In the herb propagation house at Hillcrest Nursery in Millers, Md., Emily Kelly fetches finished plugs for shipping. (Adrian Higgins/The Washington Post)

April is upon us, the month when the simple act of sticking a plant in a pot or garden bed creates such a powerful and tangible connection to spring.

Steve Hershfeld probably doesn’t think about this, but over the past four decades he has made that moment happen for countless people — millions, possibly — in the Mid-Atlantic region. At the start of another growing season, Hershfeld and some 40 employees are feverishly putting the finishing touches on tens of thousands of perennials, herbs, vegetable transplants and annuals that are beginning to supply a garden center near you.

Even seasoned gardeners don’t ponder much where these little treasures come from; the focus instead is on how these plants will work this season in their gardens. But for Hershfeld and his colleagues at Hillcrest Nursery, the generation of spring plants has consumed their working lives as far back as last August.

Located in the rolling hills of northern Baltimore County, Hershfeld’s wholesale nursery has 15 greenhouses, with a total of four acres under cover. The houses range from simple plastic-covered-frame structures — poly houses — to vast, state-of-the-art glazed greenhouses with computer-controlled venting, misting and shading systems. The company does about 75 percent of its business between late winter and May, he said.

The growers at Hillcrest Nursery in Baltimore County supply independent garden centers in the Mid-Atlantic region and have been readying for spring for weeks. These greenhouse benches hold baskets of seed-grown petunias. (Adrian Higgins/The Washington Post)

“When the winter breaks, anything that’s healthy, with a flower, we can sell,” he said. Actually, forget about the flower bit. One greenhouse full of cool-season vegetable transplants is already empty. “People are so anxious to get out and start gardening.”

In spite of the nurseryman’s mercantile delight at the season, this is a risky business. In January, under the weight of the blizzard, one big greenhouse collapsed. It was the oldest and least advanced of the houses, and efforts to thwart the snow load, by cranking up the heat, weren’t enough. “We were getting three inches of snow an hour,” Hershfeld said. A modern house will replace it.

Other pitfalls are less dramatic: cloudy weather encouraging disease, hot weather advancing plants too much, and the dreaded monster of retail and wholesale nurseries alike, a string of wet weekends in April and May that depresses sales.

Hershfeld sells to independent garden centers — not the big-box stores — and choosing what to grow is itself a major skill. The perennials, for example, must appeal to a knowledgeable garden center crowd but not be so unfamiliar that the consumer will pass them by. After four decades, Hershfeld has a honed sense of what will appeal to buyers seeking their weekend fix.

Hillcrest Nursery owner Steve Hershfeld inspects a tray of calibrachoas, a popular heat-loving annual. (Adrian Higgins/The Washington Post)

We meet grower Harry Newton at a remote section devoted to perennial propagation and get to see finished plants in one-gallon pots, ready for shipping. Newton tends to the contents of five cool poly houses in a row, each a testament to what Hershfeld and Newton think will be popular this year. Hint: Three of the houses contain varieties of coral bells or heucheras, a perennial that has become absurdly desirable in the past decade for its striking leaf markings and colors. Picking one that will last in the garden is a skill that comes down to selecting the right variety and placing it with some care — but I digress.

In addition to more heucheras, the second house is full of lavender: the classic English lavender Hidcote, the big hybrid lavandin Provence and a new hybrid named Phenomenal, which is drawing raves for its ability to thrive in hot, humid climates. Nearby, there are hundreds of gallon pots filled with the sweet iris, Iris pallida. And although its violet-blue flowers appear in late spring, it doesn’t need a bloom right now to appeal. The pots are full of the fresh spring flush of variegated, spearlike leaves — very handsome.

Steve Hershfeld inspects a basil plant for root development. (Adrian Higgins/The Washington Post)

A large part of the Hillcrest business is in herbs; herbs always have been popular, but in the age of the home gourmet, they are as coveted as ever.

Most herbs are supposed to be perennials — and surely they are in the dry heat and cooler nights of California or Italy — but here they are best viewed as short-lived propositions. Nothing looks so sad as beaten-down creeping thyme in April, and nothing looks as alluring as a fresh one in a 2 1/2 -inch pot.

Then there’s the aromatherapy. Hershfeld brushes the foliage of lemon verbena, tarragon and then rosemary. “They are coming out of a heated greenhouse; they need to be hardened off for the garden yet.” And yes, dear consumer, that’s your job.

We are in one of two 10,000-square-foot, state-of-the-art greenhouses where employees raise the herbs. On one side sit the stock plants from which the cuttings are made. On the other, the potted cuttings are quickly rooting — 105 plugs to a flat. They take two to five weeks to root, depending on the crop. The rooted cuttings either go to other houses for finishing as marketable plants or are shipped to other growers across the United States and Canada (and beyond) for them to finish in time for their spring frenzy.

In one of the perennial houses, dianthus and candytuft have bloomed and are ready for shipping. (Adrian Higgins/The Washington Post)

In another house, Hershfeld shows me annuals that are getting ready to be sold, including dozens of red-flowered geraniums in hanging baskets, started as cuttings last October.

He inspects a tray of seed-grown petunias coming into bloom. “This is a new petunia. It’s got a nice name,” he said, with a twinkle in his eye. “Cha-ching.”

A few feet along the aisles, he exchanges pleasantries with a greenhouse worker. “How are you doing, Steve?” she asks.

“I’m living the horticultural dream.”