The plant blamed for much of Kauai’s pesticide furor is a medicinal, edible shrub with a truly awful smell.

It flowered again recently, sending across the West Kauai landscape a rank stench which, in susceptible folks, causes teary eyes, headaches and nausea.

West Kauai sugar plantation workers call it spiderflower, but it is more often known locally as stinkweed. Its scientific name is Cleome gynandra.

In November 2006, students and staff at Waimea Canyon Middle School complained of a bad smell that made them nauseous and left them with throat irritation, watery eyes and dizziness. Many attributed it to agricultural spraying on a field next to the school.

The incident continues to be attributed by opponents of Hawaii’s seed industry to pesticide spraying, even though the initial field investigations by police, fire and independent botanical experts said it was stinkweed, a finding that was confirmed by a University of Hawaii study.

It flowered again within the last few weeks — the same time of the year as the initial 2006 report — and on West Kauai, people were once again complaining about the stench.

I went to West Kauai last week and collected samples in Waimea, Kekaha and Mana — some on my own and some during a driving tour with Allan Smith and Robin Robinson of Syngenta, a seed company that leases acreage in the area. We found it in agricultural fields, along ditches, and in and around numerous homes in the Kekaha area. I put my samples in plastic bags, but the stink even escaped the plastic.

Stinkweed seems to prefer the drier parts of the islands, and to do best a few weeks after a rain. It is recognized by a five-part leaf (sometimes three or seven), a long narrow seed pod and small white flowers that have long spidery parts extending beyond the petals, which accounts for why it’s sometimes called spiderweed.

One of the things that struck me about the plant is that its smell is so variable. Some specimens didn’t smell at all. Others were very strong — enough to make you gag. Some had a very intense chemical smell; others, a smell similar to the popular vegetable bitter melon (Momordica charantia).

One of the things that struck me about the plant is that its smell is so variable. Some specimens didn’t smell at all. Others were very strong — enough to make you gag.

When stinkweed was blooming in past years, Robinson said, sugar plantations would sometimes get calls about a petroleum odor, with callers suggesting there must be a fuel spill nearby.

“I can smell the sulfur in it,” said one person I asked to smell the plant sample in one plastic bag. Another said she smelled a “spicy body odor.”

This variability is confirmed on the website for Plant Resources of Tropical Africa, which says people who grow it commercially have difficulty getting consistent results as they replant the seeds, because of variability in many features of the plant.

The variability may help explain why some of the affected individuals at Waimea Canyon Middle School, who said they were familiar with stinkweed elsewhere, insisted that it’s not what they smelled at Waimea.

And there’s another kind of variability. Some people in the same place will describe different smells. Some are impacted by the stinkweed chemicals; others not at all.

“I started to get a headache. It was a nauseous headache, and I can still feel it remembering it. It didn’t come on suddenly, but I started to get the feeling something was wrong. Within an hour, I had a very bad headache and was still nauseous, clammy feeling, dizzy,” said a Waimea resident I interviewed, who asked not to be named.

The Downside of Being Downwind

The flowering alone can release a strong odor, but if the plant is rubbed or crushed, or if a field of stinkweed is plowed, the odor can be overpowering. One of the reported incidents at Waimea Canyon Middle School occurred while the stinkweed-infested field was being tilled, Robinson said.

The state Department of Agriculture issued a report earlier this year on two Waimea Canyon Middle School incidents and other school evacuations. In the Waimea cases, the department concluded stinkweed was the culprit in both.

In the first case on Nov. 14, 2006, the report noted that there had been spraying nearby with Roundup before the incident, but tests found no Roundup residue in the area. An official of the National Tropical Botanical Garden, as well as fire and police officials, “agreed that the odor was from blooming stinkweed.”

In the second case, on Jan. 25, 2008, there had been no pesticide use. Some students reported they detected a diesel smell. Syngenta confirmed it had plowed under a field of stinkweed.

The state commissioned a study by Qing X. Li, Jun Wang and Robert Boesch, of the University of Hawaii’s Department of Molecular Biosciences and Bioengineering. Their findings, issued in 2013, concluded that spiderflower/stinkweed is a powerful chemical factory.

“A closed chamber laboratory air emission study identified 29 chemicals (some with insecticidal activities) produced by stinkweed,” they said in their report. When they compared that with air samples in the field, they found all 29 chemicals in the air. When they tested the air at the school — indoors and outdoors — they found about half of those chemicals.

The chemical that the scientists felt was likely most at fault for people’s reactions is a natural, plant-based compound called methyl isothiocyanate, an organosulfur compound. Their studies showed it is significantly higher in concentration in flowers and seed pods than leaves and stems of the stinkweed.

“MITC is a highly foul-smelling, noxious chemical at high concentrations, and is cited as a potent lachrymator (eye irritant) and nose and throat irritant. Besides MITC, other stinkweed derived compounds found during this study are also known potential irritants,” they wrote.

The studies also found residues of five known pesticides, including a couple of legacy pesticides that haven’t been used for years. When they did the air quality testing, they found that “Concentrations of the pesticides and MITC were well below health concern exposure limits or applicable screening levels.”

The stinkweed plant has been in the Hawaiian Islands for more than 150 years, having first been observed in 1857. It is on all the main islands, sometimes known as honohina,iliohu, wild spiderflower or spiderwisp. It is native to central America, from Texas south to Argentina, according to the Manual of the Flowering Plants of Hawaii.

It has spread around the world, a weed in some areas and a cultivated plant in others. The leaves are eaten in Africa and parts of Asia, although it must be cooked. One Thailand website says that cooking, drying or fermentation reduces the levels of hydrocyanic acid, which it says is toxic to the central nervous system.

The website “Plants for a Future” cites some of the medicinal uses of Cleome gynandra: It says juice from the plant can be used to treat scorpion stings, a tea made from the root can treat fever and that leaves can be used to irritate the skin and reduce pain in nearby areas.

The plant is attractive to some insects, repellent to others. In some areas it is used as a mosquito and tick repellent, but when I viewed it on West Kauai, many species were severely infested with thrips, and we found an egg mass from stinkbug on the underside of the leaf of one plant.

(Notably, the stink of a stinkbug is very different from the stink of the stinkweed.)

Cleome gynandra, stinkweed or spiderflower — by any name, it smells bad. It has been an issue for Hawaii residents for decades. And it still is.