Cork is the thick outer bark of a number of oak species. Photo by Getty Images/johncopland.
For most of human history, bark has been a hot commodity. The pursuit of various barks generated astonishing wealth, sent countries to war, and pushed explorers into unknown parts. Bark’s unique chemical and physical properties, adapted over hundreds of millions of years to protect plants from threats, are the result of whatever challenges the local environment offered — fire, frost, desiccation, disease, insects, and herbivores, to name a few.
Some barks exude heavy metals, contain lethal toxins, or produce an outer layer so hard that machetes will bounce off of it. Others can cure malaria, flavor food, or float. Bark eludes artificial synthesis, despite huge possible financial rewards and almost 200 years of efforts that have resulted in seven Nobel Prizes in chemistry. Most commercial tires still require up to 40 percent natural rubber; and bark-derived aromatics, such as cinnamon and frankincense, cannot be imitated well by synthetics.
Today, the U.S. Food and Drug Administration lists more than 100 approved bark products for use in cosmetics, drugs, food, and drinks — a fraction of those used in the past. Early historical records indicate that the bark of virtually every species of woody plant was once harvested.
Despite our long-standing reliance on bark, it’s lost our interest. Only a handful of books are devoted to bark — which can be harvested from more than 60,000 species of woody plants — while you could fill a library with the books written about roses alone. Still, researchers are investigating unique bark compounds in search of everything from cures for diseases to eco-friendly building materials and clothing fibers.
I’ve focused on species with broad value and existing commercial demand. With the exception of the Amur cork, which has been banned in some states as an invasive, I recommend cultivating the following plants rather than foraging them.
Wine corks are one of the most familiar uses of cork, but it's a remarkably versatile material. Photo by Getty Images/duncan1890.
Put a Cork in It!
Aside from being great at stoppering wine bottles and ancient Greek amphorae, cork has seemingly infinite applications. Cork can be used as insulation, siding, wall covering, flooring, and as material for sinks and bathtubs.
The history of the cork oak (Quercus suber) in North America probably dates back to the early colonial period. John Bartram, a Philadelphia botanist, recorded that he saw it growing in Charleston, South Carolina, in 1765. A few decades later, Thomas Jefferson made the first known push for American production. Jefferson, who imported European wine by the barrel, hoped to establish cork for wine bottling from the coasts of Delaware to Florida. Jefferson’s dream never took off, but his failure may have been largely his own fault. While he was serving as ambassador to France, he received a memo recommending Quercus x pseudosuber, a more cold-hardy Italian hybrid — but he continued to favor Q. suber.
As dramatic as the harvest looks, cork is a sustainable product with uses that go far beyond bottle corks. It can be used to soundproof structures, provide underlayment for floors, insulate buildings, and can even be made into a leather alternative. Photo by Associação Portuguesa da Cortiça.
During the 20th century’s world wars, concerns about losing access to European suppliers led to experiments in growing cork oak and potential alternatives in 25 states. The results hint that American-grown trees can yield as well as Mediterranean, though it’s unlikely that they’ll ever compete with Spain and Portugal in terms of quality. Traditional cork-growing methods also entail decades of growth before the cork is ready for harvest. Recent attempts in Portugal to cultivate cork oaks with irrigation and fertilization suggest that the time to maturity can be shortened.
Grow Your Own Cork
- Cork oak (Quercus suber). Hardy to Zone 8, and possibly to Zone 7 with frost protection while young. Produces the highest quality cork.
- Chinese cork oak (Quercus variabilis). Hardy to Zone 5, and used for commercial cork in its native Asia.
- Amur Cork (Phellodendron amurense). Hardy to Zone 3, quick to naturalize and can become invasive. Produces thick, coarse cork that may be useful for construction.
Humans aren’t the only species to put a premium on resins and their ability to waterproof, protect against disease and pests, and glue surfaces together. Several species of bee, including honeybees, collect it as an addition to their wax for construction. Resin reduces pathogens in the hives and deters other predatory insects. Bees have used resin so effectively that a giant species that had been thought extinct was found living inside a nest of termites in a resin chamber. The colony had taken advantage of the termites as workers, but kept them out of the hive with insecticidal resin.
Natural wounds also release resin, which often traps small insects. Given enough time, the resin will turn into amber. Photo by Flickr/Hadley Paul Garland.
Conifers are the biggest producers of resins, but some other plants also produce resins. Dragon trees (Dracaena spp.) were famous in the ancient world for their red sap, which makes an excellent varnish, and they’re more closely related to palms and grasses than to other trees. Resin-bearing barks sometimes exude small amounts spontaneously or with minor handling, as anyone who’s ever climbed a pine or handled a Christmas tree will recall.
The bark of Pinus resinosa, a highly productive resin species, flakes off in gently rounded sections. Photo courtesy the Arnold Arboretum.
Harvesting resin commercially involves making large, angled wounds that meet at a point to guide the sap into a collection bucket. This method produces high yields but leaves large scars. For home use, experiment with smaller cuts 1 to 4 feet from the base of the tree. If a very small amount of resin is required, perhaps for incense or to make faux amber for jewelry, try pruning an awkward limb or puncturing the bark before you consider larger cuts.
