Is Ruellia Invasive: Tips On How To Get Rid Of Mexican Petunias

By: Kristi Waterworth

Lawn and garden maintenance can be one daunting task after another, especially if you’re struggling with plants that keep popping up where they’re not wanted. Ruellia, also known as Mexican petunia, is one of those annoying little plants that walks the line between being a beautiful ornamental and an incredibly noxious weed. They can be defeated in the home landscaping, but it takes a great deal of patience to knock them back.

Is Ruellia Invasive?

Although plenty of gardeners have cultivated Ruellia brittoniana over the years, it has since escaped home gardens and become classified as an invasive plant in nine states, stretching from South Carolina to Texas. Because of its adaptability and rapid reproduction, Mexican petunia has managed to supplant native species in many areas and across several types of natural communities.

If you want to cultivate this plant, it’s still ok to do so, provided you purchase sterile specimens from your nursery. “Purple Showers,” “Mayan Purple,” “Mayan White”, and “Mayan Pink” are common varieties that will cause a lot fewer problems in the landscape. They will still require careful disposal of clippings and cultivation, however, because even the sterile types can escape and repopulate using their rhizomes.

How Can I Kill Mexican Petunias?

If you live in one of the nine states most affected by Ruellia, you’re probably wondering how to get rid of Mexican petunias. In truth, Mexican petunia removal requires vigilant attention to the garden or lawn where they’re a problem and may become a long-term project. Because the seeds of the Mexican petunia can germinate for years after the adults are gone, it’s a battle you’ve got to really commit to.

Although pulling Mexican petunia can work for a few small plants, if you fail to dig the whole root or miss a sprout, you’ll be doing it all again soon. The best bet is to treat the foliage of the plants with glyphosate and kill them back to the root. Regrowth after the first application is expected, so be prepared to spray again each time you notice the plants setting new leaves.

If your Mexican petunias are in the lawn or other delicate area where spraying herbicides might not be a great idea, you can cut the plants back by hand. Carefully dispose of the vegetation so that it can’t have a chance to regrow. Since you’ll only be destroying the top part of the plant, you’ll need to recut it every time it starts to leaf out to force it to use its energy stores and run itself out of food.

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How do you get rid of invasive Mexican petunias?

Take steps to rid of the Mexican petunia before it develops into large, invasive colonies. Dig around the ruellia patch with a hoe or shovel and pull the ruellia plants gently from the soil to keep the roots and rhizomes intact. Double-check the area to ensure that all rhizomes and roots are removed from the soil.

Beside above, are Mexican petunias invasive? Mexican Petunias. But Mexican petunias are actually Ruellia simplex, fast-growing perennials with green to purple stems and green leaves. The wild, invasive form of this plant is so aggressive, it's banned in some states, and it's on a watch list of invasive plants in others.

Regarding this, how do you keep Mexican petunias from spreading?

There are a few ways to control Mexican petunias from overtaking other areas.

  1. Push wood planks around the plant about 6 inches deep to slow down and control the spread of the plant.
  2. Plant a larger Mexican petunia in a pot and sink it into an established garden.

What is eating my Mexican petunia?

Foliage-feeding caterpillars -- such as petunia budworms and variegated cutworms -- feed on the leaves and buds of petunias, causing jagged edges or holes in the foliage. If their numbers are high, leaf-eating caterpillars can defoliate entire petunia plants. Two to three days after ingestion, the caterpillar dies.

Why is control of wild-type and non-sterile cultivars of Mexican petunia in urban landscapes and home gardens crucial to overall control of this invader?

Managing the invasion of non-native aggressive plant species is a key component of maintaining the health and functionality of plant ecosystems. Controlling invasive ornamental species often requires multiple methods to first establish and then maintain the health of the native vegetation (Tallent-Hasell and Watt 2009 Gooden et al. 2009 Hanula et al. 2009). A key part of maintaining control of a potential invading species is reducing or eliminating the source of propagules (seeds or other plant material that could generate more plants). Urban landscapes bordering nearby natural areas are a constant source of propagules for ornamental invasive species as long as these species remain in the urban landscape. Home gardeners have the opportunity to positively impact the quality of their neighboring natural areas by removing invasive ornamental species from their landscapes. Research on the control and behavior of Mexican petunia suggests that it can be best managed with a two-pronged approach: 1) control populations in invaded natural areas with glyphosate and native revegetation and 2) reduce the urban sources of Mexican petunia propagules, particularly the wild-type and non-sterile cultivars. While sterile cultivars like 'Purple Showers' have not yet been documented in natural areas, they can spread aggressively through vegetative means. It is up to those managing urban landscapes to responsibly care for plantings of sterile Mexican petunia. The UF/IFAS Assessment of the Status of Non-Native Plants in Florida’s Natural Areas Infraspecific Taxon Protocol, a component of the UF/IFAS Assessment of Non-Native Plants in Florida's Natural Areas, reports that there is currently no documented evidence of invasion of the cultivars 'Purple Showers', 'Mayan Purple', 'Mayan White' or 'Mayan Pink' in natural areas (UF/IFAS Invasive Plant Working Group 2013).

