Getting a vertical garden to grow in the midst of a city known for its foul weather must have involved a stroke of luck for Leo Burnett, Chicago. Following its innovative "Sundial" billboard of 2006 -- which saw the shadow of the golden arches move across a lineup of McDonald's staples, according to the hour of their appropriate consumption -- Burnett pulled off yet another inventive outdoor execution for McDonald's Fresh Salads.

Occupying the same Chicago billboard across from Wrigley Field as awards-darling "Sundial," this effort to promote the fast food behemoth's salads also harnessed the power of the sun, only this time, it literally brought the "freshness" claim to life.

What's better than food porn of farm-fresh produce? Well, how about the food itself? The sign actually grew 16 types of lettuce in a vertical garden planted on the billboard. In addition to Burnett's idea-makers, the endeavor required the genius of a horticulture expert, his students, a set builder and a team of gardeners. The seeds, from all types of lettuce used in McDonald's salads, were planted in soil-based materials and arranged to spell two words. Over three weeks, the sprouts grew to fill out "Fresh Salads."

One of the largest greens distributors in the U.S., McDonald's claims to have sold more than 486 million salads since the product's inception in 2003. As for the direct results of the campaign, local nurseries told Burnett that lettuce seeds sales increased after the month the billboard ran. Hopefully Mayor Daley chalks this one up to the ad industry doing its part to green the city.