by Amy Stewart
Most nettles are low-growing plants that superficially resemble herbs like mint or basil, with toothed edges. The Australian stinging tree, widely considered the world’s most painful plant, is a member of the nettle family, but the best-known member of this family is the stinging nettle, Urtica dioica. The hairs are so fine that people unfamiliar with the plant might not even notice them. In addition to the stinging hairs, nettles can also be identified by the small clusters of flowers that emerge from the joint where the leaf connects to the stem. However, the best advice for avoiding the nettle family is to resist the temptation to stroke an unfamiliar fuzzy or hairy leaf.
SPURGE FAMILY
Euphorbiaceae
The highly irritating, milky sap produced by most members of the spurge family sets it apart. Gardeners may recognize the more common euphorbias that are popular in Mediterranean gardens, but other members of this family are not as obviously related: poinsettia, pencil cactus, Texas bull nettle, castor bean, rubber tree, sandbox tree, mala mujer, milky mangrove, and manchineel are all spurges. Many of these can burn and scar the skin, but some, like castor bean, also contain powerful poisons that can kill if ingested. At the very least, plants that produce a milky sap should be handled with care, as they may burn skin and eyes. Some spurges can be identified by their colorful bracts; for example, consider the flowers of euphorbias or poinsettias.
CARROT OR PARSLEY FAMILY
Apiaceae
This family conceals some notorious criminals among its otherwise healthy and beautiful members. Carrots, dill, fennel, parsley, anise, lovage, chervil, parsnips, caraway, coriander, angelica, and celery are all plants that a good chef couldn’t live without, but even they require some caution: many, including celery, dill, parsley, and parsnips are phototoxic, meaning that skin contact, combined with sun exposure, can cause a rash. One garden flower, bishop’s weed (Ammi majus), is so phototoxic that exposure to the seeds can permanently darken skin.
But the real danger is posed by relatives like water hemlock, poison hemlock, giant hogweed, and cow parsnip. These wild plants contain neurotoxins and skin irritants, but they so closely resemble their edible cousins that tragic mistakes have been made by hikers and cooks.
Identifying plants in the carrot family is fairly easy. Queen Anne’s lace is a typical example; like most members of the family, it produces fine, lacy foliage and flat-topped clusters of flowers called umbels, as well as a carrot-shaped root.
ILLEGAL
Khat
CATHA EDULIS
Khat played a small but pivotal role in the 1993 battle of Mogadishu in which two American Black Hawk helicopters were shot down. Gun-toting Somalian men stuffed khat leaves into their cheeks and raced around Mogadishu with a jittery high that lasted until late into the night, contributing to the violence and the deaths of American soldiers trapped at the crash site.
FAMILY:
Celastraceae
HABITAT:
Tropical elevations above three thousand feet
NATIVE TO:
Africa
COMMON NAMES:
Qat, kat, chat, Abyssinian tea, miraa, jaad
Author Mark Bowden found an interesting route into Somalia when he was researching his book Black Hawk Down: he flew on a khat plane. Because the leaves must be consumed fresh, Bowden had to pay for the amount of khat he was displacing that day. “What they did was offload two hundred pounds of khat so I could sit on the plane,” he said in an interview. “I paid for myself as if I were khat to get into the country.”
The leaves deliver a clear-headed euphoria that lasts for hours. In Yemen and Somalia up to three-quarters of adult men use the drug, stuffing a few leaves between their cheek and gum, in much the same way that coca is used in Latin America. And like coca, the khat plant has fueled wars between those who claim it is a benign cultural ritual that has been practiced for centuries and those who see it as a public health menace.
When a khat plane lands in Somalia, its cargo is unloaded and distributed in a matter of hours. Men lounge about in a blissed-out state, chewing their khat, tending to neither their families nor their jobs. Long-term use leads to aggression, delusions, paranoia, and psychosis. But the typical khat user is not deterred by these alarming symptoms. As one man put it, “When I chew it, I feel like my problems disappear. Khat is my brother. It takes care of all things.” Another man said, “You open up like a flower when you chew.”
Gun-toting Somalian men stuffed khat leaves into their cheeks and raced around Mogadishu with a jittery high that lasted until late into the night.
Catha edulis is a flowering shrub that flourishes in Ethiopia and Kenya, where it enjoys full sun and warm temperatures. The dark, glossy leaves emerge from red stalks, and young leaves may also be fringed in red. The plant reaches twenty feet or more in the wild, but only five or six feet in cultivation.
Its most potent ingredient, cathinone, is classifed in the United States as a Schedule I narcotic, putting it on the same footing as marijuana and peyote. The level of cathinone in the leaves drops sharply just forty-eight hours after harvest, a fact that turns drug smuggling into a wild race. Once the cathinone breaks down, all that is left is cathine, a very mild chemical similar to the diet pill ephedrine. For this reason, police have to move fast to get the plant to a drug lab. After forty-eight hours a major drug bust will become a diet pill raid.
