It started with an accident, says Steve Bender, co-author of Passalong Plants (The University of North Carolina Press, $22.95).

"While transplanting some mondo grass from an old homesite, I inadvertently picked up a Ruellia hitchhiker."

The freebie interloper was fine with him. "This wild petunia feels right at home in my garden."

But not everyone is so grateful to find this pass-itself-along ensconced in their gardens.

Mexican petunia (R. brittoniana), which is not a petunia, is gorgeous as a mass or accent plant. But it is also so aggressive that it can take over a garden bed and spread across lawns into neighbors' yards.

Standing nearly 3 feet high, this naturalized perennial can cast seed several feet beyond the mother plant.

As a hardy plant, Mexican petunia sometimes remains evergreen during North Texas winters, with only leaf-tip damage after a freeze.

If it dies to the ground, its resilient and colony-forming rhizomes quickly recover in the spring.

One secret to Mexican petunia's effortless land-grab is its efficiency in utilizing nitrogen and phosphorus.

This is why it out-competes R. caroliniensis, a pretty Texas wildflower blooming between April and September in the Blackland Prairie portions of the Dallas area.

So it is hardly surprising that volunteer Mexican petunias are a familiar sight in North Central Texas.

Despite summer heat and drought, their colorful purple or pink flowers can be seen ornamenting abandoned lots and even thriving in the tiny grooves separating sidewalk slabs.

There are better behaved, low-growing ruellias, such as 'Katie', also known as 'Nolan's Dwarf', which was first cultivated in Conroe, Texas.

More recently Greg Grant, an associate at the Pineywoods Native Plant Center, has introduced 'Colobe Pink', another compact also known as 'Bonita Pink'.

Great for borders, these attractive plants mound to about 6 inches high and 12 inches wide. Blooming from spring through fall, their abundant, cutely crinkled flowers appear within their foliage.

These compacts are not invasive because their trailing stems spread and set roots very slowly.

Both perform beautifully in North Texas, though they can be damaged by extended hard freezes.

But what if you prefer the majestic tall ruellias, if only they were better behaved?

Look for 'Purple Showers', a sterile hybrid cultivar of R. brittoniana (sometimes listed as R. tweediana). It is as lofty and beautiful as the seeding variety, and it is equally care-free.

Or look for 'Snow Queen', a tall, white-flowered hybrid of R. brittoniana.

Minimal invasiveness is also a feature of white-flowered softseed wild petunia (R. malacosperma), another naturalized perennial from south of the border.

Although it is commonly confused with Mexican petunia, it has wider leaves and is a little less cold-hardy.

It is also possible to grow ill-mannered Mexican petunia in a container. This ruellia is a wonderful candidate for pots placed on a deck, where it shouldn't become a nuisance.

Whether grown in a pot or in the ground, invasive Mexican petunias can be impeded by planting them in poor-draining soils and then keeping them dry for as long as possible.

It's the availability of ample water and rich soil that puts them into reproductive overdrive.

If the goal is to keep Mexican petunias in their place, exert tough love and water them only when they begin to wilt.

Bill Scheick is a garden writer and professor of American literature and culture at the University of Texas at Austin.

Archie's Gardenland, Fort Worth

Caliper, Dallas

Calloway's, multiple locations

Classic Gardens, Murphy

Garrison Gardens, Plano

Green Mama's, North Richland Hills

Grow It Land Designs and Garden Center, Coppell

Katie's Nursery & Produce, Gainesville

Lantana Gardens, Bartonville

Nicholson-Hardie, both Dallas locations

Petal Pusher's Garden Emporium, Cedar Hill

Redenta's Garden, Dallas and Arlington

Ruibal's, Dallas

Walton's Garden Center, Dallas