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Pot-Bound

The change from clay to plastic has transformed horticulture

BY ROGER B. SWAIN

A HUNDRED YEARS AGO, when you bought a potted plant from a florist it came in a clay pot. Tulips came in a bulb pan, a pot that was half as deep as it was wide. Azaleas grew in azalea pots, each three-quarters as deep as wide. African violets typically occupied standard pots, where depth equaled width. These pots were machine-made and mass-produced thanks to William Linton of Baltimore, who patented the first pot-making machine shortly after the Civil War. But what-ever the dimensions of the pots, their composition was the same—fired clay. Clay pots are like bricks; the clay used to make them can be dug from the same pit. A hundred years ago, clay pots were to florists what brick sidewalks were to pedestrians. They were unsung, utilitarian underlayments.

I grew up on a hill overlooking the world’s largest manufacturer of clay pots, A. W. Hews & Company of Cambridge, Massachusetts. Seven million flower pots a year was their peak output, with an accompanying number of cuspidors and umbrella stands. My route to grade school went past their clay pits near the present-day Alewife subway staion, a deposit that the company shared with the New England Brick Company across the street.

“Drop a clay pot and you have potsherds. Drop a plastic pot and you still have a pot.”

But by 1949, the year I was born, the brickworks was in decline. In 1952, a landslide in the 80-foot-deep pit buried the brick company’s last remaining steam shovel. The neighboring Hews pottery, which had been founded by potter Abraham Hews of Weston in 1765, made it to their company’s bicentennial. But by 1970, their last pit, too, had been filled in.

That year I had a part-time job washing flowerpots in a greenhouse on top of the Biological Laboratories of Harvard College, where I was an undergraduate. Though I didn’t recognize it at the time, the big, soapy-water-filled sinks where I worked contained the undoing of the Hews Pottery, for the pots I was cleaning weren’t clay. They were plastic.

Now, looking back on a century of horticulture and Horticulture, a magazine founded for florists, I see that I was an inadvertent witness to a revolution, that “clay or plastic” is a profound choice, that the pots I was washing were far more radical than any of the student demonstrations taking place at the time. The plastic pot changed everything.

But first a little history. Plastic itself was invented as William Linton was perfecting his pot-making machine. An Englishman, Alexander Parkes, concocted a cellulose nitrate that he called Parkesine. Unveiled at the 1862 Great International Exhibition in London, it won an award of excellence as a substitute for the increasingly scarce ivory and tortoiseshell used for brooches and knife handles. (Parkes, a prodigious inventor, had already made a more direct horticultural contribution—the spading fork, invented with metallurgist Henry Bessemer and shown at the same exhibition in 1862.)

Other plastics followed, but it was not until after World War II that plastics began to be commonly used to grow plants. Polyethylene, which had been discovered in 1933 by two British chemists, was tested as a soil mulch. Thin sheets of it spread out over the ground proved excellent for conserving moisture, controlling weeds, and warming the soil.

The first plastic flowerpots were also made from polyethylene. These were dismissed by some as mere novelties. Critics claimed that because they weren’t porous the plants couldn’t breathe. The same charge could also have been applied to the glazed clay pots already in existence, or to the steel cans being used by nurseries. In fact, this would ultimately prove to be a non-issue, Plants in nonporous pots simply need to be watered less frequently.

Tropical plant enthusiasts were some of the first to embrace the plastic pot. “Easy handling makes plastic pots highly desirable for window gardeners,” wrote William B. Carter of Tewksbury, Massachusetts, in a 1955 issue of Horticulture. “Attractive colors, some solid and some mottled with marbleized effects, add to the appearance of these new containers.”

It didn’t take long for others to see the virtues of plastic pots. For starters, these pots weren’t fragile. Drop a clay pot and you have potsherds. Drop a plastic pot and you still have a pot. Plastic pots were lighter weight, took up less space, survived winter outdoors, didn’t collect crusts of fertilizer salts and algae, and were easy to clean and reuse.