Past agreements between nursery owners and others involved in the horticultural industry have been negotiated to reduce sales of known invasive species as well as reduce introductions of potentially invasive species (Baskin 2002 Foxcroft et al. 2008 Reichard 2004). Additionally, there is increasing pressure on ornamental plant growers to restrict sales of ornamental invasive species (Coats et al. 2011 Wirth et al. 2004). However, many ornamental species are already a frequent planting in urban landscapes and have already established themselves in natural areas. The invasion of ornamental invasive species is most likely to be a serious problem where urban landscapes are adjacent to natural areas and therefore provide consistent, nearby propagule sources of the invader (Foxcroft et al. 2008). In the case of Mexican petunia, reducing the urban propagule source is likely to be a key part of long-term reduction of the species in natural areas.

Many homeowners are unaware of which species that appear in the landscape are invasive (Gagliardo and Brand 2007 Reichard and White 2001). Reichard and White reported that less than half of home gardeners surveyed said they were aware of ornamental invasive species. To add to home gardeners’ confusion, sterile cultivars of some invasive ornamental species have been developed that are safe for use in the home landscape (Tallent-Hasell and Watt 2009 Trueblood et al. 2010 Czarnecki and Deng 2009). However, while research has been done to develop sterile cultivars of Mexican petunia (Freyre et al. 2012), other research suggests that the primary method of propagule dispersion for Mexican petunia is vegetative rather than by seed (Reinhardt-Adams, personal communication). Therefore, the introduction of sterile cultivars of Mexican petunia as the only management action will likely not sufficiently reduce the propagule source for this species. We suggest removal of all wild-type and non-sterile cultivars of this species from the urban landscape will be an important part of managing this invading species in adjacent natural areas.

The sterile ‘Purple Showers’ cultivar of Mexican petunia is thriving in a Florida landscape.

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Planting Petunias as a Companion Plant in Your Garden

Petunias are popular annual flowering plants that are part of the night shade family which also includes tomatoes and potatoes. They are available in a wide range of colors and are virtually foolproof to grow since they require little maintenance. Even in dry weather, you only need to give them a good watering every 7 to 10 days.

They are some of the most popular summer flowers for gardens in northern temperate climates because of the almost limitless variety of flower colors, sizes and plant architecture. Petunias can easily be grown from seeds but transplanting the plants after purchase from a garden nursery is more prevalent. They are also popular for their abilities as companion plants.

Petunias and Organic Pest Control

Petunias are ideal as companion plants because they are nature’s version of a pesticide. They repel a variety of pests including the asparagus beetle, leafhoppers and tomato worms. They are also effective against some types of aphids and Mexican bean beetles.

Some of the plants that thrive when you plant petunias as companions include brassicas, beans, basil, tomatoes, grapes, corn and peppers. Roses also fare well when this natural insect deterrent is planted in proximity.

Protecting Your Vegetable Harvest

The brassica family includes broccoli, cabbage, brussel sprouts and cauliflower. When planting with broccoli or cabbage, petunias will increase the likelihood of you getting a good harvest because they trap cabbage worms. These “worms” are in fact a species of caterpillar that will eat through an entire crop if left unchecked so the best, organic method for you to control these pests is to plant petunias in your cabbage beds.

Tomatoes, potatoes, eggplants and tobacco are part of the same family and are susceptible to the same type of pests such as aphids, hornworms, cabbage loopers, Japanese beetles and weevils. Planting petunias or geraniums among your crops will either distract or repel these pests from your harvest plants.

Protecting Grape Vines

Grape vines attract a large variety of pests from aphids, mites and moths, to nematodes. Certain nematode species attack grape vine roots, which lead to stunted growth because the nutrient and water absorption cycle is affected. Companion planting around grape vines of petunias will protect your grapes from most of these pests and guarantee better vigor and health for your plants.

Companion planting is the best way for you to control pests in your garden naturally and cleanly. Commercially available pesticides contain chemicals that cannot be considered beneficial to your health.

Attracting Beneficial Insects with Companion Plants

Petunias are one of the most reliable companion plants due to the wide range of pests they repel. Their scent also attracts bees, butterflies and moths to your vegetable garden which are all beneficial to your plants as they help with pollination. In addition, they also add beauty to your garden. A cabbage patch takes on a whole new look if your cabbage plants are interspersed with petunias.

