Mexican Petunia (Ruellia simplex)

Britton's Wild Petunia, Mexican Bluebell, Mexican Petunia, Texas Petunia

Ruellia simplex is native to Mexico, the West Indies, western Bolivia, southwestern Brazil, Paraguay, Uruguay, and northeastern Argentina. It has been widely used as an ornamental plant and has escaped from cultivation in the United States, Australia and parts of Asia, as well as several Pacific Islands. It has become invasive in some of these areas, forming dense, single-species stands of vegetation which threaten native plants. It is mainly a plant of wet places such as ditches, pond verges, lakesides and marshes, but can survive in drier conditions.

I. Appearance and Characteristics 

Ruellia simplex, the Mexican petunia, Mexican bluebell or Britton’s wild petunia, is a species of flowering plant in the family Acanthaceae. It is a native of Mexico, the Caribbean, and South America. It has become a widespread invasive plant in Florida, where it was likely introduced as an ornamental before 1933, as well as in the eastern Mediterranean, South Asia and other parts of the eastern hemisphere.

Ruellia simplex is an evergreen perennial growing 3 ft (0.91 m) tall, forming colonies of stalks with lance-shaped leaves that are 6 to 12 in (15 to 30 cm) and .5 to .75 in (1.3 to 1.9 cm) wide. Trumpet shaped flowers are metallic blue to purple, with five petals, and 3 in (7.6 cm) wide. There is a dwarf variety that is only 1 ft (0.30 m) tall.

II. How to Grow and Care

Sunlight

Mexican petunia prefers partial shade to full sun and differs in appearance depending on the light it gets. Full sun results in purple stems, while shading produces green stems. In areas with the harsh sun during summers, it may do better in some shade. Bright, filtered sunlight is best if the plant is grown under glass.

Temperature and Humidity

These tough plants are heat and drought-tolerant. They thrive in areas with high humidity and high heat. Mexican petunias do not handle the cold well and are not frost-hardy. Be sure to protect these plants if any cold weather comes your way.

Watering

Mexican petunia likes humidity. If the humidity is high, it’s more able to tolerate dry periods. Most wild petunias prefer to be regularly watered to ensure they are kept medium to wet. Regular watering is particularly important during periods of active growth and in very hot, dry weather, water daily unless the soil is becoming boggy and waterlogged. Ruellia malacosperma, one species that is able to cope with being wet or dry, is an exception. Since this plant is fairly drought tolerant it is possible not to need to add water when it is kept outdoors.

plants may also be ready for watering if the soil beneath the surface is dry to the touch or the plant appears to be wilting. If the plant appears to be wilting, apply plenty of water. In more moderate weather, water weekly unless the soil appears very damp to wet.

Don’t water these plants in the afternoon, as the hot sun can cause more water lost to evaporation and even harm the leaves. If Mexican petunia is kept under the cover, water daily during the growing season and once a week during the winter. The pH of the water should be in the same range as for soil: 5.5-7.8, which means that most water will be satisfactory for this plant.

Soil

Mexican petunia can tolerate many different soil conditions. The preferred pH of the soil is 5.5-7.8. Soils that are fertile and organic and wet or boggy are favored. It is also able to grow well in average soil that has good drainage, providing it is kept consistently moist. Once established, it is resistant to short periods of drought. If grown in pot soil, it should have reasonable drainage, as diseases can occur if its roots are too wet for long periods.

Fertilizing

Mexican petunia can manage quite well without fertilizer. Apply a 10-10-10 mix in spring to feed the plant during its active growth period (10-10-10 means equal parts nitrogen, phosphorous and potassium: a general all-purpose fertilizer). Mix fertilizer with water. Indoor plants can be given a half dose of all-purpose fertilizer every 2-3 weeks; this is good for their leaf color. However, if too much fertilizer is applied, there will be fewer flowers.

Planting Instructions

Mexican petunia is readily available in pots or as seed, which can be sown outdoors in spring. Spring is also the best time to plant Mexican petunia in the ground. It is not particularly fussy regarding soil type and copes with either good compost or native soil. When planting in the ground, choose a site with shade to partial shade.

Topsoil or peat moss can be added to the hole beneath the plant to improve soil structure and water availability to the roots. Water well after planting.

