Adapted from somewhere else
Corrections are appreciated :)

Ruellias are sometimes called Wild Petunias, but they aren't Petunias so that's a LIE! They're named after Jean Ruelle, who was a French botanist and physician during the Renaissance Period. There are lots of plants named after botanists.
Most, if not all, will have the flowers that open for only one day—they open in the morning and fall off by the evening.

We are fortunate to have a great diversity of Ruellia species, some of which occur nowhere else in the 50 states. Take a look at these BONAP maps for a view of all the species and the ranges/distributions.

Key diagnostic characteristics

• Inflorescence structure - axilary vs panicle topping the stem
• Leaf texture - waxy, hairy, fringe on margins?
• Leaf shape, oval vs ovate vs lanceolate

Left: Red circle shows where the stem tops off with a multi-flowered inflorescence (group of flowers). This will grow into a branched structure known as a panicle. There are some flowers coming from the nodes too, but it's the panicle coming out of the main stem that's important.

Right: Large blue circle shows one of the flower buds about to come out. Note that they are coming out at the same spot where the leaves come out, known as the node. The nodes are marked with the small blue circles. This arrangement is known as an axillary inflorescence. While these flowers aren't on a stalk (sessile, botanically speaking), sometime the flowers will be borne on a stalk, like on Mexican Ruellia.

Quick and easy-to-understand illustrated glossary of leaf terminology for your convenience

Most of these characteristics can be captured and seen within 1 or two photos. I would do one showing the flowering structure (inflorescence), and one showing the leaves and their texture/hairs. For leaf texture it might be good to note that down, but that's probably a bit excessive.

Here's a few example observations to look at:
Example 1
Example 2
Example 3
Example 4

For those wanting to review in a glance. To be finished

I will organize the species based on whether the flowers are axilary or in a panicle.

This is the most common of the Ruellias. Well, most common around here... outside of Texas they, occur sparingly in a few other states and down south to Mexico. You wouldn't know if you live within Austin or Dallas or anywhere in Texas deep within its range.
The flowers will rise above the leaves in a flowering stalk, which is known as a terminal flower arrangement. Notice how the main stem continues upward and then branches out to form multiple flowers.

The leaves are very oval, with a waxy or glossy look to them. Essentially they are glabrous; they can have some hairs (trichomes if I'm being pedantic), but those hairs aren't very conspicuous or dense.

This species is named after Sister Mary Clare Metz, botanist and professor at Our Lady of the Lake University (OLLU). She has an interesting story: see here for more information.

This species looks pretty much exactly the same as Violet Ruellia... except the flowers are white. Actually, the flowers are also significantly larger than Violet Ruellia so it's useful to have a ruler when taking photos... if you manage to find one. I would actually recommend measuring the calyx lobes though... more on that below.
Flower color is a relatively reliable way of IDing this one, though apparently there is also a white form of Violet Ruellia.
Which makes that common name A LIE!
Well it's mostly not a lie but still
A LIE!!!
Anyhow... Apparently this one is also pseudo-endemic to Texas, around the Edwards Plataeu. Yes, the Edwards Plateau is very important in Texas botany. It also occurs south into Mexico. There are a lot of pseudo-endemic plants species which have populations only in Texas and parts of Mexico.

Oh, I forgot how long the corolla was compared to Violet Ruellia... hold on, let me check Shinner's and Mahler's real quick.
...there we go!

R. metziae: corolla (fused petals) 5.5-6.5 cm long, calyx lobes 14-20 mm long when in flower:
R. nudiflora: corolla to 4-5 cm long, calyx lobes 10-15 mm long when in flower
Page 214 of Flora of North Central Texas, found here: https://fwbg.org/research/brit-press/illustrated-flora-of-north-central-texas-online/

So I actually found this species recently, but I wasn't convinced it was R. metziae. So I did a little field research (for fun) and measured the corolla and calyx length for several plants, some R. nudiflora and others supposedly R. metziae. And the results were insane. The Ruellias with white flowers were all significantly larger than those with purple flowers. The calyx lobes were even crazier:

I plucked a Violet Ruellia flower just to make a point

It's not even close! The calyx lobes were almost always reliably longer for R. metziae, by 5 millimeters... which doesn't sound like a lot but it really is.
Here's the calyx with a ruler beside it:

Of course, these are only a few flowers, but I checked several plants so it's no fluke. I highly recommend reading this observation's notes for more information.

