Growing and Caring for Mexican Petunias: Everything You Need To Know

These pretty, purple perennials are resilient and cold-hardy.

Despite their name, Mexican petunias are actually not petunias. Although they resemble petunias with their cheery purple petals, Mexican petunias, also called Ruellia simplex or Ruellia brittoniana, are perennials—true petunias are usually grown as annuals. The plucky plants have vibrant lavender blooms, green leaves, and stems that can range from green to purple.

Mexican petunias are fast-growing plants native to warm parts of South America and Mexico, but they can tolerate cool temperatures in other regions of the globe. It’s a popular flower choice for gardeners because of their hardiness and their no-fuss maintenance. Learn how to care for the Mexican petunia with these tips from gardening experts.

Meet Our Expert

• Marc Hachadourian, Director of Glasshouse Horticulture at New York Botanical Garden

• Tony O’Neill, Gardening Expert and Owner of Simplify Gardening

Related: 9 Easy-to-Grow, Flowering Plants to Elevate Your Home's Curb Appeal

Care and Growing Conditions

Mexican petunias grow quickly and easily in most climates, with their main requirement being warm temperatures. They can also be overwintered in cold regions, however. The violet plants can grow well in a variety of conditions, even surviving drought conditions and floods.

Soil Requirements

“Mexican petunias aren’t too fussy about soil conditions,” says Tony O’Neill, gardening expert and owner of Simplify Gardening. “They’re adaptable and can tolerate a range from sandy to clay soils, provided the drainage is good.” For optimal growth, Mexican petunias prefer nutrient-rich, well-draining soil. However, they are versatile and can even grow submerged in water.

Light and Temperature

“Outdoors, the plants do best in a full sun location with plenty of water and food,” says Marc Hachadourian, director of glasshouse horticulture and senior curator of orchids at New York Botanical Garden. “They can grow in a wide range of conditions from somewhat sunny and dry, which will slow them down and keep them more compact, to even submerged in water as a marginal aquatic plant. The plants prefer warm conditions.”

Mexican petunias flourish with lots of sunlight, but they can grow well in full sun or partial shade. Plants in partial shade will have green stems and fewer flowers. Mexican petunias that grow in full sunlight have striking purple stems.

Water

Although hardy Mexican petunias can survive drought, they grow best with regular watering. If there is no rainfall, watering once a week is usually sufficient for them to flourish. O’Neill adds, “During extremely hot or dry periods, you might need to water them more frequently to keep the soil from drying out completely.”

Mexican petunias thrive in warm, moist conditions. With warm conditions and plenty of water, Mexican petunias can spread rapidly. “Some species can spread vigorously through seeds and other means of propagation in the right conditions in a garden setting,” Hachadourian says. “New plants can be easily removed where they are unwanted. Caution should be taken with Ruellia simplex as it has become an aggressive invasive species in warm areas like Florida.”

Mexican petunias are fast-growing plants native to South America and Mexico—in fact the hardy plants can spread so rapidly that they’re considered invasive in some regions. The state of Florida categorizes them as highly invasive.

Pruning and Deadheading

Mexican petunias grow quickly, but they are easy to maintain. Pruning and deadheading will give your plants more vibrant violet blooms. “Regular pruning and deadheading (removing spent flowers) encourages more blooms and a bushier growth habit,” O’Neill says. “You can cut them back in late winter or early spring to promote vigorous growth.” Deadheading can be done throughout the blooming season to keep the plants looking tidy and encourage more flowers.

Related: What Is Pruning—and When Do You Need to Do It?

Propagation

Mexican petunias are relatively easy to propagate from cuttings. You’ll need a healthy plant, sharp pruners or shears, rich potting soil, and a small pot. Cut the stem just below the node so it is 4 to 6 inches long. Remove the lower leaves from the cutting, leaving only a few leaves at the top. Root your plant in a pot with nutrient-rich soil, planting a few inches into the soil.

Keep your plant in a warm area with plenty of indirect sunlight and water regularly to maintain moist soil. Once roots form, you can transfer the Mexican petunia to your garden and watch it bloom.

Overwintering

Mexican petunias can tolerate cool temperatures but do not do well in freezing conditions. They are considered cold-hardy in USDA hardiness zones 8 to 11 and southward. The plant may remain evergreen throughout the year in warmer zones. “While they can take a light frost with some damage and recover, they are not winter-hardy plants,” Hachadourian says. They will likely die if exposed to prolonged freezing conditions.

In colder zones (zones 6 or 7 with milder winters), Mexican petunias can survive the winter if brought indoors (place them in areas with plenty of sunlight in the winter) or protected from freezing temperatures. Keep in mind they will die back to the ground after a frost. You can also use cuttings to propagate plants in pots over the winter.

“Ruellia can be easily overwintered as a cut-back plant, rooting cuttings which strike easily in water, or by seeds saved from the previous year,” Hachadourian says. “Warmth, sunlight, and moisture are key to keeping them over winter in a colder climate. The plants will grow rapidly and require pruning to keep them shaped and not allowed to become lanky and overgrown.”

Common Pests and Diseases

Mexican petunias are tough, usually low-maintenance plants. “Generally, Mexican petunias are quite resilient,” O Neill says. “However, they can occasionally be affected by spider mites and whiteflies, especially in hot, dry conditions. Regular watering and maintaining healthy soil can help prevent these issues. Fungal diseases like root rot can occur in overly wet conditions, emphasizing the importance of well-draining soil.”

If your plants develop brown leaves, it’s most likely due to cold temperatures. Move them to a warmer area if you can and cut off any brown leaves—the vibrant plants should thrive again.

Are Mexican Petunias Poisonous?

While they may be a threat to certain environments, Mexican petunias are not poisonous and are considered non-toxic to cats and dogs.

Related: 10 Indoor Flowering Plants to Add Color to Your Home

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