How To Grow And Care For Mexican Petunia

When you see a plant blooming its head off in a strip of compacted dirt between a parking lot and concrete wall in rainless, nasty July, you know it's a tough customer. That's just one of the reasons people love Mexican petunia. As its name implies, Mexican petunia, also known as Mexican bluebell or Britton's petunia, comes from south of the border—native to Mexico, as well as Western South America and the Antilles. They can now be found in a handful of Southern states, Hawaii, Puerto Rico, and the Virgin Islands. Mexican petunia is not a real petunia, but its flower looks like one. One accepted botanical name is Ruellia brittoniana, but you'll also see it called R. malacosperma and R. tweediana. However, taxonomists have begun to use the name R. simplex, which was the first name the species was given. Some folks say these are different plants and others say they are one and the same.

Cold-hardy in USDA Zones 8 to 11 and southward, Mexican petunia grows 3 to 4 feet tall with attractive, purplish stems and narrow, lance-shaped leaves about 6 to 12 inches long. Showy blue or purple flowers—beloved by butterflies, hummingbirds, bees, and other pollinators—appear from early summer through the fall. The flowers grow in the shape of trumpets and can either bloom individually or in clusters. Just be careful about which type you grow and where you grow it.

Is Mexican Petunia Invasive?

The fact that this plant is a survivor means it can get out of hand. It has a rapid growth rate and forms large clumps by spreading roots that are hard to kill and difficult to remove. These roots are typically large, horizontal-growing roots that often peek aboveground. And its exploding seed capsules scatter seedlings far and wide. In wet climates and unmanaged areas, it can be invasive. Indeed, Florida and a few other states consider it as such, although I hardly think it ranks up there with kudzu, popcorn tree, privet, and water hyacinth. Due to its aggressive rate of growth and difficulty to remove, some communities forbid planting it anywhere near natural areas.

So should you still plant it? Yes, as long as it's not banned in your state and you make sure to plant types that are not invasive. Fortunately, a few sterile cultivars are now available in garden centers and mail-order nurseries, which we'll tell you more about below.

How to Care for Mexican Petunia

Mexican petunia is able to grow in a variety of environmental conditions, including in differing sunlight, temperature, and moisture. It is incredibly easy to grow in full to partial sun. It's one of the few perennials that grows equally well in wet soil and dry soil, and it will typically grow to be 1 to 4 feet tall and similarly in width. I often see it thriving in traffic islands, gas station plantings, and strips between sidewalks and the curb where it gets absolutely no care. And as mentioned before, butterflies and hummers covet it. Individual flowers last for only one day, but there are always new flowers opening. Furthermore, its seed production is rather abundant and the seedlings have a high germination rate, with the ability to take root in both light and dark conditions.

Light

Mexican petunias will grow in partial shade but do best in full sun. You'll get more flowers when the plant is exposed to at least six hours of direct sunlight a day. In shade, it will grow lanky and bloom less. Mexican petunias can tolerate light shade in very hot climates.

Soil

This plant tolerates most soils, but appreciates fertile, well-drained, and moist or even boggy conditions. Mexican petunia will grow in dry or sandy soils, too. You can plant it in a rain garden, on the edge of a pond, in a traditional flower bed, or in a container full of potting soil. The plant is most likely to become invasive in wet soils in hot climates.

Water

This low-maintenance plant becomes drought-tolerant once established, though keeping the soil moist during the growing season results in the lushest growth. Mulch the plants to conserve moisture, and water new transplants regularly to keep the soil slightly moist. After it settles in, your Mexican petunia will be happiest if you water it during hot, dry weather.

Temperature And Humidity

This tender perennial is not hardy in the Upper South or Middle South, where it is generally grown as an annual. Mexican petunia dies back to the ground after a frost but will come back every year in Zones 8 and warmer. In frost-free areas, Mexican petunia is evergreen and becomes more shrubby. This plant loves humidity but is also a popular choice in dry climates due to its drought tolerance.

Non-Invasive Types Of Petunias

Fortunately, plant breeders have come up with less aggressive and sterile cultivars of the tough Mexican petunia so you can enjoy it in your garden. Here are a few safer choices:

• 'Katie': A dwarf that grows about 10 inches high and 12 inches wide, it sets few seeds and is not an aggressive spreader. Large, blue-purple flowers appear from June until frost. This is a good one for massing as a ground cover.
• 'Purple Showers': Developed by the University of Florida, 'Purple Showers' is sterile, so it sets no seed. It looks a lot like the plant pictured up top, but has larger, deep purple flowers. However, it still can spread by roots, so don't plant it in wet soil. You can order this one from Avant Gardens. It's also available at big-box stores.
• Mayan™ Purple, Pink, and White: Available in all three colors, this series was developed to produce more and larger flowers. Mayan Mexican petunias are seed and pollen sterile.