All in all, I would highly recommend checking the length of the calyx lobes with a centimeter ruler.
Here's detailed descriptions of both species:
Ruellia metziae
Ruellia nudiflora

Another difference it that this species has long decurrent leaf bases (see the link for more info). What does that mean?

See that long "stem" that I pointed out with the bracket? That's known as the petiole, the part where the leaf connects to the base. Notice how long it is, maybe even half the length of the leaf it's part of! Decurrent is what you see in the red circle, outlined by the purple lines, where the leafy bits of the leaf extends down the petiole. Long decurrent leaf base. The leaves are also a longer sort-of elliptical/oblong shape, and might even look lanceolate. Check the illustrated leaf glossary if you need to check what those mean.
If you look at the Violet Ruellia leaf image above, you can see how the leaves have a much shorter petiole and are more oval/short.

Also note both this one and Violet Ruellia have their flowers on a flowering stalk above the rest of the leaves (aka "terminating the main stem in panicle-like inflorescence")

I initially thought this one was just rare here and occurred more commonly further west of here, but turns out it also only occurs in Texas and further south to Mexico.

This one, like Metz's Ruellia and Violet Ruellia, has a terminal inflorescence, with its flowers coming out on a flowering stalk rather than from the leaf nodes (axilary flowers). However, unlike the other two, the leaves are not glossy/waxy, but more of a matte look, similar to Drummond's Ruellia. This is due to the presence of pubescence on the surface of the leaves. The leave blades are less oval in shape, more tapered towards an ovate/deltoid shape, wider towards the base and often coming to a pronounced point at the apex. They are also broader, generally about x1.25 or x1.5 longer than wide.

The flowering stalk is also densely covered in fine white hairs (canescent).

This species is particularly rare in the Austin area, so if you find it that's a real treat! On iNat there are previous observations seen to the west, in Barton Creek and Emma Long Metro Park.

This is a nonnative species from Mexico, the Caribbean, and Central/South America
It has been widely used as an ornamental and escaped from cultivated, where it is established in several southern states. It is considered an invasive in some places, particularly when it gets into riparian areas (areas near water).
Due to the many horticultural cultivars, the leaves can vary quite a bit, but typically are distinctly lanceolate (lance-shaped)

Notice how the flowers, while on a stalk, still come out of the leaf nodes

Sometimes they even tend to look almost grasslike.
(there are several cultivars)

https://www.inaturalist.org/observations/103579832

I see this plant every now and then in nurseries.

Named after the great botanist Thomas Drummond, who also has a lot of other plants named after him in Texas.

Drummond's Ruellia in a Texas endemic - aka, it occurs nowhere else but Texas
as you can see on BONAP.

mostly limited to Edwards Plateau and extending a bit up north to Dallas

From my experience it tends to like shady areas
The leaves are quite distinct:

Ovate, rounded on one end and pointed on the other. They're covered in a lot of very fine hairs, which makes them have a fuzzy feel.
The flowers also come out at the leaf nodes (where the leaf attaches to the stem) rather than coming out as a stalk from the top like Violet Ruellia. Note that they are practically sessile: no stalk/pedicel/peduncle, just coming straight out of the leaf axils.

Hairy Ruellia looks similar to Violet Ruellia, but much less common
However, the leaves are distinctly hairy as the common name actually got that right. Well, actually the top of the leaves can lack hairs,
Particularly, it has a tell-tale fringe on the leaf margins (edge of the leaf), which is a dead giveaway.

Posted on September 19, 2022 06:57 PM by arnanthescout