limit of the similarities. Ruellia is a member of the Acanthaceae or Acanthus Family. The genus has around 74 species and as one might suspect, this species is native to Mexico, as well as the Caribbean and South America. The genus name was created in 1753 by the Swedish botanist Carl Linnaeus (1707–1778) in honor of the French botanist and physician to King Francis 1, Jean de la Ruel (1474–1537). This particular species was described in 1870 by the American botanist Charles Wright (1811–1885) following a trip to Cuba. The species name refers to the arrangement of the individual leaves along the stem or branch, botanically known as a simple leaf arrangement. A compound leaf by comparison, features a number of leaflets connected to a central stem-like rachis which in turn is connected to the branch. The leaves of Ruellia are deep green in color and slender, ranging from 6–12" long by ¾" wide, although they remain a more modest 4–6" in length when grown in drier soils. If grown in full sun, the stems are often purple in color, as seen in Picture 4 below.

Zoom in Ruellia simplex.
Picture 3: Ruellia simplex as a container plant.

Unlike Petunias, Ruellia flowers last for only one day on a plant that grows to 3–4' tall and as wide (as seen in Picture 3). The 2–3" diameter flowers have 5 fused petals and are typically blue or purple in color, although white and pink forms do exist. Plants perform and flower best during the heat of summer with peak bloom from June through September. The flowers give way to bean-like pods that dramatically explode, a process called explosive dehiscence that propels the seeds to distances upwards of 10' away. Aside from reproducing from seed, broken pieces of the rhizomes also float and take root downstream, indicative of how the plants naturally grow in moist sites adjacent to streams and serve as a vector for the plant to spread.

Indeed, unlike Petunia, Ruellia can grow well in moist soils, although they also perform admirably in well-drained soils enriched with organic matter and can even endure drought. In NJ, they flower best in full sun, but will tolerate light shade, especially in hotter regions of the south. The plants have also proven to be highly invasive in the warmer climates of South Eastern US, but this is not an issue in zones 7 or colder regions where the plant is not winter hardy.

Zoom in Ruellia simplex.
Picture 4: Close-up of flowers of Ruellia simplex.

Ruellia is great in containers or in mixed borders where the round faces of the trumpet-like flowers will nicely compliment upright or spiky flowers, such as those found with Salvia, Russian Sage and Angelonia.

Clearly, a Petunia is not always a Petunia, but each genus provides great flowering solutions for your garden, whether the need be for a colorful groundcover or a shrub-like plant. Mexican Petunia is far less common than the traditional Petunia, but it provides summer color, texture and size that I have come to appreciate over the years while visiting Public Gardens. This plant now needs to become universally appreciated and grace more homes throughout NJ and beyond!

Petunia 'Night Sky.'
Picture 5: Petunia 'Night Sky.'