Sniffing Around the Herbarium: How Plant Scent is (and isn't) captured in Dried Specimens

When a corpse flower blooms, it can be smelled for up to a half mile away. But what about when that bloom is dried and attached to a herbarium paper as a scientific specimen? Do herbarium specimens smell? And what do herbarium labels tell us about plant scent?

The answers depend on the plant, and the scientist creating the herbarium label.

Why do some herbarium specimens smell?

A dried specimen of Izzy, the Missouri Botanical Garden’s third corpse flower bloom. Live corpse flowers produce an odor that mimics rotting meat to attract flies and carrion beetles. Once its bloom is complete, the smell fades, meaning dried herbarium specimens are odorless. Photo courtesy of Tropicos.

Plants use scent to talk with other living organisms. To attract pollinators, plants produce volatile organic compounds (VOCs), a mixture of different chemicals that disperse through the air. Plants also produce scents to deter pests. Think of herbs you’ve been told to plant in your garden to keep certain critters away.

So which of those smells translate to herbarium specimens?

An herbarium is a library of dried plant specimens. The Missouri Botanical Garden is home to one of the largest herbaria in the world, with nearly 8 million specimens. Learn more >

When scientists make herbarium specimens, they press a plant flat, dry it, and glue it to a special paper with a label. Scientists find that in those dried specimens, VOCs persist in some plant families and genera more than others.

Floral odors, for example, don’t persist because they are temporary scents plants produce for pollinators only when it’s time for pollination. This is also true for plants like the corpse flower, which produce unpleasant odors to attract pollinators like flies and beetles.

Which specimens still smell?

A specimen of oregano, *Origanum vulgar*e, from the Missouri Botanical Garden’s Herbarium. Oregano and other members of the mint family, Lamiaceae, hold smells for years after dried. Photo courtesy of Tropicos.

So what dried specimens do maintain an odor? The answer is as close as your kitchen. Think of the dried spices you use for cooking like oregano, cumin, coriander, and anise. Oregano is in the mint family, Lamiaceae, which comprises roughly 7,000 species. The latter three are in the Apiaceae, which comprises 3,8800 species. Many of the species we use as seasoning in these families hold their odor for years.

The drying process can also bring out unpleasant odors. Penstemon (Plantaginaceae) is a genus of about 300 species commonly called beardtongues. Some species produce no odor while alive, while others have a nice floral smell. The Missouri native species, unfortunately, smell like dog poop as its fruits mature in the summer.

“Dried plants also produce some of this odor, but it is more musty and unpleasant when preserved for the herbarium,” said Garden Scientist Aaron Floden.

Capturing Scent in Herbarium Labels

Herbarium labels noting scent “often mention unpleasant smells, like the specimen on the left (“HORRIBLE”), as well as pleasant smells like the specimen on the right (“a very sweet smell like jasmine or vanilla”). Images courtesy of Tropicos.

Although many dried plants are odorless, herbarium specimens can still capture scent through labels. Herbarium labels contain important collection data such as date, collector, location, and habitat. They can also include additional details about what the living specimen looks like or, sometimes, what it smelled like.

A team from the Garden’s Plants and People division took a first look at how botanists records scent on herbarium labels by searching for the word “smell” in Tropicos, the Garden’s online botanical database.

Ethnobotany looks at the relationships between humans, plants, and their environment, the conservation of plant species, and the preservation of traditional knowledge.

Of about 5,000 specimens with labels containing the word “smell,” “sweet” was the most common descriptor matching more than 800 labels. But scientists note smells they dislike as well. “Unpleasant” and “foul” were also common adjectives, matching about 300 specimens.

When collectors get specific about scent, they often refer to other plants — particularly familiar food plants like garlic, mint and lemon. Individual collectors can sometimes elaborate descriptions, particularly for smells that are powerful, “an irritating mixture of skunk, horseradish, cyanide, garlic and burning rubber.” Or even evoke a particular memory, like “a wonderful aromatic smell, like sweet fern in Wisconsin.”

Among the hundreds of species with scents indicated on the label, the Plants and People team noted many relatives of pungent plants that are familiar kitchen staples. This included plants in the groups Piper (Piperaceae), which includes black pepper and its relatives, and Zanthoxylum (Rutaceae), which includes Sichuan pepper and isanshō. Specimens of plants in the genus Solanum (Solanaceae), which includes tomato and its relatives, also often mentioned smells. This won’t surprise gardeners who have encountered the pungent smell of a tomato leaf or stem.

Why do scientists include mention of scent?

Some scientists include specific details about smell like “burnt biscuits and petrol oil” (left specimen) or leaves that smell like “avocado and orange” (right specimen.) Images courtesy of Tropicos.

Only a small percentage of the Garden’s expansive herbarium collection mention smell. So why do some include it while many do not?

One reason may be simply some scientists have a stronger sense of smell than others.

“When I taught field botany I always asked students to smell flowers or leaf odors, but some students detected nothing while others noted some scent, and just a few noted distinctive odors between species,” Floden said.

Floden, who does have a sensitive nose, said he in particular notes scent when it’s not typical for a species. For instance, slender mountain mint, Pycnanthemum tenuifolium, does not usually produce an odor. Some populations growing in the Ozarks, however, smell like peppermint.

Another reason may be training: if a botanist’s mentor is keen to note smell in specimen labels, mentees may follow the same path.

Expanding Ethnobotany through RSI

Some herbarium labels include information about human uses of plants related to scent, like this label noting the Zapotec use of this jasmine-scented flower to deter flies. Images courtesy of Tropicos.

Although scent be subjective, information about a plant’s scent can be extremely useful to scientists trying to understand all they can about a specific species. Scent information in herbarium labels can also provide clues about humans use specific plant species.

Soon, information about millions more plant specimens will be available online thanks to the Revolutionizing Species Identification initiative, or RSI. Funded by an anonymous $14.4 million grant, RSI will digitize 6 million specimens over six years, making critical data freely accessible to scientists, conservationists, and policymakers globally. Experts can use the rapidly digitized herbarium specimens to answer new questions about taxonomy, biogeography, evolution, ecology, ethnobotany, etc.

The Plants and People team will continue to look at this topic as more specimens come online. With these specimens newly available online, there are plenty of surprises to be found, including undescribed species or new information on the way a rare flower smelled when collected. They will also expand their search to include synonymic phrases like “aroma,” as well as other languages.

Where can I learn more about plant scents?

Visitors smell different plant scents at “Smelling the Bouquet” at the Garden’s Stephen and Peter Sachs Museum. Photo by Mary Lou Olson.

“Smelling the Bouquet: Plants & Scents in the Garden” explores the spectrum of scents plants create, inspired by the diverse live and scientific collections at the Garden. The highlight of the exhibition is the opportunity to sniff over two dozen scents, including some considered unpleasant.

The Stephen and Peter Sachs Museum is open from 11:30 a.m. to 4:30 p.m. daily and included with Garden admission.

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