The brownest thumb the world has ever known lived in a pleasant old section of Arlington, resplendent with glorious borders and gardens, aflower from azalea season to chrysanthemum -- except for his yard, which in the rainiest times looked a candidate for a California brush fire.

Even his crab grass died, yet kept reappearing, a mystery of the universe as those men who always have a three-day beard, never a two-day, never a four-day.

In desperation he brought in several loads of sludge then sold by the Blue Plains sewage-treatment plant, guaranteed to make the soil Eden-like. But everything he planted still withered in infancy. All that grew were tomatoes -- hundreds of tomatoes from seeds that survived whole in the sludge. The tomatoes soared into two- and three-foot vines, never blossomed, then withered.

Then, as he was about to call in the concrete trucks and pave the whole place, a friend gave him several shovelfuls of day lily roots dug in the woods of a tract in then- distant Fairfax County, a semi-swamp lying fallow awaiting the developers. They were planted, without hope and without preparation, in shallow holes shoveled into the wasteland.

By June the leaves were 18 inches high, strong and verdant, and Brown Thumb thought he'd better water them. As he turned on the hose, the phone rang in the house. He flung down the hose and raced indoors, to be invited to a weekend at the beach, which he promptly accepted. When he got back Monday morning, the yard was awash.

But the day lilies not only survived, they thrived, and spread. Three years later, the midsummer yard was a waving sea of orange. And it is to this day.

Day lilies are a huge family -- Hemerocallis -- with thousands of varieties of natives and hybrids; the late Walter Marsh of Oregon had so many hundreds of his hybrids patented that he ran out of names and had to resort to numbers. The predominant native in this area goes by many names, most commonly Turk's Cap, which is descriptive of the shape of the blossom; it's medium orange with black flecks.

Culivated day lily varieties range in size from dwarf (six to 10 inches) to giant (36 inches or taller). Blossom colors span the spectrum, but yellows, greens and oranges predominate. All the major nurseries in the area have several varieties always in stock, and cheap.

For more exotic kinds, send for the catalogue of Caprice Farms Nursery, 15425 SW Pleasant Hill Road, Sherwood, Oregon 97140. The American Hemerocallis Society publishes a quarterly journal. WQrite Joan Senior, Rt 2 Box 360, DeQueen, Ark. 71832.

Because day lilies are the hardiest of perennials, they're virtually labor-free. About all that's needed is to break them up every three years so they don't strangle themselves. Then you have some shovelfuls to pass on to your own brown-thumb friend.