If you see a giant flowerbed on the side of a freeway and it looks like a Toyota Prius, it’s not an optical illusion. It’s a budding form of advertising.

Greenroad Media Inc. in Manhattan Beach has begun planting seven such floral mosaics promoting the 2010 Prius across Los Angeles by the end of the month.

Greenroad has a deal with the California Department of Transportation to install up to 50 “floral mosaics” here. The mosaics cannot include any writing or advertiser logos. However, because the installations are an extension of the Adopt-a-Freeway program, Toyota Motor Sales USA Inc. can put its name on a nearby sign identifying the Prius as sponsor of the flowers.

A typical mosaic has 20,000 plants, and costs about $60,000 for the plants, sponsorship and upkeep, said Edward St. Clair, Greenroad’s chief executive. The contract is for six months.

“This is really a sponsorship program, like public television,” St. Clair said. “We offer the advertisers and the drivers of California something of value.”

Dennis Hathaway, president of the Coalition to Ban Billboard Blight, said that despite its eco-friendly disguise, floral marketing is simply more commercial advertising on the urban landscape.

“This is a Trojan horse,” Hathaway said. “In the past, sign companies have used every crack in the law to put up their signs. This program is another crack that sets a precedent other sign companies will use to get their signs on the freeways. This is a bad deal all the way around.”

The Prius mosaics have been planted on the Pasadena (110) and Santa Monica (10) freeways, and will soon start sprouting on the San Diego (405), Ventura (101), Foothill (210) and Pomona (60) freeways.