Id Lab #2: Two Common Smilax Species

Photo: A leaf of Smilax rotundifolia. Note the deltoid shape and similar length and width.

Welcome back for another ID Lab! As a housekeeping note, I had hoped to write another ID Lab post earlier, but neither my schedule nor the weather cooperated. I’ve also started adding footnotes (Chicago style, natch) where I’m using terminology or making assertions that warrant further explanation than I can cover here. Despite my scheduling woes, I did manage to get a few photos of two species of native, frequently overlooked and underappreciated vines: Smilax rotundifolium and Smilax glauca.

Commonly referred to as greenbrier and catbrier, respectively, these vines (technically, since they are woody, they are “lianas,” but no one bothers with this nomenclature so I’ll continue to use the simpler, but technically incorrect, “vine” in this post) are given short shrift, derided as “weedy,” and even get removed during overzealous invasive removal. While greenbriers can form dense, thorny thickets, they also part of our native ecology, providing forage for wildlife and creating habitat. They are unlikely to damage large trees, unlike invasive vines, so please let these misunderstood guys do their thing. Now, let’s get down to the ID!

Just like last time (I hope some of you remember all the way back to that post!) we’ll skip the leaves for the moment. Looking at the stems, you’ll notice both remain photosynthetic (green) throughout their lifespan. This is unique to this genus in our area, so if you see green-stemmed woody vines you know you’re looking at a Smilax.

The vines’ growth habit is also a good clue. S. rotundifolia is a larger, more robust plant than can from a dense thicket, or climbing “curtains” more than 20 feet tall. S. glauca generally forms smaller more delicate vines.When climbing, Smilax grows “stipular tendrils” -- highly modified stipules arising from the leaf base that wrap around branches to support the vine. While this attachment method is superficially similar to our native grapes (Vitis spp.), grapes tendrils arise from the apical meristem and are thus “stem tendrils.” This type of attachment pattern is part of the reason why Smilax doesn’t damage trees the same way that, say, Chinese bittersweet (Celastrus orbiculatus) does. Twining vines which wrap around the trunk of a tree (e.g. bittersweet) can girdle and kill even large trees; the small tendrils of Smilax don’t run that risk.

Photo: The stipular tendrils of Smilax species emerge from the base of the leaf. Seen here on Smilax rotundifolia. The stem stays green all year long.

In the two photos below, you can see the thorns of both Smilax rotundifolia and glauca. I forgot my ruler (of course) so you’ll have to take my word on this since the photos aren’t to the same scale, but the thorns of S. rotundifolia are more than twice the size of those of S. glauca. Also notice that S. rotundifolia thorns are generally sparsely arranged, emerge 90 degrees to the stem, and remain straight without any curve. S. glauca, however, has short, slightly recurved thorns which are much more densely arranged along the stem. Thorns on both species are either green or yellow on newer growth, with dark tips on S. rotundifolia, and may age to a brown or reddish color on older stems. Both species are woody and deciduous so the thorns are a great winter field diagnostic.

Photo: Large, straight thorns with dark tips arranged sparsely around the stem are indicative of Smilax rotundifolia.

Photo: The thorns of Smilax glauca are much smaller, lighter in color, slightly recurved, and more densely cover the stem.

If you encounter either of these species when they have leaves, you will understand their specific epithets. Both are roughly deltoid, have parallel venation, generally with 3 to 5 prominent major veins, but what really sets them apart is the relative length and color. Smilax rotundifolia has, as you might imagine, rotund leaves where the length is roughly similar to the width. Smilax glauca gets its name from the glaucous underside of its leaves, that is a whitish-blue bloom which you can see in the photo below. S. glauca also tends to have a leaf significantly longer than it is wide, with occasionally wavy margins, and light green blotches on the upper surface, though these traits aren’t sufficient for a positive ID on their own.

Photo: The upper surface of Smilax glauca can have irregular light-colored blotches. This leaf also has wavy margins.

Photo: The lower surface of Smilax glauca is markedly glaucous (has a whitish-blue cast). This leaf does not have a wavy margin.

Between, habit, thorns, and leaves, you should have an easy time differentiating these two, but before you go off identifying Smilax on your own, let’s look quickly at some lookalikes. English Ivy (Hedera helix, pictured below) forms somewhat similar leaves when sexually mature (notice, however, the pinnate venation), but is thornless, and grows dense aerial roots from its stem. Rosa multiflora can form a thicket, but has compound leaves and markedly recurved thorns (we’ll cover this and our native roses, in a future post). And another common Smilax is S. hispida, but its stems are densely covered in long, straight thorns, and its leaves are waxy, with deeply impressed veins. When I get a photo of that species, I’ll do a quick update.

Photo: Invasive Hedera helix (English ivy) leaves are not lobed when sexually mature and may at first appear similar. The pinnate venation (among other traits) is a dead giveaway.

So there you have it! If you have any suggestions for a future post let me know. If anyone is curious, I might do a quick post this winter about field guides and plant ID references and tools I find useful.