Ruellia, a new perennial for Eastern US gardeners
My personal experience with Ruellia began on a trip to San Antonio, Texas. while touring, Mission San Jose, I noticed an attractive but unfamiliar flower blooming near the well. I took pictures and eventually attached the name Ruellia to them. When I saw Ruellia on a nursery table last year, in Maryland, I decided to try it. I remembered the blossoms from San Antonio. How well would this plant grow, a thousand-plus miles and several heat and humidity zones away from where I'd seen it before?
Ruellia flourished in my Mid-Atlantic (Maryland) perennial bed. Granted, I crammed it in one of the hottest driest corners I could find. It was up against a south-facing foundation wall, with the fullest sun my yard can muster. The concrete wall may have given the plant a faint whiff of alkaline Southwest soil in this moist acidic area. I can't say how much the specific microclimate helped, but the Ruellia grew like a champ. Several stately stems grew and bore dark green horizontal leaves. Blooming began in midsummer; several purple flowers opened every day until fall. When a group of Dave's Gardeners visited in September, the Ruellia was still going full tilt. Any new perennial attracts attention, and fall-bloomers are extra special. Like butterflies, the gardeners flocked around the now three foot tall plant.
When real cold threatened my Ruellia, I took cuttings. They rooted easily, givng me assurance of another year of Ruellia. And when the soil warmed in April, my original Ruella began to sprout. It had survived with minimal protection. Not only were leaves popping from the base of last year's stems, rhizomes were spreading in the root zone.
Ruellia species number to a couple dozen, and they are mostly found in the southern, warmer zones. The species known as either tweediana, brittoniana or caerulea (depending on whom you ask and on which day of the week) is the source of a large handful of cultivated Mexican petunia. They are perennial in warmer zones, blooming all summer to frost in mostly sunny locations. They bear funnel shaped flowers in purple, pink, or white. Some are a larval food source for various brush-footed butterflies, like the Common Buckeye.
Garden varieties of Ruellia display either an upright or a low, clumping form. Cultivars 'Purple Showers' and 'Chi Chi' stretch two to four feet tall with strong yet airy stems that sway with the breeze. Compact dwarf 'Katie' and 'Bonita' selections grow as low, almost grassy, mounds six to ten inches high. They bear their flowers neatly above the leaves. Ruellia blooms best in full sun.
Ruellia, the unruly thug in Southern gardens and native areas
But one year's experience in one garden can never be assumed to tell the whole story of a plant. Further reading paints quite a different picture of Ruellia. What appears in my Maryland garden as hardy growth translates to overagressive thuggishness in many warmer gardens. Tall forms of Ruellia are sometimes loved but very often loathed in gardens of the Deep South. These gardeners have experienced Ruellia's penchant for throwing rhizomes far and wide, choking out any other green plant in its path. And what Ruellia doesn't throttle, it carpet-bombs with seed. I kid you not. Hatred seethes from some of the posts made by frustrated homeowners who've invited Ruellia in and lived to deeply regret it.
Like any self respecting thug, Ruellia has escaped imprisonment in backyards and is running rampant among native plants in some regions. Florida has committed Ruellia brittoniana (tweediana) to Category I on its its Invasive Plant Species List. Wild Ruellia has formed solid masses in a number of natural areas and a variety of plant communities. Ruellia spreads by rhizomes, and also sows its seed via explosive capsules. The seeds can withstand long dormant periods and float well, aiding dispersal near water.
The gardener's decision
The gardener's decision whether to grow Mexican petunia starts with location. A plant's specific site plays a key role in it's growth, as any gardener well knows. If you garden in the mostly frost free lower states, be forewarned that the tall form of Ruellia will spread aggressively by roots or seeds. A hot zone garden connected to natural water would guarantee a Ruellia outbreak. A large pot will contain Ruellia's roots but not the seeds. Dwarf varieties seem less agressive.
For the Mid-Atlantic, and other gardens to the north, Ruellia could be a tough, new summer blooming perennial or annual. It is hardy to zone 8 and likely through zone 7 or 6 with protection and careful siting. It propagates readily from stem and root cuttings. Because of its pest potential, Mexican petunia should be watched closely. Unfortunately, researchers cannot predict whether Mexican petunia could become invasive in wild areas to the north as they have in Florida. Only time will tell.
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This Youtube video from University of Florida, UFInvasivePlantsEDU, does an excellent job of explaining Ruellia's identification characteristics and showing it taking over in a Florida swamp.
http://www.youtube.com/watch?v=1NopSR-yoUY , accessed 5-27-13
Hupp, Karen V. S., et al. Natural Area Weeds: Mexican Petunia (Ruellia tweediana), http://edis.ifas.ufl.edu/ep415, accessed 5-27-13
Florida Exotic Pest Plant Council, http://www.fleppc.org
Ruellia brittoniana/tweediana is listed as Ruellia caerulea, Britton's wild petunia, in USDA PLANTS Database, http://plants.usda.gov/java/profile?symbol=RUCA19&photoID=ruca19_003_avp.jpg, accessed 5-27-13