The Monthly Fern: Bracken—dreadful or delightful?

"Shelter" by Colin.

Come my sweet and let us lie in

Some idyllic wooded glade

And let us stay til merry-made

Amid the Bracken.

For April, the South Dakota fern-a-month series features the world's most widespread fern—Pteridium aquilinum, Bracken (aka Pasture Brake, Eagle Fern, Helecho Macho, and more). Thought to be native to the Northern Hemisphere, it's now widely naturalized and known from all continents except Antarctica. In South Dakota, it grows in the Black Hills in the western part of the state (1).

Bracken is said to thrive in a variety of habitats—woodlands, fields, old pastures, thickets, disturbed soils, burned areas, and marshes. But sources vary on this. For example, some say it's intolerant of wet soil and shade; others say it can grow well in all but very alkaline soils. In any case, Bracken forms extensive colonies of robust plants to 1.5 m or more tall, from deep rhizomes to 20 feet long. The large triangular leaves (fronds) are twice or thrice pinnately compound (2- or 3-times divided into leaflets). When fully grown, the blades often bend to horizontal, shading much of the ground (source).

Leaf division is an important character in fern identification, but can be hard to understand and explain. However I will try. Below is a thrice pinnately compound Bracken leaf. It's divided into 9 large segments (one terminal), which are divided into many narrow segments, which are divided (or nearly so) into small ultimate segments. "Pinnately compound" means segments line up on each side of an axis (rachis). More here.

Bracken frond by Olegivvit (labels added).

As in most ferns, Bracken's spores are borne on the underside of leaves in clusters called sori (remember?). In Bracken, sori are continuous along leaflet margins. In youth, the sori are covered; with maturity, leaflet margins unroll, exposing mature sporangia (spore shooters).

In Bracken, young sori are protected under rolled leaflet margins, sometimes with tiny membranous indusia (flaps, click image to view); Zharkikh photo.

In this frond, leaflet margins have unrolled and lines of brown sori are visible; Zharkikh photo.

Brown "beads" are sporangia; each contains many minuscule spores ready to be "shot"; Zharkikh photo.

My first encounter with Bracken was in the Bear Lodge Mountains in the northwest Black Hills. That was at least 40 years ago, but the memory remains vivid. In a stand of tall quaking aspen, Bracken's horizontal fronds formed a lovely lacy ground cover. It was an idyllic setting, and still comes to mind when I think about Pteridium aquilinum.

That memory prompted me to search for a bit of English poetry about Bracken (it has close ties to moorlands). But it was rarely mentioned, and never in an idyllic setting (2). Perhaps poets know of Bracken's reputation. It's not a particularly nice fern, and there are many reasons to dislike it. The horrors that follow were provided by Robbin Moran, a man who who loves ferns!

Bracken grove with conifers and ferns and little else; Charlesblack photo.

The common objection to Bracken is that it's weedy—in fact a noxious invader. With its vigorous growth and colonial habit, it swamps (with litter) or shades out other species. Eradication is difficult due to its deep extensive rhizomes.

Other dangers lie hidden. Bracken is filled with nasty stuff, ostensibly for defense against insects and other herbivores. It contains at least two kinds of insect hormones, which cause uncontrolled molting and death in any insect that eats it. It also contains thiaminase, an enzyme that breaks down vitamin B1. This makes Bracken hazardous to livestock, which often find it palatable. Overconsumption will cause thiaminase-induced staggers (treatable with vitamin B1, thiamine).

And there's more. Bracken is rich in tannins and therefore bitter-tasting, which is good. For if consumed, for example in the absence of other forage, tannins inhibit enzymes critical to cellular metabolism. Bracken also produces a deadly chemical weapon, hydrogen cyanide, in response to tearing of leaf tissue, thereby deterring or killing the perpetrator.

Gosari, a popular Korean dish of Bracken fiddleheads; Hyeon-Jeong Suk photo.

Bracken fiddleheads (young shoots, also called croziers); Phil Gayton photo.

After reading of Bracken's many hazards, I was surprised to learn that people happily eat its fiddleheads. But this is dangerous too. Though cooking removes tannins and thiaminase, carcinogens remain, and increased rates of stomach and esophageal cancers have been reported where fiddleheads are popular, for example in Japan, Korea, and Britain.

The main carcinogen is the compound ptaquiloside (3). It occurs throughout Bracken plants, but is highly concentrated in young growth, in spring and early summer. Humans take up ptaquiloside mainly by eating fiddleheads, but there are other sources—airborne spores, milk and meat from affected animals, and contaminated ground and surface water where Bracken grows (source).

Now that I know of Bracken's nefarious ways, do I feel foolish about my early love affair? No, for it also offers delights, as the Radnoshire Wildlife Trust in Wales explains: "Bracken can be an important habitat in its own right.  It supports over 40 species of invertebrate, forming an important part of the diet for 27 of these while 11 are found only on bracken. It is an important breeding habitat for moorland birds ... and reptiles and mammals benefit from its shelter." It's also a great candidate for areas in reserves and gardens where nothing else will grow. Just keep an eye on it!

Finally, Bracken is beautiful, in fact so beautiful that its lovely lacy fronds make a woodland irresistible. Even Robbin Moran agrees. "The grove seems so peaceful and idyllic" he writes, though he knows that in the shadows there lurks a femme fatale.

"Afternoon light" by Colin.

PS Last month I promised to include the fern life cycle in April's Monthly Fern post. But being overwhelmed by Bracken's dark side, I've postponed "the bugbear of botany students" until May. You're off the hook for now! But this also means you must wait for an answer to the burning question: "How many average-sized fern spores does it take to fill a can of Coke?" Stay tuned.

Notes

(1) I'm surprised Bracken hasn't been reported from eastern deciduous forests in the far eastern counties of South Dakota. Looks like good habitat to me, and it grows in Minnesota not all that far away.

(2) In the absence of suitable poetry, I wrote my own (I promise I won't do it again). I did find a poet named Bracken, as well as Bracken, a literary magazine: "Bracken is green and lush, coarse and delicate, drinks from the earth, and spreads underground, more root than frond. Bracken is understory, invades, takes over, shades and protects. We seek poetry and art that will root, tender and tough, in us."

(3) At least one of Bracken's insect hormones also is a carcinogen (source).

Sources

Moran, RC. 2004. The Natural History of Ferns. Timber Press.

Royal Horticultural Society. Advice—Bracken.

Stone, J. A successful fern, or a case for control. Radnorshire Wildlife Trust (blog).