Khat dealers in Seattle, Vancouver, and New York have been busted for selling bundles of leaves under the counter in small grocery stores that cater to Somalian immigrants. In 2006 Somalia’s Islamic movement outlawed the plant in the areas it controlled and stopped all flights arriving from Kenya in an attempt to crush the use of the plant. It remains to be seen whether Somalians will give up their drug, which has been called the opium of the people.
Meet the Relatives Khat is related to about thirteen hundred species of tropical and temperate vines and shrubs, including the highly poisonous staff vine, and the equally poisonous spindle shrubs known as Euonymus.
DESTRUCTIVE
Killer Algae
CAULERPA TAXIFOLIA
In 1980 staff working at a zoo in Stuttgart, Germany, noticed an impressive strain of the tropical seaweed Caulerpa taxifolia in one of their aquariums. Usually it couldn’t handle the colder temperatures that Mediterranean fish require, but this particular specimen was lush, green, and hardy in the cold aquarium. What made it so different? Scientists believe that constant exposure to chemicals and ultraviolet light in the aquarium triggered a genetic mutation that made it particularly tough.
FAMILY:
Caulerpaceae
HABITAT:
Killer algae thrives in the Mediterranean, along California’s Pacific coast, in the oceans off tropical and subtropical Australia, and in saltwater aquariums worldwide.
NATIVE TO:
Originally discovered along the French coast, this algae is native to the Caribbean, east Africa, northern India, and elsewhere.
COMMON NAMES:
Caulerpa, Mediterranean clone
Word got around, and soon the staff at several aquariums wanted to try the plant in their exhibits. Someone brought it to Jacques Cousteau’s Oceanographic Museum in Monaco, where it escaped into the wild with just a little extra help. According to one report, in 1984 an employee cleaning the tanks tossed the leftover waste into the sea.
French biology professor Alexandre Meinesz first saw a patch of the algae growing in the Mediterranean Sea near the museum in 1989. He was surprised to see a tropical seaweed growing so vigorously in the cold water, and he warned his colleagues that the plant could become invasive.
The entire plant—its feathery fronds, sturdy stems, and tough rhizoids that anchor it to the ocean floor—is all just one giant cell that can span over two feet in length and grow about half an inch per day.
This set off a decade-long debate over the origin of the plant, the likelihood that it might become invasive, and the responsibility for combating an invasion if it ha
ppened. As committees were being formed and papers were being written, the algae made its way to sixty-eight sites around the world, covering twelve thousand acres of the ocean floor. Today, a lush, green carpet of C. taxifolia spans over thirty two thousand acres of oceans around the world, or about fifty square miles.
This is truly remarkable considering the fact that killer algae is a single-celled organism. The entire plant, including its feathery fronds, sturdy stems, and tough rhizoids that anchor it to the ocean floor, are all just one giant cell that can span over two feet in length and grow about half an inch per day. This makes it one of the world’s largest—and most dangerous—single-celled organisms.
Killer algae don’t kill human beings. The plant gets its nickname from a toxin called caulerpenin that poisons fish. This keeps marine life from nibbling on the plant, which is part of the reason it has spread unchecked in oceans around the world. The lush, green vegetation forms meadows ten feet deep on the ocean floor, choking out all other aquatic life. Fish populations are dying out, and waterways have become clogged with the plant.
This mutant aquarium strain of C. taxifolia is exclusively male, suggesting that the entire invasive population around the world stems from just one parent plant. It reproduces only through propagation: a chunk breaks off, gets chopped up in the undercarriage of boats, and then spreads throughout the ocean. The caulerpenin toxin forms a gel that heals the wound within an hour, allowing that fragment to grow and establish a meadow of its own.
Killer algae is classified as a noxious weed in the United States, which means that it cannot be imported into the country or shipped across state lines. It’s considered one of the world’s hundred worst invaders by the Invasive Species Specialist Group. Attempts to eradicate it haven’t met with much success, because chopping up the plant only helps it reproduce. One of the few success stories comes from San Diego, where an eleven-thousand-square-foot patch was destroyed by placing a tarp over it and pumping chlorine into it. Authorities haven’t claimed victory just yet: even a one-millimeter chunk of killer algae floating in the ocean could take root and spread again.
Meet the Relatives The edible sea lettuce (Ulva lactuca) and other small, green seaweeds are related to the menacing killer algae.
DANGEROUS
STOP AND SMELL THE RAGWEED
A poisonous seed will only kill you if you chew it and swallow. A painful rash can only spread if you brush up against the leaves. But some plants have figured out how to extend their reach by releasing highly irritating allergens into the air.
There’s a reason why seasonal allergies seem to get worse every year. Gardeners and landscapers, in an attempt to be tidy, prefer to plant male trees and shrubs. The females drop fruit, leaving a mess all over the sidewalk or the lawn. But a male tree produces only small, well-behaved flowers—that is, if your definition of well behaved includes spewing plant sperm into the air for weeks on end.
In the 1950s and 1960s diseased American elm trees were replaced with male varieties of wind-pollinated trees. As a result, some cities, particularly in the Southeast, are virtually uninhabitable for people with serious allergies and asthma.
