Foraging prickly sow thistle: pictures, flowers, leaves & identification for Sonchus asper

Latin Name: Sonchus asper
Common Names: Prickly sow thistle, spiny sow thistle, sharp sow thistle, rough milk thistle
Season: Early spring
Edible: Yes 
Flavor: Good
Medicinal and nutritional value: Vitamin rich, antioxidant strong, liver and kidney purifying 
Identification difficulty: Beginner

The leaves of this plant are at a good stage for harvest

Despite the texture, prickly sow thistle is one of the edible wild plants I look forward to the most every spring. 

If you can work around the prickles, which is easy enough to do when planning dishes that need to be pureed (like sauces and soups), you are rewarded with a wonderfully rich leafy green. The flavor is generally very mild with only a slight bitterness, comparable to swiss chard or belgian endive, to add complexity. Most everyone who eats leafy greens will enjoy properly prepared prickly sow thistle. 

Perhaps even better, prickly sow thistle is extremely common 

Nutritional & medicinal benefits

The sow thistle family (Sonchus) is one of the ones where a significant amount of research has been done into nutrition and potential medicinal benefits. Sow thistles have long been accepted as health foods, associated with liver and kidney purification. 

Recent studies have shown that the sow thistles, particularly the prickly sow thistle, are antioxidant powerhouses. Antioxidants have been shown to reduce the effects of aging, both on the body and mind. They also lower your cancer risk. 

While antioxidant extractions are available in pills, syrups, etc., studies show that the best way to gain the positive effects of antioxidants might simply be to incorporate a large number of antioxidant-rich foods into your diet. 

All three sow thistles were found to be rich dietary fiber and in vitamin E, though smooth sow thistle (Sonchus arvensis) had the highest concentrations. Like most leafy greens, sow thistles have a lot of valuable minerals, including potassium, calcium, magnesium, iron, zinc, sodium and selenium. Smooth sow thistle is the best of the 3 for potassium, common sow thistle has the highest concentration of iron and prickly sow thistle offers the most calcium. 

As an added bonus, sow thistles DO NOT have large amounts of oxalic acid, even though they exhibit red coloration. They contain less than 10% of the oxalic acid found in spinach or swiss chard, for example, and less than 5% of the oxalic acid in purslane. 

To learn more about the antioxidant properties of prickly sow thistle, check out this article from the National Library of Medicine. 

To learn more about the nutritional qualities of the sow thistle genus, please read this article from the National Library of Medicine. 

History as a food crop

There are three, common, wide-spread sow thistles: Common sow thistle (Sonchus oleraceus), prickly sow thistle (Sonchus asper) and smooth sow thistle (Sonchus arvensis). They are native to Europe, particularly the Mediterranean regions. They are now found throughout temperate North and South America, Australia and New Zealand, the Middle East, and much of Asia and Africa. 

A good-sized haul of prickly sow thistle leaves,
ready to cook and eat

Our earliest record of sow thistles as a food comes from the ancient Greeks, who considered it to be strengthening, and used the non-prickly varieties in salads, especially during winter time when greens were scarce.

Europeans in the middle ages valued the sow thistles for animal feed. The name "sow thistle" comes from farmers feeding the plant to sows with piglets; it was believed that this plant increased lactation.

As traditional agriculture became the standard in Europe and European colonies, like the U.S., sow thistles came to be regarded as common weeds, and a great deal is spent to eradicate them. 

But that is not always the case in areas where sow thistles have spread. 

As European agricultural practices spread with colonization, many native species of plants, which were essential food sources for the native peoples, were wiped out. Eurasian plant species had evolved a centuries-long history of competition, due to exposure to other plant species spread via trade routes. As a general rule, these plants could out-compete native species, which is why dandelions, plantains, sow thistles and others are considered common weeds today. 

But many native peoples have embraced the "weeds" traditional agriculture rejects, and have used them to supplement or replace native plants that have been decimated. Sow thistles in particular are consumed by the Māori of New Zealand and the by the native peoples of the rural Brazilian rainforest regions.

In addition to the three most wide-spread sow thistles there are many regional species, especially in Africa and some hype-local species in places like the Canary Islands, some of which are used for food. 

Prickly sow thistle identification

Note: the entire above-ground portion of the plant is edible. 

Prickly sow thistle at the perfect stage for harvesting leaves

Growth season & features

An early basal rosette, the raised leaves are good
for harvesting

Sow thistles are annual plants that begin their growing season as a basal rosette in early spring. A basal rosette is when leaves create a circle on the ground around a central attachment to the root system below. In Texas this is usually early March but can be mid-February in mild winters. In the Northeast I generally found them in mid-April. 

