Don't give up on Mexican Petunia

Taxonomists have assigned several botanical names to what most gardeners call Mexican petunia. But no matter the appellation - Ruellia brittoniana, R. simplex or R. malacosperma, this shrubby perennial is a summer-blooming plant that folks either love or hate. While some of us have grown weary with the way this ubiquitous, sometimes obnoxious "intruder" from Brazil has a propensity to hog flower beds, if assigned an appropriate place in the garden, Mexican petunia becomes an asset.

This is because Mexican petunia is good at “fixing” problem spots where nothing else thrives. It has a remarkable nonchalance toward growing conditions. Well, in Charleston gardens, anyway. It withstands searing heat and brief periods of sub-freezing temperatures plus, it doesn’t mind standing in an occasionally-flooded yard. It’s content in full sun or partial shade and scoffs at less-than-perfect soil. This is definitely a plant with a positive attitude.

However, something happened to Charleston’s Mexican petunia this past summer. Its leaves and stems became oddly fuzzy and blotchy and remained looking bedraggled throughout the season. Fortunately, the appealing purple/periwinkle blue blossoms (there are also pink and white varieties) were never compromised, but the overall attractiveness was certainly diminished. It wasn't bad enough to make gardeners yank out their plants, but I doubt any ever made it into a bouquet.

I first noticed this affliction in my backyard garden in early summer, except I blamed this Jackson-Pollock-effect it on my house painters. (Sorry, guys.) I thought they were flinging watery paint-soaked brushes in the direction of my stand of Mexican petunia while cleaning up at the end of the day. However, this wasn’t enough to make me complain because I figured the “artwork” was temporary and would eventually fade away.

But shortly after, I noticed the same blotchy patterns on swaths of Colonial Lake’s Mexican petunia that grow near Rutledge and Broad Streets. That was when I realized that marauding house painters were not to blame, and I began looking for the similar evidence in nearby landscapes. This was an easy and pleasant assignment because I see plenty of Mexican petunia during my morning runs around the lower Charleston peninsula. I never encountered a single, blemish-free plant all summer.

Although I initially thought this botanical disorder was caused by a fungus, this is not the case. A native microscopic mite is the culprit. Also known as gall mite, these little buggers were the cause of these unsightly streaks that greatly diminished the appearance of Mexican petunia’s stems and leaves. This fuzzy blotchiness is simply the result of the plant’s defensive reaction against Eriophyid’s invasion into its cell tissues.

I’m cutting back my Mexican petunia this week (mid-December). Usually, I wait until mid-February to perform this chore but, because this past growing season was not kind to this plant from Brazil, I’m willing to risk an early cut-back. The stems and leaves are not going into my compost pile, however. I’ll bag them and set out this debris for the city’s yard-waste pickup.

Here are three reliable websites that discuss Mexican petunia and Eriophyid:

https://ipm.ucanr.edu/PMG/GARDEN/PLANTS/INVERT/gallblistermites.html

https://content.ces.ncsu.edu/horticultural-oils-for-ornamental-plants

https://nwdistrict.ifas.ufl.edu/hort/2023/08/02/my-mexican-petunias-are-fuzzy/