The colorful, prolific, shade-loving impatiens

Alan Arrowsmith, a vice-president of Pan-American Seeds here, has just one word to describe the rise of impatiens in public esteem: ``Meteoric. There's been nothing else quite like it,'' he says. The English-trained horticulturist maintains there's barely a home anywhere that wouldn't be brightened by the infusion of color that impatiens brings to a garden, particularly its shady spots. And if repeat sales at garden centers around the country are any indication, those gardeners who have tried this plant of the tropics obviously agree. It has been the No. 1 bedding plant in the nation for the past two years and a dominant increase item at garden centers in each of the preceding 12.

There are many reasons for the flower's popularity, but two stand out: Impatiens covers itself with color while asking little in the way of attention from the gardener except periodic watering. Performance comes without effort, making it, in the words of a friend of mine, ``the perfect Yuppie plant.''

When I visited the Pan American trial grounds last summer, Mr. Arrowsmith listed the qualities of impatiens in this order:

The plant is a prolific bloomer. A massed planting is as spectacular as anything you can expect from those long-time favorites, marigolds and petunias.

It is self-cleaning. Dead flowers fall off naturally.

It quickly eliminates competition. Simply, there is no weeding.

In all but the most sterile soils it requires no fertilizer. Fertilizing, in fact, tends to spoil the effect by causing vigorous leaf growth that hides the flowers. In Mr. Arrowsmith's words, ``the best fertilizer is H2O.''

Finally, and perhaps the most important quality of all, impatiens loves shade even while it tolerates some sun. This fact alone makes it preeminently suited to urban gardens and those suburban homes where maturing trees overshadow everything else in the yard.

Breeders have been improving the plant over the years so that newer varieties now branch out naturally without the need to pinch out the tops. It fills in rapidly to provide a carpet of bloom within weeks. Ultra-dwarf varieties grow from 8 to 10 inches tall; other compacts from 12 to 18 inches. Plant them close and they grow tall, give them more space and they spread (to about 15 inches across), Mr. Arrowsmith points out.

Shade also regulates height: the more of it there is, the taller the plants grow. Should they get too tall, there's a simple Arrowsmith solution: ``Cut them back.''

The impatiens is now available in every color except true blue and yellow. A suggestion from Mr. Arrowsmith: ``Use pastels in the shade because they stand out at dusk as though someone has turned on a light. They seem to leap out at you.''

While regular impatiens is employed to provide color along sun-short, north-facing walls, under shade trees, or in hanging baskets on the porch, there is a newer variety that can tolerate full sun. It was discovered in New Guinea and was first introduced into the United States in 1970. Breeders have turned it into a more compact plant in recent years.

The New Guinea varieties flower almost as prodigiously as their shade-loving counterparts, but have one other distinct plus: They boast a variegated leaf that would earn them a place in many gardens whether they flowered or not.