Sunday, April 13, 2014

Plant Story - Yarrow, Achillea millefolium, a plant species complex

Achillea millefolium, yarrow
Achillea millefolium, yarrow
Yarrow, Achillea millefolium (sunflower family, Asteraceae) is a small perennial plant found around the world. It is a medicinal herb in both modern and traditional medicines (see previous post: link) and is an attractive, easily-grown garden flower and in some places, a weed.

Very few plant species are listed as native to Europe, Asia and North America but that is the case for Achillea millefolium.  

Actually, yarrow is what botanists call a species complex. I will outline the situation as I understand it, because this sort of complexity is surprisingly common in plants, although yarrow does it particularly well.

First, an overview of the plants: 
yarrow, Achillea millefolium
yarrow, Achillea millefolium

The genus of yarrow, Achillea, is native across the Northern Hemisphere. There are perhaps 110-140 species, most of them in Eurasia. Only three species of yarrow are considered native to North America, siberian yarrow (A. sibirica,  northern US and Canada), Chinese yarrow (A. alpina in Canada but not the US) and common yarrow (A. millefolium), which is in every state and province. Common yarrow, Achillea millefolium, although just one of 110-140  species in the genus, is very widespread. In fact, its range is probably as great or greater than all the rest of the genus combined. 

Second, common yarrow:

Most likely, common yarrow, Achillea millefolium originated in Asia and spread out from there a long time ago. It is a moderately weedy plant that grows well in disturbed sites. Natural disturbed sites are found where rivers and lakes rise and fall or where big animals congregate, for example at watering holes. Additional disturbed sites are created by human settlements. Hopping from one disturbance to another, yarrow probably spread through meadows and along forest edges. But it was also adaptable, so it climbed mountains, flowering sooner in the shorter growing seasons at high elevations, or descended to the seaside, growing tall and skinny in the dense grass. At some point in crossed into North America and spread out there. Over those vast distances differences developed between yarrows, and the ones of New York are more like the ones of Kansas than they are like the yarrows of France, Ukraine or Yunnan, but the differences are subtle. 
Yarrow in the grass,
Tierra del Fuego,
southern Argentina

Understanding yarrow was made more difficult by people! We have used it medicinally for millennia in Europe, Asia and the Americas. In fact, yarrow was buried with a Neanderthal in Iran about 65,000 years ago and found on the teeth of Neanderthal skulls in Spain, dated to 40,000-60,000 years ago. Plants that people use are carried around with them and, often, planted in places where they did not naturally occur. Thus, yarrow has been introduced by humans into places as far apart as Greenland and Patagonia, New Zealand and Hawaii.
yarrow, Achillea millefolium
yarrow, Achillea millefolium

Third, the variation in common yarrow:

Wherever they grow, yarrow plants respond to differences in the environments with changes such as flowering sooner or growing taller, genetic changes which adapt the plant to its particular environment but do not indicate different species. Generally a close look shows that all the way up the mountain, time to flower gets shorter to match the shorter growing season. The plants at the extremes might seem different enough to be two species, but continuous variation makes any line totally arbitrary. 

There is another form of genetic variation in yarrow--at the chromosome level. Genes in an organism are on chromosomes. Higher animals and higher plants (that is, protozoa and algae, among others, don't fit my generalization) have chromosomes in pairs, which allows recombination in their offspring. Each new animal or plant gets half its chromosomes from each parent. The simplest system--and the one that seems self-evident to us as animals--is that there is one pair of each chromosome in every individual (except sex-determining XY chromosomes) . 

Plants have long been known to have individuals and species that have more than 2 copies of each chromosome. When all the chromosomes (the whole genome) has doubled, there are 4 copies of every chromosome and so of every gene. That's called tetra (for 4)-ploidy. Two copies is called diploidy. Some yarrows are diploid, but others are tetraploid. And yarrows don't stop there. Some have six copies (hexaploid) and others eight copies (octoploid) of all their genes. In animals polyploidy usually causes a breeding barrier, so that tetraploid animals form a different species from their diploid relatives, but plants, including yarrow, mate promiscuously across chromosome numbers. Tetraploids do mate successfully with diploids--and with other tetraploids and with hexaploids and octoploids (and with the odd-numbered combinations, but those are uncommon.) Consequently chromosome numbers add variation, they don't define species. 

Botanists have wrestled with this problem for a long time. They don't yet understand what polyploidy does for plants--is it advantageous or a mistake that isn't fatal?--but it is certainly a problem for defining species. Different botanists have come to different solutions looking at the same information.  Achillea millefolium has been separated into something like 40 different species and lumped into one. By experts. Presently all over the world  the plants are usually combined into one species, the common yarrow, Achillea millefolium.

