Just sit back and watch the grass grow. And grow.

Grass-covered lawns rank as one of America’s largest “crops,” amounting to at least three times the acreage planted in cotton, the U.S. Department of Agriculture says. There are nearly 40 million acres of turf in the U.S., according to estimates by the Journal of Environmental Management.

Despite their maintenance, traditional lawns far outnumber the horticultural alternatives — rain gardens, rock gardens, water features and garden rooms, among them.

And there are many ways to get a good one.

“We have seen this reflected in consumer purchases of grass seed, which increased 17 percent in 2006,” said Su Lok, a spokeswoman for Scotts Miracle-Gro Co. in Marysville, Ohio, which describes itself as the world’s leading supplier and marketer of consumer products for lawn and garden care.

“What we have seen is the trend of consumers who want products and services that make caring for their lawns and gardens easier,” she said.

(But not quite as easy as allowing sheep to be your lawn mowers, as they were at the White House during Woodrow Wilson’s tenure.)

Enter the world of high-class grass — turf varieties resistant to nearly all forms of neglect and trauma including repeated cuttings, persistent drought, insects and disease.

Many lawn and garden companies like Scotts Miracle-Gro market their grass seed in one-container-does-all blends of varieties that complement one another and aren’t easily susceptible to attacks from fungi, parasites and other plant pests. They have started offering these combination during the last decade or so.

“We’ll mix three or four different varieties of the same species — Kentucky bluegrass, perennial ryegrass and fine fescue,” Eric Nelson, Scotts Miracle-Gro director of turf grass development, said.

“Each has its strengths and weaknesses but they’re shored up by the other varieties in the package. That blended grass seed will produce a lush green lawn that should continue to perform,” he said.

Most grass seeds used in the 1930s through the 1950s were common types — bluegrasses, fescues and such. In 1962, “Windsor” Kentucky bluegrass was patented by Scotts, making it the first patented turf grass variety.

Scotts’ “Picture Lawn,” “Family Lawn” and “Play Lawn” seed products were introduced to the retail market in 1956, Lok said. The brand names were descriptive. “Picture” for appearance; “family” for active use and “play” for athletic lifestyles.

But those products gave way in the late 1990s, with blends more capable of responding to different growing conditions within the same yard coming into the picture. Today’s varieties have more heat tolerance, therefore requiring less watering and less frequent mowing, for example.

“Pricing depends upon the variety,” Lok said. “The premium blends are a little more expensive but still amount to one of the cheapest components in the total lawn management package.”

By that she means far cheaper than fertilizers and soil amendments — and they’re not all that pricey unless you’re dealing with acre-sized lots.

Fertilizers and such have become more technical, too. Rather than applying the same weeding and feeding solutions everywhere, you can now buy them tailored to fit variations of light or shade (trees, buildings, latitude or slopes), wear, nutrients (or not), moisture (or its lack), soil types and temperature extremes.

For those who want to choose specific grass seeds, here is a primer:

Among choices for grass seed, Kentucky bluegrass, for example, is a common cool-season grass appreciated for its longevity and ability to fill bare spots quickly. But it doesn’t tolerate shade, has little appetite for salts and requires frequent attention.

Bermuda grass is a popular low-growing option that can handle a lot of wear and tear, yet it often builds thick layers of water-robbing thatch and craves direct sun.

Many fescues are drought-resistant and shade-tolerant but prone to damage from foot or other traffic. Perennial ryegrass, meanwhile, spreads quickly and handles heavy use but requires repeated mowing.

All those genetic pluses and minuses may explain, in part, why Scotts Miracle-Gro sells more than 80 different varieties of grass seed products.

The concept of an ideal lawn is all in the eyes of the beholder, said Nathaniel Mitkowski, an assistant professor of plant pathology at the University of New Hampshire, where turf grasses have been studied and improved since the early 1890s.

“I know a fellow who likes fescue and moss. He lives on a property surrounded by trees. He doesn’t have any dogs or kids and it’s a perfect fit for him. If you put down 3,000 square feet of Kentucky bluegrass sod, it would be a disaster for the way he lives. Then he’d have to mow it and maintain it.”

The lesson is to think hard about what you want from your yard before seeding or sodding it, Mitkowski said.

“Have a plan. Once you figure that out, choose the proper grass varieties. People who want utilitarian lawns for active kids or lifestyles are very different from the people who want a trophy lawn that no one will be allowed to walk on.”