A Native Alternative

Another choice is to go native and plant a charming little species indigenous to the Midwest and South called Carolina wild petunia (R. caroliniensis), pictured above in my garden. It grows only a foot tall and blooms off and on all summer. It will spread by seed somewhat, but not enough to be annoying. And it's just as tough as Mexican petunia. You can order it from Woodlanders. It does well for me in sun and light shade and is more cold-hardy than Mexican Petunia.

Pruning Mexican Petunia

After the foliage dies from frost, cut Mexican petunia back to the ground. This plant doesn't require pruning where it's evergreen, but you can snip off stems after they flower to encourage more blooms or lightly trim the whole plant to encourage bushier growth. If you are growing a Mexican petunia that produces seeds, it's best to remove the seeds as soon as they appear. You can also pinch off faded flowers to prevent the plant from setting seed.

Propagating Mexican Petunia

Because Mexican petunia spreads so readily through its roots, you'll likely have an opportunity to disperse more clumps around your garden or share it with friends and family — unless it's a total thug, in which case you may want to throw it out. Keep in mind that propagating trademarked cultivars is prohibited.

Dividing plants is generally most successful in the spring and on cloudy days. The mother plant will send out horizontal rhizomes that develop into new shoots. If you want to separate them, cut the rhizome connecting the plants with a garden knife or sharp spade. Then dig up the new shoots for transplanting in the desired location.

You can also divide a large clump following these steps:

1. Dig up a healthy clump of Mexican petunia with a shovel or spade.
2. Separate the clump into two or more plants, carefully separating foliage and cutting the roots apart with a garden knife.
3. Replant the clumps where desired and water well.
4. Unless you have rain, continue watering daily until the plants recover. In hot weather, cutting back 1/3 of the foliage can help transplants survive the shock.

Mexican petunia is also easy to grow from cuttings, so easy that rooting hormone isn't required. Here's how you do it:

1. With pruners, cut the tip of a healthy stem 4-6 inches long, just below a node.
2. Remove leaves from the bottom half of the stem, as well as any flowers.
3. Stick the stem 2 inches deep into sterile, light-weight potting mix. You can use a 4-inch pot for each cutting, or use a tray and space each cutting a few inches apart.
4. Water thoroughly so that the potting mix is moist. Cover the cuttings and pots with clear plastic.
5. Set in bright, indirect light indoors or in a sheltered, shady spot outdoors (only if the weather is warm but not hot). Keep potting mix moist. You can remove the plastic after the plants have developed roots. Transplant in the garden after the plant grows larger.

How To Grow Mexican Petunia From Seed

Mexican petunias are easy to grow from seed, especially since the seeds don't require cold treatment to germinate. Seed from dwarf cultivars growing in your garden can produce a completely different-looking plant. If you want to grow 12-inch plants, buy seeds for 'Southern Star,' which comes in purple, white, and pink. You can start them indoors in late winter. Growers recommend sowing seeds in a moist seed-starting mix, spacing them 12 inches apart, and lightly covering them with the mix. Place in a location that stays between 72 and 76 degrees. Keep the seed-starting mix moist until germination, which takes 7-10 days. But according to the University of Florida, Mexican petunia seeds aren't that picky about light exposure or temperature. The important thing is to plant the seeds within six months for the best germination.

Potting And Repotting Mexican Petunia

Growing Mexican petunia in a pot eliminates any concern about this plant spreading out of control through its roots. When potting or repotting your plant, choose a container that is about 2 inches wider in diameter than the nursery container. Use rich, high-quality potting soil with good drainage. Water regularly, as keeping the soil moist will result in the most vigorous growth and blooming.

Overwintering

Mexican petunias cannot survive cold winters in the Upper and Middle South. The good news is you can overwinter your plant indoors if you live in Zones 6 or 7. If it isn't already in a container, dig up your Mexican petunia and plant it in one. The plant can be trimmed back if leggy. Set the container indoors in bright light. Keep soil moderately moist. After temperatures have warmed in spring, you can reacclimate the plant to outdoor conditions over a week or two, starting by placing your plant in a shady spot for an hour and gradually increasing light exposure and time outdoors.

Common Pests & Plant Diseases

Mexican petunia is a fairly carefree plant without any serious diseases or problems. In certain climates, tiny mites can infest the plants, creating white splotches that look like paint on the leaves. Remove severely affected leaves. Spray the plant with horticultural oil, repeating a week later if necessary. In hot, sunny weather, it's best to spray in the evening or switch to an insecticide that is effective for mites.

How To Get Mexican Petunia To Bloom

The trumpet-shaped flowers of Mexican petunia usually appear in late spring and continue through the fall. In frost-free climates, this plant can bloom year-round. However, there are a few reasons your plant might not be blooming. Mexican petunia blooms best in full sun; move to a sunnier location if your plant is in shade. If you have had a long dry spell, giving your plant some water could help it produce flowers. You can also try cutting back stems that have already flowered so that the plant will grow more flower buds.