The Botanerd’s Handy Guide to Thalictrum Species

After a quick happy hour dinner and art walk on Friday evening, Steve and I settled in at home, each with our laptops, him working on something for his employer, and me trying to understand the genus Thalictrum and identify some plants.

Do we know how to party or what?

I wasn’t getting very far, because my pictures were taken on a windy, overcast day, and high ISO = noise = can’t see details. So I turned to Steve and asked “hey hon, would you be willing to be my assistant on a short hike tomorrow?”

Steve has little interest in botany, but he’s a good sport. Saturday morning, with rain threatening, we hiked quickly to the mystery Thalictrum meadow and took measurements, looked at leaflets with the hand lens, and got some slightly better photos.

I’ll spare you the details of keying out the species, which was Thalictrum coriaceum, or maid-of-the-mist.

Because I know other wildflower enthusiasts who’ve had difficulty with this genus, I’ve put together a handy guide to identifying the five Thalictrum species found in the Maryland piedmont.

 

If it looks like this, it’s T. thalictroides (rue anemone). I’m not trying to be flip, but rue anemone is the only one of the five species to bear perfect (ie., bisexual) flowers, so it can’t be confused with any of the others.

pistillate flowers of Thalictrum coriaceum

 

 

 

T. coriaceum and T. dioicum have nearly identical flowers. Both species are dioecious, meaning that plants bear either pistillate or staminate flowers. If you see the flowers in late March or early April, and the plants are no taller than two feet, they’re probably T. dioicum.

staminate flowers of Thalictrum coriaceum

 

If you see the flowers in late April or later and the plants are around three feet tall, they’re probably T. coriaceum.

a proximal leaf of T. coriaceum clasping the stem

 

 

 

 

As a further check, examine where the leaves meet the stem. On T. coriaceum, the proximal leaves (the ones closer to the base of the stem) should be clasping, like this.

a sessile distal leaf of T. coriaceum

 

 

 

 

The distal leaves (closer to the tip of the stem) will be sessile, like this. More specifically, according to Weakley’s Flora, the leaf immediately below the lowest flowering branch will be sessile. If you’re wondering how this can be considered sessile, read on.

click on this one!

 

 

The attractive thing about these Thalictrum species, at least to me, is the intricate, airy leaves, which are described as triternate, or triply compound.

 

 

Steve stretches out a leaflet for me to photograph

 

Each leaf has three stems, called petiolules (sub-petioles), that end in leaflets; each leaflet is divided again into three, and each of those divisions is divided once more into sub-leaflets.

An Erigeron shows what a sessile leaf looks like

 

 

 

Sessile generally means that the leaf tissue is directly connected to the main plant stem, without a petiole.

 

 

 

With triternate leaves, the fact that all three petiolules meet at a point directly adjacent to the main stem makes the leaf sessile. Note that in the photo showing a clasping leaf, there’s a little collar of leaf-like tissue surrounding the stem at the place where the petiolules meet it. That collar is lacking from the sessile leaf in the following photo.

The last two species are T. revolutum and T. pubescens. Both usually flower even later than T. coriaceum, and both usually stand taller (up to six feet or more). Examine the undersides of the leaves. T. pubescens will usually be slightly pubescent (finely hairy), though it may be glabrous (smooth). T. revolutum will have small, round bumps or stipitate glands, which look like hairs with teardrops on them. You will probably need a hand lens to see these details. And remember to always examine multiple leaves and see what features the majority of them have.

Crush a leaflet between your fingers and smell it. T revolutum will have a skunky odor. Although the specific epithet revolutum means that the leaves turn downwards at the edges, a useful mnemonic is to remember the word revolting.

I regret that I have no pictures of this plant to show, but I’ve never seen it in the wild. From what I’ve seen on various internet sites, the flowers of this plant look much like those of T. coriaceum, except they’re white.

T. pubescens can be dioecious or polygamo-dioecious (see this post for explanations of these terms). I believe these to be the staminate flowers, but am not sure; I took this picture last summer before delving into Thalictrums. On my to-do list for later this year is to find a stand of these and examine them more closely. And then write about it, of course.

 

When confronted with a large number of details I often make spreadsheets in order to make sense of it all. The information in this chart comes from Flora of the Southern and Mid-Atlantic States, Flora of North America, Illinois Wildflowers, and to a very small extent, my own observations. Any errors are mine.

  T. coriaceum T. dioicum T. pubescens T. revolutum
height to 30″ 25.5″ to 59″ 20″ to 118″ 36″ to 84″
stems glabrous glabrous or glandular coarse glaucous or slightly pubescent
leaves
   proximal      (toward base) petioles clasping petiolate petiolate; petioles often pubescent
   distal (toward the tip) sessile or nearly so petiolate sessile
   basal? no yes yes  no?!
leaf at lowest flowering branch sessile petiole 3-7 cm
leaflets ovate, obovate, suborbicular ovate, obovate, suborbicular ovate, obovate, suborbicular ovate, obovate, suborbicular
3 – 9 lobes 3 – 12 lobes 1 – 5 lobes 1 – 5 lobes
lobe margins crenate lobe margins often crenate lobe margins entire lobe margins entire
leaflet undersides glabrous or glandular glabrous or glandular finely pubescent to glabrous stipitate glands or papillae
flowers pistillate: purplish, standing pistillate: purplish, standing white to purplish pistillate: greenish
staminate: purple with yellow anthers, drooping staminate: yellow, drooping filaments standing staminate: white, drooping
flowering May – July March – April May – July May – August

A final note: Thalictrum is one of those taxonomically challenging genera; different authors recognize different characteristics and different names. This guide is probably not much use outside the Maryland piedmont, where more or different Thalictrum species may be present.

3 thoughts on “The Botanerd’s Handy Guide to Thalictrum Species

  1. I bought a white thalictrum in about 2004 that had a wonderful sweet smell like albizzia. sadly it died out and i have never seen any other thalictrum that has such a sweet smell. do you know which one it is? i live in maryland.

  2. Pingback: DPP | Elizabeth's Wildflower Blog

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