HOW COME?: As grass heals, it emits one swell smell

Surveys of favorite smells put "freshly cut grass" near the top of the list, along with scents such as sun-dried laundry, baking bread and sizzling bacon. An intact blade of grass smells earthy and green. But it takes cutting to release the heady fragrance that makes us think "summer."

Grass isn't the only plant to smell distinctly different before and after cutting. Think of onions, virtually scent-free in their netted bags at the supermarket. Cut into an onion, however, and its broken cells release pungent sulfurous compounds, which spread as a gas into the air. The gas is so irritating that it brings tears to our eyes, but it's just an onion's clever defense mechanism - a kind of plant-y tear gas.

Something similar happens when grass is trimmed or torn. Left to its own devices, lawn grass will grow only so tall and then go to seed, developing a spray of wheat-like seedpods at the end of each blade. Regular cutting prevents grass from reaching the seedy phase, in the process creating a fragrant haze of chemicals that remind us of summers past.

According to researchers, when grass is cut, the plants release a long list of compounds to heal the injury and prevent more damage. So a growth-stimulating acid revs up the production of new cells to close the wound. And other chemicals, including hexenyl alcohols, are secreted to kill invading bacteria and fungi, heading off an infection. (It's the grass' job, after all, to grow tall enough to set seeds, which can blow here, there, and everywhere, changing vacant lots into mini-prairies.)

According to scientists, the smell of cut grass is created by vapors emitted from torn blades. Chopped grass releases volatile organic compounds, or VOCs, including methane (as in swamp gas) and acetone (as in some nail-polish removers). These gases are a byproduct of what's going on in the injured plant. But they may also act as signaling molecules, producing a defensive (chemical) response in surrounding plants.

Of the many compounds released, an aromatic chemical called cis-3-Hexenal is mainly responsible for the signature scent of grassy "greenness." So when the intoxicating smell of cut grass wafts into your window, think of it as an alert to the rest of the yard: Prepare to be trimmed!

Despite the heady fragrance, many scientists suggest thinking of cut grass as a kind of chemical polluter. Researchers have found a hundredfold increase in VOCs in the air after grass is cut and clippings dry compared with an unshorn lawn or field. Chemicals released by cut grass react with the air to form ozone, and may make up one-third of the VOCs in the air around cities, helping produce summer smog.

On the other hand, scientists say that the fragrance of cut grass, along with VOCs emitted by pine trees and other plants, help us de-stress. (Just think of the deeply relaxing smells of the woods on a hot day.) By reducing stress, the scent of grass and other plants may help preserve the brain's memory functions.