PPeter Raven, president emeritus of the Missouri Botanical Garden, has led efforts to track biodiversity. (Missouri Botanical Garden /Missouri Botanical Garden )

Peter Raven might not be a household name, but he is a giant among botanists. The U.S. National Arboretum sought to raise his profile recently by honoring him with its Medal of Excellence, of which he is only the third recipient.

He co-wrote a standard and enduring textbook called “The Biology of Plants” and as the 39-year president of the Missouri Botanical Garden drove major research efforts into the plant biodiversity of the Americas and China.

Taking stock of the entire known plant life of a region, its flora, is methodical and painstaking work, but Raven and his scientific collaborators realize that it is the foundation of any conservation effort. You have to know precisely what you have, and what you are losing, before you can seek to save it. Raven “has made enduring contributions to the planet and its inhabitants,” said Colien Hefferan, the arboretum’s director.

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Among Raven’s projects: “Flora of China,” “Flora of Bolivia,” “Flora of Panama” and more.

“In terms of cataloguing the biodiversity of the New World, he’s a massive influence for the better,” said Kevin Nixon, a professor of plant biology at Cornell University.

Raven orchestrated these scientific projects from the helm of the Missouri Botanical Garden, but he is also credited with reviving the fortunes of the St. Louis institution — and, by example, others around the world — through his emphasis on ecological research.

In an age of habitat destruction and climate change, this long and steady work of documentation has become vitally important and prescient.

Raven, now 78 and president emeritus of the botanical garden, has also been a driving force behind “Flora of North America ,” a monumental effort to document and describe the wild plants of the United States, Canada and Greenland. Sixteen of the 30 volumes have been completed.

What, other than the remaining volumes, is still undone? The 22-volume “Flora of China” might give us a whole survey of that vast country’s plant species, but we have no corresponding sense of what we all grow in our gardens.

Raven told me that there is a clear need for a flora to name and describe all the cultivated plants that are in our gardens, common landscapes and the nursery trade.

It seems bizarre that we may have a better taxonomic understanding of, say, a wild datura in Central America than a lavender hybrid planted in Silver Spring. For all the wholesale nursery “availability lists,” for all the groaning perennial benches at the independent garden center, for all the inventory in the garden sections of the mass merchandisers, there is no current, complete and botanically sound catalogue of all these plants.

Such a scientific listing may seem unnecessary, but “we really don’t know how many species are in cultivation in the United States right now,” Nixon said. He estimates that as much as 20 percent of the planet’s plant genera are represented in American gardens. “That’s an incredible reservoir of biodiversity, when you think about it,” he said.

Such a flora would have untold scientific and horticultural value.

The last flora, actually a lesser beast called a plant dictionary, was published in 1976 as the third version of a guide first assembled by the great American horticulturist Liberty Hyde Bailey (1858-1954). His Hortus came out in 1930, the second version in 1941. The third was published in 1976 with the work of his daughter Ethel Z. Bailey and faculty at Cornell’s College of Agriculture and Life Sciences.

Hortus Third runs to 1,290 pages, weighs close to seven pounds and describes 34,305 plant entries. And yet it is hopelessly out of date. Since its publication, horticulture has changed in a million ways: The global commerce in plants has increased, and gardeners have shifted their gaze to ornamental grasses and herbaceous perennials and fostered a renaissance in annuals and tropical plants.

The nursery trade has seen its own revolutions, with the consolidation of growers and retailers, the advent of sophisticated branding and marketing programs that have expanded the range of plants to consumers, and the Internet’s effect on specialty nurseries and plant societies.

“We cultivate now much more than we did in 1976, especially with the increase in international trade,” said Robert Naczi, curator of North American botany at the New York Botanical Garden. And with the advent of DNA science, many plants have been reclassified; the old lily family is now split into eight or nine families, he said.

In 1976, there was also little regard for the invasiveness of cultivated plants; in some quarters today, it’s the paramount concern among native plant gardeners and ecologists.

A flora of cultivated plants would identify earlier the small fraction of garden plants that can become a major environmental problem, said Alan Whittemore, a botanist at the National Arboretum. Now, he said, field biologists “aren’t picking up invasive plants until they’re totally out of control, and the reason is they can’t identify them. It can be years before [a problem plant] gets into the scientific literature,” he said.

Why don’t we have a Hortus Fourth? “It would be a monumental undertaking,” Naczi said.

Typically, institutions need staff to coordinate the creation and publication of a flora, relying on experts such as Nixon — he’s an authority on the oak genus — to provide text as they can. “Flora of China” took 25 years. Another obstacle is that botanists have an affinity for studying wild flora, not the varieties and hybrids developed by breeders for horticultural use.

Raven believes the National Arboretum, as the horticultural branch of the Agricultural Research Service, would be naturally positioned to lead a multi-year effort to catalogue cultivated plants. It is also one of the few national institutions that has a dried plant collection, an herbarium, of cultivated plants that could act as a yardstick for such a flora.

But don’t hold your breath. The arboretum, run by the U.S. Agriculture Department, has been chronically underfunded for years and is now closed to the public three days a week to save money.

“It’s an important project and something we think about doing.” said Hefferan, who retired June 28. “To be honest, it’s probably not something we are able to take on right now.”