The plant will very quickly grow raised leaves from the rosette that stretch upward. This is the best time to gather the leaves, when they aren't against the ground (the leaves on the ground tend to be tough and fibrous) but before the stalk is branching. 

I also harvest the leaves when the central stalk has formed but is young, 4-6 inches long, even if it already has flower buds. But once the plant starts branching the leaves usually loose flavor and become stringy. 

This plant to the left is already branching at the top. The leaves will generally be less flavorful at this stage, but this plant seemed to be unusually healthy and well-watered, and some of the leaves were still tender and not stringy. 

If left unchecked, prickly sow thistle can grow to around 3' 6" tall (about a meter), but will most often be under half that height in areas that are mowed or otherwise landscaped. 

Not every plant will produce branches though all will produce a cluster of flower stalks at the top of the central stem, if allowed to grow long enough. 

Leaf shape & features 

The leaves are the most important Identification feature for prickly sow thistle. 

Leaf progression. When very young (left), the leaves are spoon-shaped and not lobed.
As the plant matures the leaves will become increasingly large, dark, lobed and the prickles will become more defined. All of these leaves are from a young plant. 

The lower leaves in the basal rosette are deeply lobed

Each leaf in the basal rosette stage is long and narrow, about 3 times as long as it is wide. The leaves are deeply lobed, which means they appear to have cut outs on the sides; they somewhat resemble dandelion leaves. 

Leaves from a 8" tall plant, dark green and
purple, from cold expos

The leaves are generally bright green but will be darker in cooler weather. If they are exposed to near freezing conditions during their growth they will have red or purple zones, or even become purple entirely. This isn't a concern for the forager, as the purple leaves are just as edible. The central midvein of the leaf is thick, well-defined and white but will turn pink, red or purple in cold weather. 

The leaves on the upper stalk or branching stems are a different shape from the leaves at the base. These grow as teardrops, are not deeply lobed, and the rounded base clasps around the stem. These leaves are smaller, less flavorful and often papery. 

Leaves as they appear on the upper stalk or branching stems.

The entire margin (edge) of each leaf is rimmed with prickles where the leaf pinches itself together into soft spikes. Unlike thorns, prickles are not especially hard, and will bend easily if pressed. When touching them I would describe the sensation as prickly (lol) rather than painful. 

This is one way to distinguish a sow thistle from a true thistle as true thistles have hard spines or spikes on the leaf edge that will pierce the skin if pressed. But more on that in the look-a-like section below. 

While in the elevated basal rosette shape, the prickles on the leaves create rather beautiful fractal patterns as viewed from above. 

Stem/stalk features

The central stalk of the plant is hollow, ribbed or lined on the outside, translucent green, patched or streaked with red/pink and exudes copious amounts of thick, opaque white sap when damaged. This sap will oxidize when exposed to air and turn the plant matter brown. 

One important ID feature of prickly sow thistle is that the stalk will snap easily and cleanly, rather than bending or stretching. 

Flower & buds

What appears to us as a single yellow flower is actually many dozens of ray flowers clustered together into a single flower head. Sow thistle flowers closely resemble dandelion flowers, and, like dandelions, this clustered growth will ensure each flower releases dozens of seeds, which will also turn into fluffy white tufts to be born away by the wind. 

Unlike dandelion flower heads, which contain so many individual ray flowers that they bend back over themselves and their seed stage resembles a sphere, sow thistle heads always resemble a disc or an 80s flattop hairstyle. You can easily see the green bracts from which the flowers come below, and the seed head will only ever be a half-sphere. 

In this image, the yellow circled are flower buds,
while the rest are spent flower heads.

The plant will almost always produce multiple flower heads from the central stalk and if conditions are ideal will also have several branches each ending in multiple flower heads. 

The flower buds are shaped like fat hourglasses and will be tightly tucked in on the top, with no yellow or white showing until right before blooming. The spent flowers most often look like teardrops or diamonds and will usually have a tuft of darkening yellow at one end.

From left to right: youngest bud, mature bud, bud about to open, spent flower,
spent flower beginning to close, fully closed spent flower

After growing as a bud, blooming, and tucking the flowers back into the calyx in a teardrop, the spent flower will once again re-open to release the seeds which are at the base of white cottony tufts, denser and more opaque than those of a dandelion. 

A healthy, well-developed plant might have 4-7 flowers, 2-4 spent flowers and a dozen or more buds simultaneously, and will continue to bud and flower throughout the growing season unless the plant is cut or the weather becomes too hot or dry. In the Northeast, un-mowed sow thistles would often survive until the frosts in October, but here in Texas they generally die off in late spring, June at the latest. 

