THE AMERICAN GARDEN is as much a melting pot as the American population. We have been happily assimilating plants and people since the days when Thomas Jefferson, a prominent early planter, created his 1,000-foot long garden on a sunny terrace at Monticello and traded seeds, bulbs and plants with friends on two continents.

The third president's correspondence, notes Allen Lacy in his collection of amusing and informative garden essays, is full of letters on such weighty issues of state as cucumbers, cherries and magnolia trees, which he helped transplant to Europe, and flowers like Spanish broom, which he helped establish in this country.

And it was Jefferson, when U.S. minister to France, who smuggled rare rice seed out of Italy in his pocket, which he thought might do well in the marshes of South Carolina. But then garden theft is a whole separate chapter in Farther Afield, the second gathering of Lacy's gardening columns from The Wall Street Journal and assorted gardening magazines.

What sets Lacy above the common garden-variety journalist is not just his good humor and good sense -- he loathes red salvia -- but his fascination with the origin of the plants we grow. This scholarly interest is perhaps understandable in a New Jersey college philosophy professor who only trips among the tulips as a sideline.

Whenever you buy six-packs of petunias and impatiens at your local garden center, a biologist friend told Lacy in 1982, "chances are more than fair that the seed came originally from (Claude) Hope's finca in Costa Rica." Thus came about Lacy's visit and his longest essay, on the Seed King of Costa Rica -- the expatriate Texan who came to grow quinine in World War II and stayed to create a mountain seed farm that is also producing new varieties of snapdragon, lobelia, coleus, amaryllis, gerbera, delphiniums and even vegetables like a patio tomato "with a genuine old-fashioned tomato flavor."

It is a fascinating study of how modern plant hybrids are born, in a sort of floral Shangri-La where crops grow continuously since there "is almost no variation in either day length or temperature throughout the year, no matter what the season." IN A GLIMPSE into the workings of the venerable English seed firm of Thompson & Morgan, Lacy recounts the role in developing new plant varieties that can be played by amateur gardeners, including little old ladies in tennis shoes. The new strain of huge, hardy and long-lasting Iceland poppy Thompson & Morgan is bringing out was developed by Erma Westcott of Portland, Oregon, through almost 50 years of hand pollination in her small back-yard garden before her death in 1982.

Called the Oregon Rainbow strain at her request, "hers may be one of the finest achievements in recent years by an amateur hybridist," Lacy was told.

With trips to gardens around the world, from England's Sissinghurst Castle to the Netherlands, Switzerland, Spain, Virginia apple orchards and his own New Jersey back yard, Lacy's delightful, light-hearted rambles are almost always informative and fun to read -- though not all may share his occasional enthusiasm for huge, stately plants like yucca and fritillaries, the veteran floral troops of Victorian public gardens.

While most of Lacy's book, like its predecessor, Home Ground, is composed largely of Wall Street Journal columns, you won't find his column any longer among the stock options and pork belly futures. It was uprooted this spring, on the eve of his book's publication, and transplanted across Manhattan to The New York Times' Home Section.

WHICH brings up another new garden book, This Is the Way My Garden Grows . . .(And Comes into the Kitchen) by Barbara Dodge Borland. It comes with a blessing from The New York Times garden editor and was written by the wife of the late Hal Borland, whose nature pieces graced The Times' editorial page for several decades until his death in 1978.

Barbara Dodge Borland is also billed as a garden columnist, having written a column of the same title ("This Is the Way My Garden Grows") for an unnamed newspaper.

This is a thin, quirky vegetable gardening book which promises a flourishing garden with little weeding, watering or effort and almost no bugs -- the latter's absence due primarily to the liberal use of one insecticide, Rotenone, which she apparently sprinkles like salt on almost everything.

But Borland writes with gusto and this is a full-speed-ahead-and-damn-the-weeds book. The weed problem she solves by a little hoeing with a "heart-shaped hoe" and ignoring them. They provide shade for her vegetables, she says, and support for plants like peas.

It's the kind of advice that could give shy, beginning gardeners the courage to do what needs to be done, though perhaps without so much Rotenone. The insecticide, while plant-based, biodegradable and less dangerous for warm-blooded animals than its chemical counterparts, is still highly toxic to fish in runoff and of course to insects bad and good, including bees.

The book does leave you with a vivid picture of Borland, who says she always transplants in the rain, runs her "fingernails through a bar of soap before gardening (your nails will thank you)," and sews "G" in red floss in her blue jeans to help her distinguish her gardening and non-gardening pants.

Her house also may look a bit "eccentric," she says, because she dries herbs by hanging them in brown paper bags and letting them "dangle from any doorway in the house. . . just reach out and tap the bag as you pass. It's a wonderful system." She suggests guests may want to "play the game too."