A handful of cars, among them the 2010 Toyota Prius and the 2011 Fisker Karma, offer "solar roof" options that sandwich electricity-generating photovoltaic cells in a thin glass covering. Now Subaru, whose sensible all-wheel-drive station wagons appeal to the environmentally conscious, has upped the ante.

The 2009 Subaru Forester will offer a "green roof" option that mimics those found in designs for green homes or commercial buildings. Like those on other vehicles, the Forester's Green Roof helps keep the car's interior temperature down. In this case, its temperature rises less than that of the metal roof it covers, and the thick turf layer acts as insulation to keep the cabin cool.

Where it goes one further is in reducing ambient carbon dioxide levels. Unlike the Prius, Subaru's green roof actually offers an environmental benefit when parked. The company estimates it cuts the Forester's CO2 emissions by roughly 10 grams per 100 kilometers (the international unit for measuring CO2 impact).

Subaru's breakthrough was in developing a polymer durable enough for automotive use that could be molded to retain the 3-inch layer of dirt, water, and seed that combines to produce vegetation. Those elements sit atop a 1.5-inch-thick mesh that leaves space for the root system to develop. The mesh also includes drainage channels that route non-absorbed rainwater into the car's standard roof gutters. The entire ecosystem rests on a waterproofing membrane that lines the polymer tray.

The first version will offer a blended seed mix called "New Forester Wildflower" seed mix. This NFW mix will be joined in future months by a variety of vegetation options, including specific blends keyed to the vegetation zone in which the vehicle is likely to spend the most time. Customers in desert regions may be offered an artificial-turf option.

According to Subaru, it got the idea from its partnership with Greensgrow, a recognized leader in urban farming. "When we became involved with Greensgrow’s green roof programs," said spokesperson Aprilla Furst, "it became clear to us that having a Subaru “Green Roof” Forester was a natural extension of our relationship.”

[UPDATE: Sigh. Subaru of America has apparently been getting calls from customers asking about this, so they've asked us to insert the legal language that came at the end of their spoof press release. Here it is:

Not really. It’s an April Fool’s joke. We don’t sell such a product and we don’t advocate you try to put soil on top of your cars. The Legal guys make us add this.

It's a joke, folks. It's a JOKE!]

2010 Toyota Prius solar moonroof

2010 Toyota Prius solar moonroof