A Comprehensive Guide to Growing and Caring for Mexican Petunias

Although they share a name with petunias, Mexican petunias are actually not true petunias. They resemble petunias with their vibrant purple flowers, but are scientifically known as Ruellia simplex or Ruellia brittoniana. These hardy plants are perennials, unlike true petunias which are typically grown as annuals. They feature bright lavender blooms, lush green foliage, and stems that can vary from green to purple.

Mexican petunias, native to warm regions of South America and Mexico, are fast-growing and adaptable. They can also withstand cooler temperatures in other areas of the world. Gardeners love these flowers for their resilience and low-maintenance needs. Follow these expert gardening tips to successfully care for your Mexican petunias.

Growing and Care Conditions

Mexican petunias are fast-growing and thrive in most climates, with their primary requirement being warm temperatures. They can also survive the winter in colder regions. These resilient plants can flourish in a variety of conditions, including droughts and floods.

Soil Requirements

“Mexican petunias aren’t picky about the type of soil,” says Tony O’Neill, gardening expert and owner of Simplify Gardening. “They’re flexible and can tolerate soils ranging from sandy to clay, as long as the drainage is adequate.” For best results, Mexican petunias prefer rich, well-drained soil. Nevertheless, they can adapt and even grow while submerged in water.

Light and Temperature

“Outdoors, Mexican petunias thrive best in full sun with plenty of water and nutrients,” says Marc Hachadourian, director of glasshouse horticulture and senior curator of orchids at New York Botanical Garden. “They can grow under a wide variety of conditions, from somewhat sunny and dry, which slows their growth and keeps them more compact, to being submerged in water as a marginal aquatic plant. These plants prefer warm conditions.”

Mexican petunias thrive in full sunlight, but they can also do well in partial shade. Plants grown in partial shade will have green stems and fewer flowers. When exposed to full sunlight, Mexican petunias display their striking purple stems.

Watering Requirements

While Mexican petunias can survive drought, they perform best when regularly watered. If there's no rainfall, watering them once a week should be enough for them to thrive. O’Neill advises, “During particularly hot or dry periods, you may need to water them more often to prevent the soil from drying out entirely.”

Mexican petunias thrive in warm, moist environments. With plenty of warmth and water, these plants can spread quickly. “Some varieties can spread aggressively through seeds and other forms of propagation in ideal garden conditions,” says Hachadourian. “New plants can be easily removed from unwanted areas. Be cautious with Ruellia simplex as it has become an invasive species in warmer regions like Florida.”

Pruning and Deadheading

Mexican petunias grow rapidly, but they’re easy to care for. Pruning and deadheading can help encourage more vibrant violet blooms. “Regular pruning and deadheading (removing spent flowers) will promote more blooms and a fuller growth habit,” explains O’Neill. “Cutting them back in late winter or early spring can encourage strong new growth.” Deadheading can also be done throughout the blooming season to maintain a neat appearance and stimulate further flowering.

Propagation

Mexican petunias are relatively simple to propagate from cuttings. For this, you'll need a healthy plant, sharp pruners or shears, nutrient-rich potting soil, and a small pot. Cut the stem just below the node, making it 4 to 6 inches in length. Remove the lower leaves, leaving a few at the top. Place the cutting in a pot with rich soil, burying it a few inches deep to root.

Place your Mexican petunia in a warm spot with plenty of indirect sunlight and water it regularly to keep the soil moist. Once the roots have developed, you can move the plant to your garden and enjoy its vibrant blooms.

Overwintering

Mexican petunias can endure cooler temperatures but struggle in freezing conditions. They are considered cold-hardy in USDA hardiness zones 8 to 11 and further south. In warmer zones, the plant may stay evergreen throughout the year. “While they can tolerate a light frost with some damage and recover, they are not winter-hardy,” says Hachadourian. Extended exposure to freezing temperatures will likely cause them to die.

In colder zones (zones 6 or 7 with milder winters), Mexican petunias can survive the winter if brought indoors (place them in sunny areas during winter) or shielded from freezing temperatures. Keep in mind that they will likely die back after a frost. Alternatively, you can propagate them using cuttings during the winter months.

“Ruellia can be easily overwintered by cutting back the plant, rooting cuttings in water, or by saving seeds from the previous year,” explains Hachadourian. “The key to overwintering them in colder climates is warmth, sunlight, and moisture. These plants will grow quickly and require regular pruning to maintain their shape and prevent them from becoming leggy or overgrown.”

Common Pests and Diseases

Mexican petunias are hardy, low-maintenance plants. “Typically, Mexican petunias are quite tough,” says O’Neill. “However, they can occasionally be affected by spider mites and whiteflies, particularly in hot, dry conditions. Consistent watering and healthy soil can help avoid these issues. Fungal diseases, such as root rot, may develop in excessively wet conditions, so it’s important to have well-draining soil.”

If you notice brown leaves on your plants, it’s likely due to cold temperatures. Try moving them to a warmer location and trim off any brown leaves—the plants should recover and thrive once again.

Are Mexican Petunias Poisonous?

Although Mexican petunias can pose a threat to certain ecosystems, they are not poisonous and are considered safe for both cats and dogs, as they are non-toxic.