Clay pot manufacturers didn’t give up without a fight. Brickbats were thrown. One manufacturer of clay pots famous for its annual customer-appreciation banquets categorically refused to invite any florists that had even a single plastic pot in any of their greenhouses. But such efforts only slowed progress. The number of plastic pots manufactured in 1960 exceeded those made from clay, and it hasn’t been close since. We all laughed at the advice given to Dustin Hoffman as Benjamin Braddock in the 1967 movie The Graduate. But in horticulture you could have taken “plastics” to the bank.

What exactly happened? Plastic pots were light enough to hang from the ceiling. What would macrame have been without them? Mailorder nurseries stopped shipping bareroot plants because plastic pots with soilless mixes are light enough to mail long distances. With plastic, container gardening became everyperson’s pursuit, not limited to the estate gardens with their lead urns, stone troughs, and imported Italian terra-cotta pots.The nursery industry began to grow caliper-size trees in plastic nursery tubs that allow trees to be moved anywhere and be transplanted anytime.

“The number of plastic pots manufactured in 1960 exceeded those made from clay, and it hasn’t been close since.”

Where seedling tomatoes used to be grown in large wooden trays, cut apart with a knife and wrapped in newspaper for sale, by the early 1960s they were being sold in plastic six-packs. The pack system, with its individual root zones, makes transplant shock almost obsolete. And the cost of producing those transplants dropped in the 1970s because of plug trays, where up to 512 individual seeds can be perfectly sown and germinated, each in a separate little micropot.

Not everyone concedes the superiority of plastic when it comes to displaying plants.

There are still those who would rather showcase their prizes in earthen pots. But the very plant that is lovingly shown off in a clay pot probably started its life in plastic, probably reached the customer in plastic.

Today, polyethylene, polypropylene, and polystyrene are all used to make plant pots. All three are tough, impact-resistant plastics. Excess strength comes with a price, however. If it seems that plastic pots are getting thinner and thinner, it is because many of them are. Manufacturers are trying to stretch the plastic resins to hold as many plants as possible. Their motivation is to keep cost down. Costs might be reduced if the pots were reused, but few of them are. Those plastic pots I was washing in college are an exception now. Most plastic nursery containers wind up being discarded. There are stacks of them curbside every trash day.

Any plastic pot can be washed out and reused, and few home gardeners ever have a need to buy one anew. Recycling them, however, is more of a challenge. For starters, there is the issue of contamination—foreign material adhering to the plastic’s surface. Also, plant pots have typically been exposed to sunlight, which causes ultraviolet degradation of the polymers. And different plastics, even if they are stamped with the same recycling code (e.g., #1, #2, #3, etc.) can’t automatically be blended and remelted because of differing melt indexes, which affects how they can be refabricated. Like it or not, landfilling or burning for energy recovery is often the most economical method of disposal.

There are, however, businesses committed to the conservation of this resource. The easiest products to make are those with broad design tolerances, such as plastic landscape timbers. The thicker and more solid the product being made, the more likely it can accommodate the mixing of a variety of different plastics, as well as associated soil or potting mixes. There is considerable skill involved in combining unrelated feedstocks. One reprocessor described his work as “mushing the plastics together,” but to do it successfully requires skill in chemistry, engineering, and economics.

“With plastic, container gardening became everyperson’s pursuit, not limited to the estate gardens with their lead urns, stone troughs, and imported Italian terra-cotta pots.”

In Taunton, Massachusetts, scarcely 40 miles south of the A. W. Hews pottery site, is a young company called Selec Tech, Inc. Located on the site of a former cast-iron stove foundry, the company has been recycling commingled dirty plastic since 1994. Their secret is to clean the used plastic only to the degree necessary — that and the ability to blend multiple plastics to yield a color other than black. They make a variety of horticultural products: landscape timbers, a plastic half whiskey barrel, and EcoPlanters. The last come in 14–, 18–, and 24– inch diameters, and look for all the world like thick-walled terra-cotta pots. Until you pick them up. One-third as heavy as clay, these pots are completely frostproof, are said to withstand even a sledgehammer blow, and come with a 20-year warranty. Sales, I hear, are good. It’s proof that when better flowerpots are made, even better clay pots, they are going to be made from plastic. H

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