A botanist’s dream: Herbarium houses 300,000 specimens

Room 1026 in the Katherine Esau Sciences Laboratory Building is not your ordinary space. It’s home to the UC Davis Center for Plant Diversity Herbarium and a world of 300,000 preserved plants, including lichens, algae, mosses and other specimens.

The facility features plant presses, a botany library, an extensive reference collection, a climate-controlled storage area and a "dirty room" for new specimens. Over the past year, staff have welcomed more than 1,000 visitors hosting tours, events, researchers, teachers and students. 

“Herbaria are a collection of pressed, dried plant specimens,” said Dan Potter, the Herbarium’s director and chair of the Department of Plant Sciences. “They’re collected from all over the world and used for identification. It’s really a valuable resource.”

Herbaria date back to the 1700s and originally served as a resource for doctors and others to learn about plants used to make herbal treatments. The roots of the UC Davis herbarium date to 1922 and its University Farm days, when botanist and author W.W. Robbins created a personal reference collection. 

It has grown over time as agronomy, vegetable crops and viticulture collections merged. It is now the fourth-largest herbarium in California, serving students, researchers, community members and even veterinarians seeking answers about all things botanical.

Some of the collection’s specimens, like a maidenhair fern, date to the 1870s. “In the age of extinction, a fair number of species are only in herbaria now, where they can last for hundreds of years,” Herbarium Curator Alison Colwell said. 

Old and new school

Herbarium staff, students and volunteers rely on a mix of old and new technology to preserve plants for current and future generations. Specimens are dried, pressed flat and secured on sheets of watercolor paper in a process dating back to the 1500s. 

They are labeled with the species name, location of discovery, the person who found or donated the specimen and other information. Some pages feature small packets that hold plant pieces for DNA tests, while a barcode can be scanned for more information. 

“The specimens are made to show the kind of traits taxonomists use to tell plants apart,” Colwell said. “There’s truly quality information in these specimens.”

Delicate petals, stems and leaves are arranged neatly on pages, like silhouettes captured by an artist. Researchers can use these to measure petal length, width of leaves and types of hairs. 

“I enjoy mounting these beautiful plants,” said Zihan Wang, a sophomore student volunteer, who was cataloging specimens recently. “It’s like an art for me.”

 Wang, who is majoring in neurobiology, physiology and behavior, found her way to the herbarium when she was thinking about changing her course of study to plant biology. As part of her shift, she delicately wiped insects and other debris off the specimen sheets before placing them in a lightbox to take high-resolution photos that would be scanned, digitized and uploaded to the Consortium of California Herbaria database. 

Images of about 90,000 specimens are available digitally and more are added each week.  

“We do a lot of imaging of our specimens because that is what gets them out into the world,” Colwell said.

Wang is one of 15 to 30 interns, work-study and volunteer students who work at the Herbarium each year, Potter said. 

During COVID-19 closures, staff continued to work even while the facility was not open to the public. For students who had to attend classes remotely, Potter devised a way to hook up a microscope to a web camera so he could teach plant identification online. “We used a lot of images of herbarium specimens,” he said.

Cold storage and preservation 

Specimens are stored long-term in the collections room, a warehouse-sized space filled with rows of large metal cabinets that can be rolled back and forth to access items. The room is kept below 60 degrees to keep insects that may get inside from endangering the specimens.

“We have a case of fleeces and sweaters to lend to visitors,” Colwell said. “Most herbaria don’t have the luxury of climate control. We are very lucky in that case.”

A separate “dirty room” is equipped with a drying oven and freezer where samples are placed to kill pests. “This is how we keep larvae from getting into the cases,” Colwell said. “If any specimen stays out overnight, it goes back in the freezer for two weeks.”  

A resource near and far

The collection houses 150,000 specimens from California, 75,000 from North America and thousands more from around the globe. It garners attention worldwide.

Entomologists visit for ecological research. “When they look at this, they see things a botanist does not,” Colwell said, pointing to a mounted plant. “They see stippling and insect activity.”

Mohsen Mesgaran, an assistant professor in plant sciences, consulted the herbaria as part of his research on invasive plants and dormancy periods worldwide. 

“Herbarium records have been incredibly useful for researchers trying to answer various ecological and evolutionary questions,” Mesgaran said. “These records are essentially detailed archives that tell us where and when different plant species have been found, which is a key piece of information for studying nature.”

Herbarium students recently collected and preserved seeds of 30 species of rare plants for the California Plant Rescue project. A group from France examined and took samples of grapevine leaves from the early 1900s for clues to the origins of Pierce’s disease. And, crowdfunding is supporting an effort to digitize and “virtually repatriate” records of African specimens in partnership with African botanists.  

Colwell fields queries from students doing research and veterinarians interested in plants that can poison livestock and pets. She also gives tours, pointing out how to preserve cacti (slice and section), tips to preserve succulents (a few seconds in the microwave helps) and spotting signs of insect damage (holes and powder).

“There’s tricks of the trade, as it were,” she said.