Mexican petunia needs to be contained to area

My experience with Mexican petunia is that it grows like a weed, and I consider it the broadleaf version of Bermuda grass, aka devil grass. If it’s contained in an area and prevented from spreading into the landscape, I think it will be OK.

Q: We have Mexican petunias, and all have suffered frost damage. How far should we prune them back?

A: Contain Mexican petunia in a localized area, so it doesn’t spread. Cut it back close to the ground, about 2 inches tall. For every single cut, three new shoots will grow. That improves its density.

Cutting off flowers that have finished blooming will reduce the spread of seed to new areas. Thoroughly clean afterward any equipment you might use, so you don’t spread it to a new location.

My experience with Mexican petunia has not been a great one. It grows like a weed, and I consider it the broadleaf version of Bermuda grass, aka devil grass. If it’s contained in an area and prevented from spreading into the landscape, I think it will be OK. Problems arise when it is planted in a shrub or flower bed.

Most of what’s available at nurseries will grow 3 or more feet tall. They can be cut back any time of the year, and they will regrow. There are supposed to be some varieties available that grow only about a foot tall. I don’t know if they are available here. Mexican petunia will spread into areas by seed or by underground stems called rhizomes. Some people will mistakenly call the plants rhizomes “roots,” but they’re not.

Use compost as a fertilizer application because mineral fertilizers, unless they are slow-release, will cause Mexican petunia to grow excessively. These plants like improved soil, and an application of compost helps with that. There might be fewer flowers produced if you overapply a fertilizer.

You can tell if the plants have plenty of nutrients by the color of the leaves. They will be dark green.

As a sidebar to this discussion, the reason they thin out and get scraggly is because they are not pruned and fertilized regularly. Cut them back and apply a fertilizer and see what happens after they are watered.

Q: I moved into a new home with exterior plants I am unfamiliar with. Because my cat has access to the enclosed backyard, I was wondering how I could determine whether any of these plants could be toxic to him.

A: One way or another, probably about 60 to 70 percent of all landscape plants are poisonous. Some are more poisonous than others. For instance, the toxicity of oleander, African sumac and cycad (sago palm) stems, leaves, flowers and seed has been well-documented. The berries produced by nandina (heavenly bamboo) have been reported to kill wild birds that eat them. Mushrooms that pop up from the ground after a rain generally cause animals to vomit but do not kill them. Don’t forget interior plants because a lot of those are poisonous as well.

The website by the American Society for Prevention of Cruelty to Animals has many poisonous and nonpoisonous landscape plants listed.

Q: My peach tree was in full bloom when that freeze hit in early February. This is the second time in three years that I had my peach crop wiped out by freezing temperatures. I’m thinking seriously about replacing it. What varieties bloom a little later than the first week in February?

A: As a general rule of thumb, the early-producing peaches also are the first to bloom. Later-producing peaches, like in July, August and even September, typically flower later.

Early-producing varieties like Earlitreat, Flordaprince, Flordaking and May Pride deliver peaches in early to mid-June in our valley. They flower consistently starting the first week of February. Even the early apples like Anna and Dorsett Golden push flowers around the third week of February here.

The location of a fruit tree in a landscape can encourage or discourage its flowering. For instance, locating a fruit tree in a warm microclimate of the yard causes it to flower early. Locating the same peach tree in a colder part of the yard might delay its flowering as much as two or three weeks.

To wipe out a crop, it usually requires repeat freezing temperatures while it is in flower. The flowering period can last two to three weeks depending on temperatures. February 2019 was a good example. Three different freeze events came through the valley a week to 10 days apart.

Many of the popular fruit and nut trees such as peach, apricot, plum, pluot and almond produce flowers first and then push leaf growth shortly after that or around the same time. Fruit trees like pomegranate, jujube, quince and persimmon flower much later and seldom get bothered by late spring freezing temperatures.

We forget that there is still a 5 percent chance of frost occurring after March 15 here.

If the tree has been growing less than three years, it’s possible to move it to a cooler location in the landscape. The time to do that is now or when temperatures start to cool down in the fall toward the end of September.

If you move the tree, remember to cut the major limbs back considerably before moving it. That helps compensate for the loss of roots that occur during the move.

Q: I have a dwarf peach tree that was about a foot tall when I planted it last year. It has grown about 6 inches more now. When can I prune it so it doesn’t grow taller than me? I am 4 feet 10 only and I don’t want it to grow very tall because it will be very hard for me to cover it with a net in future years. I want it to stay small.

A: By calling it a dwarf peach tree you mean it’s a genetic dwarf, not a semi-dwarf. It might be better to call a dwarf peach tree a miniature fruit tree instead. That is why it grew only about 6 inches last year.

