13 de febrero de 2022

Thistles with white in their leaves

Hello! This post is to talk about the thistles that have white on the upper surface of the leaves. Where these species co-occur, they are often mis-identified with each-other, especially prior to flowering, so I am putting together an ID guide to them here.

This post applies to the Americas, both North and South, but is probably still useful for sub-Saharan Africa, Australia/New Zealand, and other places where this same suite of exotic thistles have been introduced. Basically, this post doesn’t work for Eurasia and Mediterranean Africa, where there are more members of all of the genera mentioned below and thistles are generally just significantly more complicated.

There are an assortment of thistles that have white patterning on their leaves. The most conspicuous of these is Silybum marianum, and, consequently, many thistles with white on their leaves are identified as that species, here on iNat. However, the situation is complicated by the presence of these other thistles as well. The purpose of this post is to shed some light on the variety of thistles that have white in their leaves and how to use the different patterns of white to help make IDs.

In the Americas, we have six species of thistles with white in their leaves and two genera of thistle-look-alikes (that are actually not even in the same family!) that are often confused with thistles before their flowers open. In this post, I’ll be using scientific names throughout. This is partly because different common names apply in different areas (and the same common name can apply to multiple species of thistle) and partly because it is easier for anyone who is running this through google translate into a different language to not be confused by English common names.

I will eventually get to all of these species here, but initially I will only be posting about three species. I'll come back and edit this post periodically until it is complete, but with it being spring in California right now (February 2022), I wanted to get the most important ones done first!

Species currently covered here:
Carduus pycnocephalus
Carduus tenuiflorus
Silybum marianum

With these thistles, there are a few things to consider. I’ll break each of these down, within each species profile, below.

  1. Location. Geography is helpful, as not all of these species occur everywhere.
  2. Where the white on the leaves is located.
  3. The amount and strength of the white.
  4. What to look for if there are any reproductive parts (and other useful ID notes).
  5. Whether the thistle is in cultivation or not.

Let's do the Carduus thistles first!

(also, I'm sorry the images are a bit squished! I'm not really used to embedding them using HTML, and I'll try to get them fixed at some point. In the meantime, the observations they come from are mentioned immediately under each photo, so you can click there for better resolution)


Carduus pycnocephalus
English common names include:
Italian Thistle, Italian Plumeless Thistle, Slender-winged Thistle, Slender Thistle, Plymouth Thistle

Location:
This species is found nearly exclusively in the Mediterranean climates of North and South America, but there are records in the Willamette Valley and also Texas (US), Puebla (México), Chimborazo (Ecuador), and near/along the Río de la Plata (Argentina/Uruguay).
[link to iNat map] [link to USDA PLANTS profile]

Where the white coloring is found and the strength of the white coloring:
When this species has white in the leaves , it is often confused with Silybum marianum as a rosette. The white coloring is on and immediately touching the main and secondary veins. The color varies from a fairly white to just a light green and may be absent altogether, but it is always weaker than the white color of Silybum marianum. There is also sometimes some white on the pointy tips of the leaves, as there often is with Carduus thistles.

One good way to figure out if the plant you are looking at is this species or S. marianum is where/how the white is positioned. In Carduus pycnocephalus, the white, if present, is almost always only on the main and secondary veins and is nearly never on the third-order veins. On S. marianum leaves, the white is always on the third-order veins! When S. marianum has very little white, the white is absent from the main and/or secondary veins and is found only on the third-order veins. See the portion of this post with S. marianum to see differences in the distribution of white on the leaves.

A Carduus pycnocephalus rosette, showing the distribution of white on the primary and secondary veins.
Note the white coloration on the main and secondary veins as well as the leaf tips. [source observation]

A Carduus pycnocephalus rosette, showing more white coloration than normal, but within the variation for this species. Note the distribution of white on the primary and secondary veins, with a small amount on the third-order veins..
This rosette shows more white coloration than normal, but within the variation for this species. Unusual for this species, note the small amount of white on the third-order veins. This is the most white that is really ever seen on Carduus pycnocephalus [source observation]

Reproductive parts/other ID notes:
Heads are (usually) in clusters at the end of stalks. Clusters contain anywhere from 1-5 heads. Phyllaries of heads are generally more narrow and less overlapping than found in C. tenuiflorus and there is often some spider-webbing of hairs between and around the phyllaries.

Leaves of C. pycnocephalus are often less hairy than those of C. tenuiflorus on the top, which gives them a darker green color and those of C. tenuiflorus a more gray-green color. Leaves of C. pycnocephalus are also more “textured” in appearance than those of S. marianum, giving S. marianum leaves more of a smooth and glossy appearance (though C. pycnocephalus leaves can be glossy, but aren’t really generally super smooth looking).

Cultivation:
This thistle is not cultivated and is classified as a noxious weed in several US states.