Commercial resin production requires mature trees, because of the size of the wound required to collect large quantities of resin. For home use, you can trim small branches or puncture the trunk of the tree, to minimize the damage and stave off disease. Photo by Flickr/Jacob.
Grow Your Own Resin
- Pine (Pinus spp.) Trees in the pine genus are easy to grow, widespread in North America, very cold-hardy, and the most resinous option by far.
- Sweet gum (Liquidambar styraciflua). Hardy to Zone 7. Can be cultivated for a culinary resin or a substitute for ambergris, an expensive perfume ingredient.
- Myrrh (Commiphora spp.) and frankincense (Boswellia sacra). Both are hardy to Zone 10, but fussy to grow. Their resins produce an intoxicating incense and are used in cosmetics too.
Rubber to the Road
For plants, latex serves the same function as resin: It gums up the mouths of insects and hits them with poisons or repellent chemicals. Like amber and cinnamon, it’s been compared to gold for its economic value.
Pre-Columbian cultures in South America were the first to harvest latex for rubber. They manufactured balls for games and religious ceremonies, as well as waterproof gear and containers. After American inventor Charles Goodyear discovered a method for processing latex into a more durable rubber in 1839, it earned the moniker “white gold.” European entrepreneurs flocked to South America to produce rubber, building lavish European-style palaces in the jungle and enslaving native peoples for labor.
Latex-producing plants tend to be tropical, and processing natural latex into rubber is expensive and labor-intensive. Instead, try growing figs or a hardy rubber tree for smaller quantities of latex. Photo by Getty Images/georgeclerk.
Today, tire manufacturers still use up to 40 percent natural rubber because it resists deterioration better than synthetic alternatives. However, rubber is only one of many uses for latex. Thousands of plant species produce some form of latex, with uses that range from chewing gum and food thickener to latex gloves and electrical wiring. Most woody plants that produce bark latex can only survive to about Zone 10, and won’t be very productive in a pot. But North American gardeners in more temperate areas still have interesting options.
Fig latex can substitute for rennet in cheese production, and you can chew the latex from hardy rubber trees like chewing gum. Photo by Ben Whitacre.
Grow Your Own Latex
- Edible figs (Ficus carica). Grown outdoors as far north as New York City, figs release a flow of latex when injured during the growing season. The ancient Greek poet Homer mentions using fig latex as a substitute for rennet in cheese production, and recent research supports this use, though it might be problematic for folks with latex allergies. It can be harvested by tapping, but you might also be able to harvest enough for home use by pruning new growth that hasn’t yet hardened off.
- Hardy rubber tree (Eucommia ulmoides). Hardy to Zone 4, this traditional Chinese medicinal tree produces a latex that can be extracted to make rubber. However, harvesting large amounts requires a costly extraction process, as the latex won’t flow when tapped. Hardy rubber trees are better used as attractive landscape specimens with medicinal inner bark and a small supply of chicle-like latex that could be used as chewing gum.
- Guayule (Parthenium argentatum). A woody shrub native to the American Southwest, but potentially hardy to Zone 7, guayule bark must be pulverized to remove the latex, but once extracted, it can be used to create hypo-allergenic latex and tire rubber.
The Spice of Life
At the time of the conquistadors, La Canela, the mythical land of cinnamon, was as eagerly sought after as El Dorado, the land of gold. While cinnamon isn’t even worth its weight in fresh basil today, the Asia native is worth growing like an herb. Not only is it easy to cultivate outdoors to Zone 9, or indoors in a pot, but fresh cinnamon tastes better than quills imported from Asia.
You can grow tropical true cinnamon in a container. Cassia may be more cold-hardy, but will still need winter protection in most of the United States; some people can also detect a difference in flavor between the different species of cinnamon available. Photo by Getty Images/DavorLovincic.
Some connoisseurs distinguish between true cinnamon (Cinnamomum verum) and cassia or Chinese cinnamon (C. cassia), but today, the handful of commercial cinnamon species are recognized as varietals rather than different spices. To harvest, simply cut back a 6-inch limb or stem as needed, score it along the length to the wood, pull the bark away, and then peel the rough outer bark from the soft, aromatic inner bark. During periods of rapid growth, the inner bark may pull away as readily as string cheese. Commercial growers coppice the plants to produce many small, easily harvested shoots from a single, mature tree.
Edible cinnamon is the soft inner bark of various species of Cinnamomum; the outer bark must be peeled away. Photo by Ji-Elle.
Grow Your Own Cinnamon
- True Cinnamon (Cinnamomum verum) is hardy to Zone 9.
- C. cassia may withstand colder climates.
- Other flavorful barks worth cultivating include the cinnamon-flavored Carolina allspice (Calycanthus floridus; the only part considered safe to eat is the inner bark, so caution is advised); sweet birch (Betula lenta); and shagbark hickory (Carya ovata).
Photo by Sam Mugraby.
Ben Whitacre is a hobbyist gardener who’s worked at Arnold Arboretum, Mount Auburn Cemetery, the American Horticultural Society, and Monticello.