Here is reality," says Charles A. H. Thomson, retired colonel (army intelligence) and political scientist (The Brookings Institution and

the Rand Corporation). His bony hand points at his garden, 100 feet wide and 185 feet long. "Whatever there is is transitory," he says, "but the garden is reality."

Thomson is 70, pink-cheeked, long-shanked and light-hearted. For the past four decades, he has been a Montgomery County gardener: a pillar of the Men's Garden Club of Montgomery County and a founder of the Potomac Lily Society. He is a gardener's gardener. A devout student of horticulture, he is also a gregarious neighbor who thrives on swapping bits of information on tulips and cabbages best for this area and its proverbial clay.

"I like clay, he says. "It's easier to start with clay than with sand--clay has a lot more nutrients. The problem is how to loosen up clay to take advantage of the nutrients."

Each of his plants is identified by a metal or plastic label stuck in the ground. But he need not read the labels. He knows the names of every one of the many hundreds of hybrids in his garden-- names like 'Empress of Ireland' and 'Ice Follies,' 'Salmon Trout' and 'Caesar's Brother.' "Breeders really go wild naming their hybrids," he says.

When called upon, Thomson rattles off the Latin names of his plants--names like Lycoris squamigera and Leucojum vernum. He pronounces Latin with a Yankee precision, without a hint of an incantation, priestly or scholarly. Knowing nomenclature comes with the territory.

"As an undergraduate, I studied Hegel, Kant and the English idealists--monism versus pluralism, materialism versus idealism," he says. "But that the world is one and truth is universal came to me late in life, in the garden. It came home to me in the garden that the world you can see and touch contains things you can't see and can't touch. Some people say: the real world is real and fantasy is not real. The garden tells you, 'Listen, buster, fantasy is part of reality too.'"

Thomson calls himself a romantic gardener. "Gardening is a matter of feeling and knowing," he says. "But feelings are more important to me. I am always thinking that if I don't do it on the right date I may still have weather that makes me right. Science has its place. But all science is partial truth or truth-in-becoming, and I don't get too involved with either.

"Some people have an instinct for what to grow where and when. We all try to read the signs-- some people are better at it than I. It is important not to get upset if an early warm season is followed by rains, even snow. But May 15 is a good date to keep in mind to set out your tomatoes."

Thomson began gardening in 1942, on the same quiet Kensington street where he lives now. After building his first home, on a lot 125 by 225 feet, he told his wife Adele that he would only mow the lawn. "I was interested in squash and boating," he says. "But the lot had to be cleared of sumac and pine. There were also some daffodils. That's how I put my foot in the quicksand known as gardening."

He worked in the Pentagon at the time. World War II was on, and his specialty was psychological warfare. "I never saw the end of anything I worked on," he says. "But then I'd go into my garden. If I planted it right, weeded it right, it told me I was right.

"I had the sun at the back of my head and no one had to concur with my decisions. I needed no evidence submitted from third and fourth parties. The garden made its own tangible statement."

After the bombing of Hiroshima, he remembers being "hit hard" by thoughts of a nuclear judgment day. He wondered whether it made sense to plant that spring. "Then suddenly a great peace descended on me," he says. "If I plant these gladiolus bulbs, I said to myself, I have a good chance that in 60 to 90 days I'll see beauty, and there is a strong possibility of their beauty returning year after year. If I don't plant the bulbs, I'll never see their beauty. So I planted them, and I have raised many varieties of gladioli since."

After the war, there was an interlude of nine years in California--"to gain perspective," he says wryly. "But all my horticultural stuff is keyed to Washington. I won't move back to Southern California. I like the change of seasons here."

Adele Thomson practices ikebana, the Japanese art of flower arrangement. "I am the raiser," he says, "and Adele is the pruner."

In their home, flowers are imprinted on throw cushions, carved into the sofa and the armchairs, and painted on canvases and plates.

Thomson owns eight shovels, five forks and six spades, including a hand-carved, oak-handled beauty made early in this century and inherited from his father-in-law. Asked to name his favorite tool, Thomson points to a Cape Cod weeder with a blade shaped like a question mark, a Dutch scuffle hoe that cuts weeds off at the soil surface, and a 6- foot high, 10-pound digger bar of solid steel "with a lot of authority."

Since his retirement in 1975, Thomson has been a full-time gardener and garden lecturer. "I make a noise now and then," he says. "I want to make sure that Brookside Gardens--a horticultural treasure --receives a fair break from Montgomery County. I write letters. I hot up the garden clubs about senior citizens and property owners and people who need to do challenging things when their jobs fall apart."

Thomson's yard is a sprawling lawn broken up by free-form islands of azaleas and rhododendrons, and by rectangles such as a 10-year-old asparagus bed and a raspberry patch. He has a thriving vegetable plot--double-dug and divided into 5- foot-by-5-foot modules that are not to be stepped on so as not to compact the soil. He deals with 20 seed companies, some of which ask him to try out their new varieties.

At most any time of the year, the vista of Thomson's yard is dominated by scattered drifts of bulbous plants: irises, lilies, squills, tulips and, above all, daffodils. Thomson is a daffodil man--a connoisseur of the subtle relationships between the cup and the perianth--the petals surrounding the cup. He talks of split coronas--the cup's flattened, fringed edge--and he looks forward to the breeders introducing more pink--called pink but in fact orange--in the blossoms that were either white or yellow until about 30 years ago.

Thomson raises all 11 of the daffodil's major classes and about 75 varieties within those classes. "There have got to be a hundred clumps of daffodils in my yard," he says. "It's my mainstay. I first started planting daffodil bulbs in 1942 and planted some every year since then. I must have grown hundreds of varieties.

"I like all varieties. Some will fade quickly, others hold their gold right to the end. This is delicate, that is rambunctious. Oh, I like variety!

"The daffodil is my flag, my family crest. So to speak. The Thomson crest is probably thistle rampant. My father's forebears were Lowland Scots--cattle thieves and smugglers."

He can't praise enough the daffodil, the genus of plants which botanists speaking Latin named after Narcissus, the Greek boy who drowned while admiring his own image in the water. "Look what happened to poor old Narcissus when his self-love got the better of him," Thomson says. "But look at what a lovely flower he left us."