Good observations to check:
https://www.inaturalist.org/observations/105485472
https://www.inaturalist.org/observations/105593845
https://www.inaturalist.org/observations/105563520
https://www.inaturalist.org/observations/106384694
https://www.inaturalist.org/observations/106257128
https://www.inaturalist.org/observations/106213380
https://www.inaturalist.org/observations/106571779


Carduus tenuiflorus
English common names include:
Winged Plumeless Thistle, Slender-flowered Thistle, Slender Thistle, Winged Slender Thistle, Corsican Thistle, Sheep Thistle, Shore Thistle, Scotch Thistle (though this name is more commonly applied to Onopordum acanthium and Cirsium vulgare)

Location:
This species is found mostly in the Mediterranean climates of North and South America, but there are records in the Willamette Valley and a decent population in Texas (US), Estado de México (México), and one record near the Río de la Plata (Argentina/Uruguay).
[link to iNat map] [link to USDA PLANTS profile]

Where the white coloring is found and the strength of the white coloring:
Generally, this species lacks white on the leaves. However, like other Carduus thistles, there can be white to light green at leaf/spine tips. The upper leaf surface of this species is usually hairy, which conceals any color of the leaf surface. In cases where those hairs are mostly absent, there can be lighter coloration in a similar pattern of distribution as C. pycnocephalus, but the white color of C. tenuiflorus is never as white as it can be in C. pycnocephalus (which itself is never as white as S. marianum). So, generally, this species isn’t one with lots or any white in the leaves.

A Carduus tenuiflorus individual, with leaves and flower heads, showing the hairy leaf upper surface, which lacks white coloration.
Note the hairy leaf surface, which obscures any white coloration on the upper leaf surface. [source observation]

A Carduus tenuiflorus individual, with leaves and flower heads. At the lower part of the photo is a leaf which is largely lacking hairs (unusual for this species) and shows the distribution of white coloration.
Note the large leaf that is from the rosette on the lowest side of the plant in this photo, which is largely (and unusually) lacking the hairs on the upper surface of the leaf. This is a good example of how the white coloration appears in this species if it is visible, and note the similarities in the distribution and intensity with Carduus pycnocephalus. [source observation]

Reproductive parts/other ID notes:
Heads are in clusters at the end of stalks. Clusters contain anywhere from 5-20 or as many as 30(!) heads. Phyllaries of heads are generally fairly broad and can be lightly overlapping. Heads also generally don’t have much or any spider-webbing sort of hairs.

Leaves of C. tenuiflorus are often more hairy than those of C. pycnocephalus on the top, which gives them a gray-green color and those of C. pycnocephalus a more dark-green color.

Cultivation:
This thistle is not cultivated and is classified as a noxious weed in several US states.

Good observations to check:
https://www.inaturalist.org/observations/77563642
https://www.inaturalist.org/observations/77516321
https://www.inaturalist.org/observations/76949690
https://www.inaturalist.org/observations/75558798
https://www.inaturalist.org/observations/75127314
https://www.inaturalist.org/observations/75097727
https://www.inaturalist.org/observations/74989941
https://www.inaturalist.org/observations/74058365
https://www.inaturalist.org/observations/46412885


Silybum marianum
English common names include:
Milk Thistle, Blessed Thistle, Blessed Milk Thistle, Mary’s Thistle, Marian Thistle, Variegated Thistle

Location:
The largest wild populations of this species in the Americas are in the Mediterranean climates of North and South America. On the west coast of North America, there are naturalized populations north through the Willamette Valley and similar habitats all the way to Vancouver, British Columbia (Canada). There is also a large naturalized population in Texas (US), in and around the Estado de México (México). There are also small populations at scattered locations in eastern North America, on the west side of the Andes in South America, and in the pampas and espinal areas of Argentina. Additionally, because this species is cultivated, it can pop up as a waif pretty much anywhere that it is possible to cultivate it.
[link to iNat map] [link to USDA PLANTS profile]

Where the white coloring is found and the strength of the white coloring:
The white on the leaves of S. marianum is usually very bright and apparent. The white can vary from only light spotting to extensively white. The white coloring is always present on the third-order veins first, so plants with minimal white will have white on those veins. For plants that have higher amounts of white, the coloring will be present on the secondary and main veins as well. In general, the white coloring on leaves of S. marianum is always a stronger, more brilliant white than nearly any other thistle that we have in the Americas. The only other thistle that gets this brilliant of white color is Carduus nutans, which doesn’t have white on the veins at all (the white color on that species is only on leaf margins and spines). See notes under Carduus pycnocephalus.

A seedling of Silybum marianum, showing the pattern of the white color being more present on the third-order veins before the secondary veins and primary veins.
Note how here the white color is mostly on the third-order veins and a bit on the secondary veins. This is a different order of where the white is more present than the Carduus thistles. The white color here is also more intense than found in Carduus pycnocephalus. [source observation]

A close-up of a Silybum marianum rosette, showing extensive white patterning on pretty much all of the veins.
The white color here is extensive on the leaves, covering all the third-order veins and the secondary veins. This is a very clear example of both the intensity and distribution of the white coloring in this species. [source observation]

Reproductive parts/other ID notes:
Heads are large with phyllaries that are broad and the bases of which are spine-tipped where they meet the head. Heads are solitary and not clustered at the end of stalks.

Leaves of S. marianum are usually glossy and smooth in appearance, whereas those of C. pycnocephalus are usually more textured in appearance (though they may appear glossy).

Cultivation:
This thistle is occasionally cultivated for both medicinal and horticultural purposes. This means that it can be found occasionally as a waif escaping from cultivation pretty much anywhere.

Good observations to check:
https://www.inaturalist.org/observations/106128574
https://www.inaturalist.org/observations/106038579
https://www.inaturalist.org/observations/106571811
https://www.inaturalist.org/observations/106214150
https://www.inaturalist.org/observations/106571879
https://www.inaturalist.org/observations/105938651
https://www.inaturalist.org/observations/105742151


I've deactivated comments on this post but if you have feedback on this post, I would love to hear about it! There's a version you can comment on [at this link].

Ingresado el 13 de febrero de 2022 por cwarneke cwarneke

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