The first year of pruning is aimed at training the tree’s future form and not aimed at fruit production. You want limbs to come from the trunk as low as possible without the fruit touching the ground.

Remove limbs from the trunk that are lower than about 12 inches from the soil. Remove a branch that is crossing another branch or rubbing against it. Branches should be about 10 to 12 inches apart all through the tree canopy.

In miniature peach trees, the ends of the branches often times grow into a crows foot of four or five small shoots. These are the places where flowers and fruit will be produced. Reduce the crows foot so that only one or two remain and remove the others. When the fruit is about the size of a quarter, you might have to remove other fruit so that the remaining fruit grows to a normal size.

Q: We recently purchased a house and I would like to begin identifying the specific plants, shrubs and trees on our property. Can you recommend a publication that would help me to do that?

A: Wait until these plants produce leaves and hopefully flowers. The easiest way is to identify with a combination of leaves, stems and flowers.

Take several pictures of these plants, including leaves and flowers, to a local nursery or make a visit to the master gardeners help desk just south of McCarran International Airport. You can call the help desk at 702-257-5555 and make an appointment.

There are several online websites and publications, but many do not have pictures, so it is sometimes hard to confirm. Two places that I would consult online is the Southern Nevada Water Authority searchable plant database and the Arizona Municipal Water Users Association list of landscape desert plants.

Once you have a name for a plant, confirm it on a search engine by looking at images for these plants online. It is more helpful if you have the scientific or Latin names for these plants rather than just the common names.

Plants that originate from desert regions usually require less soil amendment at the time of planting. They are usually more tolerant of rock applied to the surface of the soil, aka rock mulch. Plants that originate from wetter regions usually perform better in the long run with wood chips applied to the soil surface rather than rock.

One More Summer Sizzler -- 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 comes from south of the border. (No, I'm not talking about that tacky tourist park on the NC/SC line where Pedro sells sombreros the size of beach umbrellas.) Mexican petunia is not a real petunia, but its flower looks like one. The accepted botanical name is Ruellia brittoniana, but you'll also see it called R. malacosperma and R. tweediana. Some folks say these are different plants and others say they're all the same one. Grumpy says, "Who the h*** cares?" I mean, really. We're busy people.

Cold-hardy in UDSA Zone 7 and southward, Mexican petunia grows 3 to 4 feet tall with attractive, purplish stems and narrow, lance-shaped leaves. Showy blue or purple flowers, beloved by butterflies and hummingbirds, appear from early summer through the fall. How showy are they? Well, when you find pots and pots of Mexican petunias in bloom in front of Home Depot and Lowe's, you know know even non-gardeners find them fetching.

The Good, the Bad, and the Pretty

First, the good. Mexican petunia is incredibly easy to grow in full to partial sun. It's one of the few perennials Grumpy knows that grows equally well in wet soil and dry soil. I often see it thriving in traffic islands, gas station plantings, and strips between sidewalks and curb where it gets absolutely no care. And as I mentioned before, butterflies and hummers covet it. Individual flowers last but one day, but there are always new flowers opening.

Now, the bad. The fact that this plant is a survivor means it can get out of hand. It forms large clumps by spreading roots that are hard to kill. And its exploding seed capsules scatter seed far and wide. In wet climates and unmanaged areas, it can be invasive. Indeed, the state of Florida considers it as such, although I hardly think it ranks up there with kudzu, popcorn tree, privet, and water hyacinth.

So should you still plant it? Yes, as long as you plant types that are not invasive. Fortunately, Grumpy knows of a few that are now available in garden centers and mail-order nurseries.

Here's the first one. It's a dwarf called 'Katie.' It 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. Niche Gardens is a good mail-order source.

A second Mexican petunia to consider is 'Purple Showers.' It looks a lot like the plant pictured up top, but has larger, deep purple flowers. Developed by the University of Florida, 'Purple Showers' is sterile, so it sets no seed. 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.

Your third 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 some, but not enough to be annoying. And it's just as tough as Mexican petunia. I got mine from Jean in Tennessee, who's a wonderful person despite being a die-hard Volunteer fan (Oh, the shame!). You can order it from Woodlanders. It does well for me in sun and light shade and is more cold-hardy than its Mexican cousin.

Don't Forget the Tomato Contest!

I know it's been a tough summer for growing tomatoes, but Grumpy believes in you! So be sure to enter Grumpy's Big Fat Tomato Contest before August 31. Prizes will be awarded for:

It's your chance to achieve tomato immortality. Click here for instructions.

Watch the video: Ruellia. Mexican Petunia -Groomed in pot Vs Growing wild - your view?

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