Homeowners are surprisingly reluctant to remove these trees. One allergen expert remembers a family with a huge male mulberry tree in their garden. After blasting the tree with a hose in a misguided attempt to wash off the pollen, both the husband and wife felt their throats close and had to lock themselves in the bathroom all night just to be able to breathe. The pollen had germinated in water, releasing even more allergens than before.
Consider banishing these plants from the yard:
RAGWEED
Ambrosia spp.
A versatile weed that flourishes throughout the United States and across Europe. A single plant can produce a billion grains of pollen during a season. The pollen remains airborne for days and can travel several miles, affecting some 75 percent of allergy sufferers and creating cross-allergies with foods that have similar proteins, including cantaloupe, banana, and watermelon. Ragweed releases more pollen when carbon dioxide levels are higher, so global warming will only make the situation worse.
YEW PINE
Podocarpus macrophyllus
A shrub or small tree popular as a street tree or as a foundation plant in landscapes, this plant is a heavy pollen producer, and the fact that it is often planted right under windows in suburban landscapes means that allergy sufferers may wake up with a sore throat that will only get sicker if they spend the day in bed.
PEPPER TREE
Schinus molle or S. terebinthiefolius
A controversial landscaping tree that can be invasive and cause a nasty skin rash. The berries are poisonous if eaten. The male trees send copious amounts of pollen into the air over a long blooming season. Because it is related to poison ivy and other members of the toxicodendron genus, people who are especially sensitive to those plants will also suffer around pepper trees. It produces an oil that can vaporize into thin air, causing people to develop asthma, eye inflammation, and other reactions just from being nearby.
OLIVE TREE
Olea europaea
Olive pollen is so highly irritating, owing to the number of different allergens it contains, that some cities are trying to banish the tree entirely. The city of Tucson, Arizona, has passed an ordinance banning the sale or planting of olive trees.
MULBERRY
Morus spp.
One of the most potent sources of spring allergies, this plant sheds billions of pollen grains that linger on patios and get tracked indoors.
HIMALAYAN CEDAR
Cedrus deodara
A fast-growing cedar reaching up to eighty feet tall and forty feet wide, found in gardens and parks throughout mild winter areas in North America and Europe. The small, male cones shed pollen in the fall. Many seasonal allergy sufferers are sensitive to cedar, making this an unbearable tree to be around.
BOTTLEBRUSH
Callistemon spp.
A popular, showy shrub in North America, Europe, and Australia. The long, bristlelike red stamens release golden pollen from the tips. The pollen is triangular in shape and lodges in the sinuses, making it a particularly vicious allergen.
JUNIPER
Juniperus spp.
This evergreen is a serious but overlooked source of allergens. The males produce cones, along with large quantities of pollen. Some junipers have both male and female organs on one plant (monoecious), which means that they might produce some berries but will also shed pollen.
BERMUDA GRASS
Cynodon dactylon
One of the most popular grasses for lawns in the South and warm-weather climates throughout the world, it is also the most allergenic. It blooms steadily, and the flowers often grow so low that lawn mowers miss them. New varieties don’t produce any pollen at all, but older varieties are so problematic that some cities in the Southwest have banned them.
DESTRUCTIVE
Kudzu
PUERARIA LOBATA
Kudzu to the rescue!” So proclaimed a 1937 Washington Post article about the powers of this exotic vine to control erosion. And indeed, for almost a hundred years the vine enjoyed the enthusiastic support of American gardeners and farmers.
FAMILY:
Fabaceae
HABITAT:
Warm, humid climates
NATIVE TO:
China; introduced to Japan in the 1700s
COMMON NAMES:
Mile-a-minute vine, the vine that ate the South. To the Japanese, the word kudzu means “rubbish,” “waste,” or “useless scraps.”
The Centennial Exposition, held in 1876 in Philadelphia, was a carnival of wonders. Roughly ten million Americans were introduced to the telephone, the typewriter, and a miraculous new plant from Japan: kudzu. Plant enthusiasts loved the flowers’ fruity, grapelike fragrance and the fact that the vine could scramble over a trellis so quickly
Soon farmers realized that livestock would eat the vine, making it a useful forage cro
p. Kudzu gripped the soil and stopped erosion. A government program encouraging the use of the vine gave kudzu all the encouragement it needed.
Kudzu had other plans for the South. The vine made itself at home, growing up to a foot per day during the warm, humid summers. This plant is born to run: Over two dozen stems emerge from a single crown, and each of those vines can stretch to one hundred feet. A single massive tap root can weigh up to four hundred pounds. Each individual leaf can twist and turn so that it receives the maximum amount of sunlight, making the vine particularly efficient at harnessing the sun’s energy and keeping rays from reaching the plants below it.
Kudzu shrugs off cold weather and spreads by underground rhizomes and seeds, which can survive for several years before sprouting. It strangles trees, smothers meadows, undermines buildings, and pulls down power lines. Southerners say they sleep with the windows closed to keep it from sneaking into the bedroom at night.
The vine covers seven million acres in the United States. The damage it has caused is estimated in the hundreds of millions. At the Fort Pickett military base in Virginia, kudzu overwhelmed two hundred acres of training land. Even M1 Abrams battle tanks couldn’t penetrate the rampant growth.