A Neotropical Savanna

Many years ago when I lived in New York City, I went birding with a small group of Audubon pros. I’ll never forget the time when one woman said, pointing to a bird whose name I’ve forgotten,

There it is – on the verbena bush!

We were in Central Park and there were lots of bushes around – and I had never heard of a verbena bush and I certainly would not have been able to recognize one. (I think I did find the bird, though.) At any rate, whenever I think about the Verbenaceae (or verbena) family these days, that incident always comes to mind, and my memory searches through those sunlit bushes, trying to guess which one is the verbena.

The memory is particularly amusing because, according to wikipedia, most plants in this family are tropical – not your basic NYC habitat. Nevertheless, the family is widespread, as we’ll see later. Here in Panama, the lantana (Lantana camara) that grows wild around our yard belongs to the Verbenaceae, as does Michael’s sandpaper vine, Petrea volubilis, and even the teak tree (Tectona grandis).

As does this plant that I’ve seen growing along roadsides, in an opening in the depth of a pine forest, and in the gardens of many of my neighbors.

It’s commonly called porterweed – Stachytarpheta mutabilis (Jacq.) Vahl.

So what do all these plants, including a tree, have in common, that they should be grouped in the same family?

Verbeneaceae – The Verbena Family

The family has about 100 genera and 2600 species, commonly having quadrangular twigs and/or aromatic herbage [University of Hawaii]. If this makes you think of mint, which has its own family but which may be closely related, then look for these additional features, listed the Delta taxonomic identification site and which are illustrated using the porterweed.

• leaves opposite - here you see the leaves attached to opposite sides of a porterweed stem (also a mint feature).

• young stems tetragonal - actually, on the porterweed, every stem I saw, young or old, had edges like these, and of course, this is a mint feature as well.

• flowers usually in inflorescences (clusters) - in porterweed, the flowers are clustered on a long stalk. (Mint inflorescences are either at the end of a stem or adjacent to leaves. Verbena inflorescences have their own stalk.)

• flowers have bracts (bracteate) – bracts help protect the developing flowers; here they cover the flower buds before the bloom and remain behind after the bloom. (No bracts in mints.)

• petals overlap like tiles (imbricate), corolla usually tubular - it’s a bit hard to see the tile-like overlap of the top part of the petals, but you can easily see the tube they make. (Mint flowers are more irregular than this.)

• pollination by insects (entomophilous) - just search for “porterweed” & “butterfly” at Flickr and you’ll find more than 50 images of butterflies feeding on porterweed nectar. (Mints are pollinated by birds as well as bees – and that’s enough for the mint comparisons.)

• very widespread temperate and tropical but absent from central and northern Eurasia – and in the map below is the widespread-ness, showing that it’s not so odd after all that there would be a verbena bush in Central Park.

The Stachytarpheta Genus

Now that it’s clear that the porterweed fits comfortably in the Verbenaceae family, what is it that separates porterweeds from the other 99 genera mentioned by the University of Hawaii?

It’s those long spikes that you see in the first image of the post. The genus name comes from the Greek stachy, meaning “spike,” and tarpheta, meaning “thick.” [Hammer] All members of the genus have them, and a couple of the other common names for the plant in English reflect this appearance: “snakeweed” and “rat’s tail.”

Two other characteristics that separate Stachytarpheta from other genera in the family are [Gentry]

• evenly serrated (sawlike) edges to the leaves - click to enlarge the image and you’ll even see a little tooth at the end of each sawtooth.

• leaf blade decurrent (extending down the leaf stalk or petiole) – here you can see the blade extending down without even enlarging the image.

Both these leaf characteristics are easy to recognize in the field. There are other, detailed, differences between the flowers of Stachytarpheta and other genera, but the long spikes for the inflorescences and the two leaf characteristics are enough in our area for you to identify the genus with confidence.

The “porter” part of porterweed had me puzzled until I read at several sites that some of the species are used to make a porter-like brew for medicinal purposes, the most authoritative source being one from the University of Florida. The concoction is supposedly used for fever, a wash for skin irritations, relief of constipation, and for worms in children [Hammer].

The species mutabalis

The species of Stachytarpheta that is most common around here is S. mutabalis, the one used to illustrate this post, with a pink flower and leaves about 10-20 cm long (smaller toward the top of the plant). A blue-flowered species that may be S. frantzii has smaller and slightly less hairy leaves is also in the area. Since they both attract butterflies, it is not at all surprising that a local “weed” has become a common garden plant, especially one with such an interesting appearance.

Stachytarpheta mutabilis is a neotropical native, being distributed throughout Central America and into South America, but having been spread to Hawaii and eastern Australia.

Verbena and Stachytarpheta

I may never get back to Central Park to look for that verbena bush, but I did find a drawing at the Delta taxonomic identification site that has both a Verbena species and a Stachytarpheta mutabilis drawing side by side. At least I can see them together on my desktop.