Mexican petunia is a lovely perennial with willow-like leaves and petunia-like purple flowers. It’s resilient, but can sometimes grow ‘too’ enthusiastically. Here’s everything you need to know about growing this desert-adapted plant so you can decide whether it’s right for your garden.
Mexican petunia (Ruellia simplex) is a herbaceous perennial that has an upright, bamboo-like form. Its slender, willow-like, dark green leaves are often tinged with purple. Its lavender-blue flowers resemble petunias, hence the plant’s common name.
It’s a favorite among many gardeners because it’s beautiful, low-maintenance, readily spreads to fill in bare spots, and is hard to kill. It’s so desert-tough that it’s one of the most popular landscape plants in Dubai!
It survives in a wide variety of conditions and is one of the few desert landscape plants that tolerates drought as well as it does flooding.
Its flowers attract bees, butterflies, and hummingbirds.
Why I Like This Plant
- Dark green foliage, pretty purple flowers
- Exceptionally resilient
- Low maintenance
- No thorns, low litter, non-toxic
- Attracts pollinators
- Deer resistant
Things to Watch Out For
Mexican petunia’s resilience can also make it a nuisance. It spreads rampantly by underground rhizomes forming dense colonies. If you try to remove it, even small pieces of roots left behind can regrow.
Additionally, it reseeds aggressively. Seeds are explosively expelled, flying all over your yard, and can persist in the soil for years. Consequently, some gardeners struggle keeping it in bounds and ultimately regret planting it.
Optimal Growing Conditions
If you’re thinking of adding Mexican petunia to your garden, you need to find a suitable place that will keep your plant healthy and looking good… while minimizing maintenance for you.
Here are the key factors to keep in mind.
Temperature
Mexican petunia can be grown in USDA Hardiness Zones 9 – 11. It loves the heat and is cold-hardy down to 15℉. It can be grown in Zone 8 but suffers from freeze damage during the winter. In mild winters, it can bloom all year.
Sun Exposure
Plant Mexican petunia where it gets either full or part sun. They don’t mind the reflected heat from a wall. More sun encourages more blossoms. Don’t plant in full shade, where it gets leggy and has few flowers.
Size and Growth Rate
Mexican petunia is considered a moderately fast grower that generally reaches a mature size of 3’ wide by 3’ tall. But the more water it receives, the faster it grows and it can reach a size of 6′ by 6′.
Soil
Mexican petunia prefers rich, acidic or neutral soil, but will tolerate typical desert soil, provided it’s well-draining.
Other Location Considerations
Since Mexican petunia prefers rich, moist soil and can grow aggressively when planted in the ground, you can solve a lot of problems by growing one in a pot. They make fantastic container plants.
Mexican Petunia:
The Essentials
| Common Name | Mexican petunia |
| Scientific Name | Ruellia simplex |
| Origin | Mexico, South America |
| Plant Type | Shrubby perennial |
| USDA Zones | Zones 9 – 11 |
| Cold Hardy | To 15℉ |
| Flower Color | Purple |
| Flower Season | Spring, summer, fall |
| Mature Size | 3’ high x 3’ wide |
| Growth Rate | Moderate |
| Sun Tolerance | Full, part, reflected sun |
| Water Needs | Low to moderate |
| Pests & Diseases | None |
| Garden Friendly | No thorns, non-toxic |
| Cautions | Aggressive, potentially invasive |
| Wildlife | Attracts butterflies, bees, hummingbirds Deer resistant |
How to Plant
When planting any perennial in the desert, dig a hole twice as wide as the root ball but no deeper. Even though thought Mexican petunia prefers rich, moist soil, amending the soil is not necessary — this plant is vigorous enough without amendments.
When to Plant
The best time to plant Mexican petunia is in the fall. This gives it three seasons to grow roots and get established before the following summer.
The second best time is in spring, the earlier the better. This still gives your plant time to get established before the intense heat of June arrives.
How to Care for Mexican Petunia
Whether you’ve recently planted a Mexican petunia or have an existing one in your yard, here’s how to take care of it to keep it healthy and looking its best.
How to Water
Once you’ve got your Mexican petunia in the ground, watering is your most immediate concern. Here’s a recommended watering schedule for new shrubs planted in the spring or fall.
| Weeks 1 & 2 | Every 3 – 4 days |
| Weeks 3 & 4 | Every 6 – 7 days |
| Weeks 5 & 6 | Every 7 – 10 days |
| Weeks 7 & 8 | Every 10 – 14 days |
After week 8, gradually extend the time between waterings.
Since Mexican petunia survives in both wet and dry conditions, it’s not too fussy about watering, once established.
It lets you know when it needs more water by wilting dramatically. Don’t be alarmed if this happens, it perks back up. (I had a neighbor whose potted plant went completely limp at least 4 times a week, but it always rebounded.)
If given enough water, it can take over your yard, which may or may not be what you’re going for.
Should You Fertilize?
There is no need to fertilize Mexican petunia. Plants do just fine in native soil. (Fertilizing them can make them a bigger nuisance than they already are.)
How to Prune
Prune in late winter or early spring to control size and remove winter damage. You can also prune anytime your plant is looking rangy or to encourage more blooms.
Plants respond well to an occasional severe rejuvenation prune (a few inches above ground level) just before new spring growth appears.
Mexican Petunia Cultivars
There are a few cultivars of Mexican petunia that you may encounter when shopping.
Ruellia simplex ‘Katie’ is a popular cultivar that grows around one foot tall by 2 feet wide and is well-behaved in the garden. It’s sometimes called Katie ruellia.
If you’re concerned about reseeding, look for plants in the Mayan series. These cultivars are sterile and non-invasive, and come in purple, white, and pink.
Don’t Confuse Mexican Petunia With…
Mexican petunia is sometimes confused with desert ruellia (Ruellia peninsularis). Desert ruellia is more shrub-like and has smaller purple flowers than Mexican petunia. In many ways, this is a more suitable landscape plant for desert gardens, since it’s a Sonoran Desert native and has no invasive or aggressive tendencies.
Plant Lover Facts
Mexican petunia goes by a few additional common names including purple ruellia, Mexican bluebells, and desert petunia.
Mexican petunia has undergone many scientific name changes. The correct name is now Ruellia simplex, but several former names still persist including: Ruellia brittoniana, Ruellia malacosperma, Ruellia caerulea, and Ruellia tweediana. So if you encounter any of these, know that they all refer to the same plant.
Ruellia is the “wild petunia” genus that has 366 members.
The name petunias might lead you to believe that it’s related to the petunias (genus Petunia) that are commonly grown as annuals. But they are only called petunias because of their petunia-like flowers and are not remotely related botanically.
Mexican petunia is native to parts of Mexico, Central America, and South America, but has naturalized throughout the world.
In the US, it has escaped cultivation in Alabama, Georgia, Hawaii, Louisiana, Mississippi, and South Carolina and is now considered invasive in parts of Texas and “highly invasive” in Florida.
Plants are most invasive in wet areas, so it’s unlikely to become invasive in arid areas of the southwest.
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Author Bio
Deane Alban is the creator of Southwest Gardener. She is a science writer with a bachelor’s degree in botany from the University of South Florida. Gardening is her lifelong passion. She’s been gardening in Tucson for 15 years.
