Spring is upon us: The dogwoods are out, the azaleas are blooming, the tulips are up and so are the weeds. There once was a time when this season produced visions of formal beds and English gardens, of miraculous conversions of the back yard into horticultural splendor. This year, however, I'm thinking of condos.

My affair with the garden began years ago, after my firstborn was about 4 or 5 years old, and I received one of Thalassa Cruso's books for Christmas. By February of that year I had purchased approximately one of every item featured in the Burpee catalogue, and by May I had young perennials growing in the greenhouse. At the rate I was going, Thalassa Cruso was going to have to watch out.

As the years passed, however, I made some discoveries, one of which is that gardening is cyclical. I do not, for example, enjoy gardening when I am pregnant. My younger son was born in September and my gardening that year consisted mainly of resting in a lounge chair and reading planting directions to my spouse: "Dig hole 2 feet wide, by 2 feet deep . . . . " Nor do I enjoy gardening for the first two or three years after my children are born. But suddenly, in the fourth year, various nesting or nurturing or propagating instincts emerge in search of an outlet, and suddenly I'm back into gardening. By this time, however, weeds have taken over the beds and we have to start over.

Some years ago we moved to a larger house, bringing with us a number of azaleas that had been growing in a nursery in our old garden. These were quickly planted in a newly dug bed that ran the length of the front walk. More beds were put in beneath front windows and along the carport. In time, my son the teen-ager became a terrific source of cheap labor, so my ambitions knew few bounds. More beds went into the back yard, and in went rhododendrons, more azaleas and roses. Lots of roses.

Then my son the teen-ager spent two summers working for my brother's nursery in Massachusetts. Each time he returned with landscaping schemes that would make Versailles look plain. More beds were dug: in went more roses, hydrangeas, more daffodils, tulips and perennials.

The trouble was nothing died. For mysterious reasons that have nothing to do with green thumbs, everything grew. And grew.

There had been plenty of space between the azaleas on the front walk when we planted them. So much space, in fact, that I decided to fill it with perennials. By last summer, they had taken over the sidewalk and one day I came home from work to find my son the landscaper digging them out, vowing to get rid of them all. (They got moved to yet another bed in back.)

My son asked a number of penetrating questions after my husband and I separated, one of which he asked shortly before he left for college. It was: "Mom, how are you going to keep this place up without me?"

Last fall, of course, that was an easy question to answer. All you have to do is worry about the lawn. This past weekend, however, I was forced to face the question square on. All around me lawn mowers were humming, neighbors were out planting, my weeds were growing and my son the landscaper was away at college.

A young lady, one of four siblings who have taken up the lawn-mowing business, arrived to do my lawn for the second time this year. Her father, who had given me an estimate for lawn mowing the first time one of his daughters did it, had expressed some amazement on the first visit at the size of my grass. He commented on it again.

While she mowed the back, I went out in front and started to tackle the weeds. The payoff for years of horticultural success is that I am the only one who can identify what should be in various beds and what shouldn't. The moment had clearly come when I had to decide whether to move into a condo or salvage the house from the weeds. I began thinking about my interest rate.

After an hour or so of weeding, I began thinking that a higher interest rate would mean a bigger deduction on taxes.

After about two hours of weeding, I began thinking that perhaps I should have taken up jogging after all. The ache in my lower back was becoming acute, and my legs were threatening to give way. Jane Fonda might be in shape to weed, but I was not.

Suddenly the lawn mower stopped, and the young lady and her father appeared. Once again, he allowed as how he could not believe how rapidly my grass had grown. It had grown far faster than anybody else's in the neighborhood, he said.

"Just think," he said. "It's only April. You must have warm soil."

Just my luck.