Look-a-like plants

Thanks to the prickles, there really isn't anything poisonous that you could confuse prickly sow thistles with, which is why I consider them to be a beginner plant. Common and smooth sow thistles are a little more complicated, but I'll cover them in another post. 

Prickly lettuce, edible

The easiest plant to confuse with prickly sow thistle is prickly lettuce, Lactuca serriola, is just as abundant as prickly sow thistle. It's also edible, and is the wild ancestor to our modern cultivated lettuces; if you confuse prickly lettuce for prickly sow thistle you will be unharmed but disappointed. The wild lettuces are quite bitter. 

Prickly lettuce also has prickles on the leaf edges (though much smaller and less defined than those of prickly sow thistle), and has a similar leaf shape, white sap and growth habits. The two plants are related, both in the family Asteraceae, the aster or daisy family. 

Prickly lettuce has a line of soft spines down the back of each leaf mid-rib, and this sets it apart from prickly sow thistle, which lacks this feature. 

True thistles, plume thistles & cotton thistles, edible

Members of the genera (plural of genus) Carduus and Cardueae (true thistles), Cirsium (plume thistles), and Onopordum (the cotton thistles), also somewhat resemble prickly sow thistle, at least when quite young. They also grow as a basal rosette and also have translucent, hollow stalks (though the walls are much thicker), but lack the copious, opaque white sap. 

The easiest way to tell the thistles apart from prickly sow thistle is generally to touch the spines. Most thistle spines definitely prick, a sensation of pain rather than simple unpleasantness; pressed hard enough they can piece the flesh. 

All members of the above four genera are edible, if you remove the spines. Thistle leaf midribs are quite good, a favorite wild food of mine. The stalks are covered in a bitter skin that must be pealed, other than that they are also edible and tasty. The flower buds may be pealed, though there wont be much left when you are done, and they taste like artichokes, which are a type of thistle. 

Common and smooth sow thistles, dandelions, chicory, smooth lettuces, and others, edible

One of the many wild lettuces

Oddly enough, you are less likely to mistake prickly sow thistle for one of its close cousins, like common or smooth sow thistle, than you are to mistake it for a distant cousin like prickly lettuce. This is because these plants lack prickles, spines or any similar growth pattern.

Common sow thistle is much more difficult to identify for this reason, and also because the leaf shape changes dramatically over the course of its growth cycle. There are many plants in the aster family which have deeply lobed leaves, grow from a basal rosette, and have copious amounts of white sap from the stalk. At first glance you might confuse prickly sow thistle for dandelions or other wild lettuces, but the lack of prickles will quickly clear up any confusion. 

How to harvest and use prickly sow thistle in the kitchen

As mentioned before, the entire above-ground plant is edible. 

Flavor-wise, I prefer prickly sow thistle to it's cousin, common sow thistle, I find it to be much less bitter and the leaves have more moisture, creating a better texture.

The most abundant food part, and my personal favorite, are the leaves, which are a mild leafy green. The prickles will soften on young leaves that are blanched, steamed, added to a casserole or sautéed, but for older leaves you should plan a recipe that involves pureeing the leaves, like a sauce or a soup. 

Some people find the sensation of even the youngest leaves to be unpleasant when only lightly cooked. My husband is one of these, but he does enjoy the plant when it's completely pureed. 

In the image to the left, this core of leaves clustered around the stalk, taken from a plant around 6" high that has not yet branched or flowered, is, in my opinion, the most flavorful part. Still, as young as these leaves are, they will be best pureed. 

 For most people the flower buds are their favorite part. It's important to review the picture in the buds section above to not confuse the bud with the spent flower head. 

There's nothing poisonous or toxic about the spent flower head, but it does have an unappetizing texture, being soft and mealy. 

Flower buds can be tossed directly into a salad or soup but are most popular pickled and used as a substitute for capers. 

The stems are also best pickled, though due to their thin walls it's important to not over-cook them and turn them into mush. The thin flesh is both a blessing and a curse when pickling. On the plus side, the stems drink up the pickling flavor very quickly, and are ready to eat after only an overnight or so. But on the downside there isn't much food there to make the effort worth it. I will only make quick fridge pickles with sow thistle stems, and even then I generally only take the time if I am also pickling something else. 

The flowers can be tossed into salads or breaded and fried. If you remove the bracts, the white and yellow flower rays can be added to baked goods, cheeses, or really used any way you would use the similar dandelion flower. Unfortunately, sow thistle blossoms lack the wonderful flavor of their dandelion cousins. 

To see how I've used sow thistles, click here.