If you visit the north side of the park on a warm, calm afternoon these days you may catch Fennel Fever. You’ll be walking past thick stands of a lacy green forest taller than your head, and suddenly your lungs fill with a heady perfume. It’s like licorice, or anise, but more penetrating. Your sinuses clear, as if you were inhaling a sweet menthol, and then it reaches your brain. The powerful scent triggers a chain reaction of neurochemicals: organic molecules that sharpen your senses, tickle your erotic zones, banish depression and other negative mentalities, and flood you with good feelings. You’ve caught Fennel Fever, febricitantem faeniculi.
The fennel plant is native to the Mediterranean. It played a role in the ancient civilizations of that cradle of Western culture. Classical Greek legend prized it as the vessel by which the god Olympus conveyed fire to mortals. The herb grew profusely in a region of the Peloponnesus where the Olympic games were held, and the fragrant plant lent its ancient Greek name, marathon (μάραθον), to the long-distance foot race from which the athletic contest sprang. So, if you run a marathon, you’re running fennel.
Because it’s not native to California — it’s been here “only” about 125 years, probably as an escapee from imported garden plants — it’s attracted the ire of botanical nativists. However, the plant has proved highly resilient and has outlasted several generations of fennel haters. The CalIPC website (IPC = Invasive Plant Council) says that fennel will invade areas where the soil has been disturbed and can outcompete and replace native plant species. It does this in part, say botanists, by “exuding allelopathic substances that inhibit growth of other plants.” I’ll get back to those allelopathic substances a bit later.
Note for now that the IPC experts warn that “In areas where fennel stands are already well established,management will require a long-term commitment of time and resources.” This is because fennel tap roots go deep, and chopping, burning, or chemically defoliating the plant above ground will merely clear the air for a more vigorous regrowth the following season. Even where fennel control is successful, says the IPC, the result is almost always an invasion by other non-native species.
We’ve seen a small sample of this truth here in the park, where the management power-mowed an eight foot wide strip of fennel in the summer and fall of 2016, only to foster a bumper crop of the nastiest invasive thistles the following year. Fortunately, this year the fennel has come back and reclaimed the strip. The IPC experts conclude, “It is probably impossible to completely eradicate fennel from wildlands …” Given the park’s limited resources, a serious and successful multiyear effort to replace the fennel with a native plant (and which native plant would that be?) is not in the cards. Hopefully, parks management has learned to leave the fennel alone. As the Beatles song says, Let it Be!
Although the plant may be an undocumented immigrant, in the park it forms an essential home for several native avian species. The Red-winged Blackbird, in particular, comes to the fennel forest in the northwest quarter of the park every spring to mate, breed, build nests, and raise its young. Their arrival seems to be timed, to a degree, to the maturity of the fennel. This year, when the fennel on the north side for some reason was slow to rise and become dense, the blackbirds sent several scouting expeditions early in the year, but withdrew after a short visit. The mass of the flock arrived later than usual, and stayed for several weeks after the Summer Solstice, contrary to their usual habit of leaving within a day or two of the astronomical event.
The blackbirds build their nests deep in the density of the fennel forest, using old and fresh fennel leaves as nesting materials. The mothers feed their young with insects that feed on fennel. The birds’ droppings fertilize the soil. Thus the birds repay the fennel for its hospitality.
When the blackbirds depart, the fennel forest teems with sparrows, finches, warblers, towhees and others. They eat the fennel flowers fresh from the crowns, and scavenge for insects. Once the fennel flowers mature into seeds, in late summer and fall, the forest becomes a cornucopia of inestimable bounty for feathered seed eaters right through the winter. Without the fennel, many of these bird species could not live here. There are so many seeds that you and I can safely harvest a few fennel seeds for our kitchens without impacting the birds’ well-being.
Now, about those allelopathic substances. A scholarly article by F. and Z. Cheng in the journal Frontiers in Plant Science points out what observant gardeners have known since ancient times: plants have an influence on their neighbors. This influence may be positive, by promoting pollination and soil fertility or inhibiting insects, fungi, molds, weeds, and other harmful influences on nearby growth, or it may be negative, by doing the opposite. So, for example, gardeners plant marigolds next to vegetables for their protective effect against harmful bugs. The substances by which a plant influences its neighbors are called allelochemicals. Scientific notice of this phenomenon dates to the 1930s but in-depth study is more recent, dating from the mid 1990s, when chemical detection and identification technology matured. The article gives an impressive list of some known allelochemicals. They include highly complex organic acids, alcohols, aldehydes, ketones, lactones, fatty acids and many others. The Chengs note that both donors and receivers of allelochemicals may include animals.
A 2011 study of the effect of the chemicals contained in fennel seeds showed a powerful weed-killer effect. The seeds of four plants (wild barley, perennial ryegrass, wild oat, and dandelion) failed to germinate, or germinated poorly, when treated with an extract of fennel seed. The authors cite related studies tending to show that fennel is among the top ten medicinal plants with the highest inhibitory effect against the germination of a number of common weeds, as well as of lettuce.
Another study in the same year tested the inhibitory effect of fennel extract on the germination of 34 different plants found on Santa Cruz Island in Channel Islands National Park. It found, supporting the IPC’s observation quoted above, that fennel significantly inhibited the germination of native species, but the introduced species found on the island were immune.
Fennel is not alone in its ability to inhibit germination of some other plants. Many others have similar capabilities. There is now a growing literature exploring the allelopathic relations between plants. The hope that drives this research is to replace conventional manufactured chemical pesticides, herbicides, and other agricultural chemicals with crops whose allelopathic action provides the same benefits without the environmental toxicity. One day, perhaps sooner than later, fennel may become an important industrial crop supplying natural herbicides.
I’ve written earlier about fennel’s culinary and traditional uses in folk medicine. Google “fennel plant” and you’ll come up with dozens and dozens of websites about these topics. What I can add here is a link to a substantial scholarly literature about the medicinal uses, including both the traditional wisdom and the modern biochemical and pharmacological assays. A good introductory essay is the 2016 study by Rather et al. in the Arabian Journal of Chemistry. It validates many of the traditional medicinal beliefs on the basis of modern analysis of fennel chemistry. Worth reading.
So, as I walk past the lush stands of mature fennel on the north side of the park, I am impressed by the beauty of the plant and of the forest it creates. Its stalks have the elegance of bamboo. Its tender fronds describe the most delicate lace. The geometry of its floral crowns recalls an explosion of fireworks. Its flowers display among the richest flavors of gold in the floral world. And then there is the perfume. I now know that this powdery essence is probably allelopathic. I also know, from the Chengs’ article, that allelopathic substances can influence animals. I am an animal. What exotic neurochemicals are in that floral incense? Whatever they are, they give me Fennel Fever, an illness of which I don’t want to be cured.