Mexican petunia attracts butterflies and hummingbirds. It’s best to plant in the ground, as rapid growth can cause it to outgrow containers quickly. That said, containers are useful planting options to restrict the spread of this plant.

Another method to reduce spread is to plant it behind solid edging, or have a regular maintenance routine to pull up every unwanted plant. Planting near wetlands is not recommended, as it is likely to colonize the area very rapidly. Although sterility stops spread by seed, it remains aggressive in its spread of roots.

Pruning

Mexican petunia does not require much pruning. Remove dead leaves and flowers after flowering for aesthetic purposes, and to stop it going to seed, which may result in spreading. Pinch young shoots to encourage branching and more flowers. Remove and dispose of the debris to avoid attracting unwanted bugs or animals. Damaged or unwanted branches can be pruned back to ground level in late winter or early spring.

Propagation

These plants can be easily and quickly propagated through stem cuttings and rhizome division. This is best done in the summer.

To propagate by cuttings, you will need sharp, clean garden snips, a small pot, well-draining, and rich potting soil:

  • Using sharp garden snips, cut a stem about six inches long just below a node.
  • Remove the leaves from the bottom half of the cutting. 
  • Dip the cut end into rooting hormone and shake off the excess powder. 
  • Fill a small pot with rich, well-draining potting soil and plant the cutting a few inches deep. 
  • Keep the pot in a warm area with bright, indirect light and water regularly, keeping the soil moist. 
  • Transplant into the garden once a root system has formed.

To propagate through rhizome division, you will need a shovel:

  • Gently dig around the plant to loosen the soil. 
  • Using the shovel, slice through the rhizomes to create multiple, separate plants.
  • Lift the divided plants out of the ground and transfer them to their new location.

Potting and Repotting 

Mexican petunias can be kept in pots, which is an ideal option for those living in areas with cold winters since the plant can be moved. However, because of its rapid growth rate, Mexican petunias will need to be repotted often. To do this, choose a pot that is at least 2 inches larger than its current pot. Gently dislodge the plant from its pot and set it into the new pot. Fill the pot with well-draining, fertile soil and bury the root system to the same depth it was before. Water thoroughly. 

Overwintering

When grown in warm climates with mild winters, Mexican petunias do not require extra winter care. Simply cut back on watering, only when the soil is dried out. If cold temperatures are threatening your area, it is best to move Mexican petunias indoors because they cannot handle cold temperatures.

Pests and Diseases

Mexican petunias are difficult to kill and can withstand droughts, floods, and high temperatures.1 Though hardy, gardeners have encountered a few issues with these plants. One of the most common issues is brown leaves.

Brown Leaves

If your plant has brown leaves, it has most likely experienced cold weather and has sustained damage as a result. Move the plant to a warmer area and remove the damaged leaves. The plant will regrow full and lush, if it receives the warmer temperatures it requires

How to Get to Bloom

Mexican petunias are known for their petunia-like, purple, trumpet-shaped flowers that last about a day. Though the blooms themselves die off quickly, Mexican petunias continue to prolifically produce their vibrant blooms for quite some time, usually from early summer to fall. They do not have a strong aroma.

To encourage flower production, prune the plant after blooming. Cut down the flowering stems to encourage new growth. Deadhead the spent blooms to make room for future blooms. And be sure to place the Mexican petunia in an area that receives plenty of sunshine, as this will encourage abundant flowering.

III. Uses and Benefits 

Mexican petunia is grown ornamentally often for its attractive lilac flowers. However, this plant is an aggressive grower so using a container to control growth is strongly advised. Other quick-growing flowers like butterfly weed and pale purple coneflower make good partners for mexican petunia. It is well suited to prairie, meadow, and rock gardens.

Mexican Petunia (Ruellia simplex) Details

Common name Britton's Wild Petunia, Mexican Bluebell, Mexican Petunia, Texas Petunia
Botanical name Ruellia simplex
Plant type Annual
Hardiness zone 8a, 8b, 9a, 9b, 10a, 10b
Growth rate Fast
Height 3 ft. 0 in. - 4 ft. 0 in.
Width 3 ft. 0 in. - 4 ft. 0 in.
Sunlight Full sun (6 or more hours of direct sunlight a day)
Soil condition Clay
Flower color Purple/Lavender
Leaf color Green