About species complexes:

Since botanists are uncomfortable with species that vary a lot and contain polyploid series, they often call such plants a species complex. This term recognizes that, for example in the case of yarrow, habitat, distance and chromosomes cause the plants to vary, but in ways that do not suggest separate species. If yarrows in Germany cross with each other despite diploids, tetraploids and hexaploids in the population and yarrows of multiple ploidy levels in Iran do the same thing, and if between the moist forests of Germany and the dry uplands of Iran the plant characteristics change gradually, then it is difficult to draw a line somewhere and find different species. 
yarrow, Achillea millefolium
yarrow, Achillea millefolium

I'm a supporter of species complexes, because when people try to split apart a group like yarrow into component species, they have to find characters for recognizing one species from the other. The interbreeding of yarrows makes that very difficult, and so the characters the botanists can find are small and subtle. Small subtle characters make it difficult for nonspecialists to separate species. Chromosome number, for example, is important genetically, but you need a good microscope and a healthy plant to count chromosomes. So on your hike you'd be forced to say, "well, that's either Carpenter's yarrow or Potter's yarrow depending on the chromosomes." And if you don't take it home to study, you will never know which one. I prefer characters that allow field identification.

My attitude doesn't sit well with plant-centric botanists. If the plants recognize species, usually by not crossing, then we should recognize species, even if the two species are absolutely identical except for the breeding barrier, they would argue. I agree in principle, but in practice I want easy characters for identification.

Lacking easy characters to sort them out, the whole mess is called Achillea millefolium or the Achillea millefolium complex

Species complexes are relatively common among plants. Widespread, well-known plants are particularly likely to resist subdivision into moderate-sized species. Not all species complexes are a polyploid series, some just have lots of variation but the distribution of that variation frustrates attempts to divide the complex into component species. But polyploidy is plant biology's dirty little secret: polyploidy is difficult to work with, difficult to understand (animal biologists find it incredible that plants of different ploidy levels form one species) and yet quite common. 

Native and introduced
yarrow, Achillea millefolium
yarrow, Achillea millefolium

 The USDA plants map says Achillea millefolium is both native and introduced in the U.S. LINK  That should be a contradiction--how can something be both introduced and native?--but for yarrow, it applies. Plants of common yarrow were brought by Europeans to North America, where they hybridized with the native common yarrow, making the species complex even more complex. As individual plants and as a species, Achillea millefolium, is both native and introduced to North America.

Common yarrow, you will have noticed, has conquered the world, and whether it is a single species or a group of closely related species or something in between, you have to acknowledge its success. 

Comments and corrections welcome.

Note added for thoroughness: neither Carpenter's yarrow nor Potter's yarrow exist; nor is it likely that two species will be absolutely identical except for a breeding barrier. I was illustrating my points.


Neanderthals and yarrow:
Popular article:  Barras, Colin. Neanderthal dental tartar reveals evidence of medicine 18 July 2012 New Scientist online:
Technical papers:
Hardy, Karen et al. 2012. Neanderthal medics? Evidence for food, cooking, and medicinal plants entrapped in dental calculus. Naturwissenschaften. Vol. 99 (8). AUG 2012. 617-626
Leroi-Gourhan, A. 1975. The flowers found with Shanidar IV, a Neanderthal burial in Iraq. Science 190:562–564.

Yarrow across the world
Achillea millefolium at USDA Plants link

Smithosonian National Museum of Natural History Flora of the Hawaiian Islands online;
Natural History of Iceland site:

Kathy Keeler


  1. Thank you for this article/explanation which was interesting to me even as a common (yarrowlike) person!

  2. Very informative, Kathleen. I was researching whether yarrow was native or introduced, and your blog popped up! I'll be linking back to and crediting your article in the blog post I'm writing. It was good to meet you, albeit briefly, at the Capital Region Fling this month!

  3. Yours is the first explanation that makes sense. Even botanist friends have not been able to de-confuse me. Thank you so much.

  4. Thanks...whenever I write about constantly changing taxonomy as a barrier to teaching an increasing number of people interested in the plant world I get slapped down. I still can't pronounce what used to be "Aster" lol. I like the "complex" idea, because complex is what plants are. Maybe a number system for the types for study?

  5. AchilleaBunne🐰🌿September 29, 2020 at 3:56 PM

    Excellent article. Coming across your blog page was like finding a small treasure chest today.

  6. This blog is an incredible resource, thank you for sharing it.
    - When we say that when europeans brought yarrow to the Americas, it mixed with the existing native yarrow. How did the native yarrow get here?

    1. I think it came when people arrived in the Americas. Much earlier Neanderthals used yarrow as a medicine. So likely the people who became Native Americans were using yarrow as a medicine too, and brought it, on purpose or because seeds fell where they lived.

    2. amazing - thank you! I'm an artist and am really interested in this. I just ordered your book to learn more, I love the way that you write and think about about plant life.