Aztecs tapped the sap of Hevea brasiliensis much as American Indians bled sugar maples for their sweet syrup. But the hevea rubber tree yielded something quite inedible — a thick, milky liquid we call latex. The Aztecs used it to make a hard, dense ball for playing games.
In the 19th century, inventors and engineers figured out how to make rubber flex (add sulfur under heat) and to modify it from a solid ring to a tube filled with air.
These breakthroughs led to the world as we know it — on the move on pneumatic tires. Passenger car tires are made from a mix of natural and synthetic rubber — mostly the latter and derived from petroleum. Truck tires need a greater ratio of natural rubber, and aircraft tires are made wholly of natural rubber. The successful landing of a wide-bodied jet, with its enormous loads, is in large part because of the skill of the pilot and the existence of a spindly jungle tree native to the Amazon. If you don’t want to take a jet, you can see a hevea growing in the Tropics, the central conservatory of the U.S. Botanic Garden at the foot of the U.S. Capitol. Most people pass it by, absorbed by the more imposing palm trees.
On a recent spring Saturday, the botanic garden was the scene of the announcement of another rubber breakthrough — the successful harvesting of rubber from a scrubby little shrub named guayule (why-YOU-lee). This modestly attended talk was given by Gene Lester of the Agriculture Department’s Agricultural Research Service. I got a sense the world was missing something momentous. We’re talking here of the ongoing evolution of the wheel, which is the history of the connecting of human societies, generally considered a good thing.
The world has shrunk. So too has our understanding of the plant kingdom and how dependent we are upon it. Where do people think rubber comes from? “The rubber fairy,” said Lester, who directs the research service’s product development programs.
The hevea tree, which is related to the milky-sapped poinsettia, is king of the rubber world — other plants can yield latex, including dandelions — but hevea is by far the predominant source of commercial natural rubber. Though the tree is native to the Amazon, most hevea plantations today are in Thailand, Indonesia and Malaysia. For all its market share, hevea has its problems (beyond a leaf blight disease in its original environment).
It grows in a fairly narrow equatorial band, takes almost a decade to reach maturity and requires daily scrapings of its exudations. A rubber plantation requires a lot of labor, though I’m sure they’re working on a robot.
The need for rubber has never been greater. Lester told the audience that the world has more than 1 billion vehicles along with 39,000 aircraft flying about 4 billion passengers a year. Lester shared estimates that by 2030, the number of vehicles will double. The number of air travelers will be just shy of 8 billion.
A corresponding increase in hevea production would undoubtedly require the clearing of more tropical rain forest at a time when the planet needs every oxygenating jungle it’s got.
Enter guayule. The shrub is native to northern Mexico and the Southwest. As an aside, it has very few close relatives, but one is a great garden plant, a hardy perennial named wild quinine, with clusters of white flowers for weeks in early summer. It does well in difficult, hot and dry sites and draws loads of pollinators. It should be used much more than it is.
Guayule is not as pretty, but it is at least as tough. It grows in hot, dry regions, and if it were to become a major source of rubber, it would flourish in areas of the country where crop options are limited to such things as alfalfa and cotton, both of which require more precious water.
Although the rubber tree is carefully cut and tapped, the guayule is chopped to the ground and its branches mashed up to extract the latex. The plant then regrows and can be harvested several more times before depletion. It also produces valuable byproducts, including fuel.
Under a $6.9 million government grant, ARS researchers joined other plant scientists and industry experts as part of a consortium looking at the feasibility of guayule production. The five-year effort included the genomic sequencing of Parthenium argentatum for use in picking plants with desired genetic traits — drought tolerance and high latex yield, for example.
“We think it’s good for the globe and good for farmers, especially in the desert Southwest where water is dear,” said Colleen McMahan, the ARS’s lead scientist for domestic natural rubber.
As part of the study, Cooper Tire & Rubber Co. in Findlay, Ohio, made several hundred tires from guayule rubber and tested them successfully for performance and safety. “A modern radial tire has 27 components, and every one is designed to do something different,” said Chuck Yurkovich, senior vice president for global research and development. “What we had to do was reformulate every component to make it work.”
But they succeeded, and “if there was a supply of this material, we could and would put it in our tires,” he said — more specifically, a supply made affordable by scale of agricultural production and processing plants. The economics and investments aren’t there yet.
William Niaura, director of collaborations and open innovations for another tire company, Bridgestone Americas, said in an email that guayule would supplement rather than replace hevea rubber. It will take until the middle or end of the next decade for the shrub to be farmed in sufficient quantities, he said, and further development is needed to market its byproducts to make it economically viable.
But guayule rubber is already making its way into high-end products because it is considered purer than hevea rubber, offers superior tactile feel for surgical gloves — brain surgeons take note — and doesn’t induce the allergic reactions of hevea.
While we wait for guayule rubber to sink or swim, it may be time to plant and enjoy its close cousin, wild quinine. Parthenium integrifolium is guaranteed to transport your mind to a beautiful, swaying, buzzing wildflower meadow, away from the prospect of a world with 8 billion fellow airport users.
Unsupported pepper plants will flop and may break by late summer. They should be given support when young. Regular tomato cages are too small for most tomato plants but are ideal for keeping pepper plants upright. Pinch out the first flowers and developing fruit to allow pepper plants to get better established in early summer.
— Adrian Higgins