I love green roofs. And I love innovative mass transit

Still, when I first heard about PhytoKinetic's efforts to put green roofs on buses, I assumed it was something out of The Onion.

Or maybe "Portlandia."

Mobile green roofs
The idea, as detailed in media outlets ranging from the UK's (usually green skeptic) Daily Mail to the sustainable design folks at Inhabitat, is to install "air purifying" green roofs on city buses. Here's how the project's founder, Marc Granen, explained the logic to the Daily Mail:

The lungs of a city must grow at the same rate as its population, but much-needed green areas are not always available. PhytoKinetic has grown out of this supposition, with the goal of delivering a practical and tangible solution. If finding new urban spaces for gardens is problematic, we can use spaces that already exist, such as the roofs of city public transport. Our mission is to expand the garden area in urban areas, increase the absorption of CO2 and give public transport a new ecological and tourist attraction.
Reduced interior temperatures
The gardens, which are planted in lightweight hydroponic foam, can be irrigated using the condensation from the buses' own air conditioning systems. There may indeed be significant benefits from cooling the interior space — Granen claims that temperatures inside a prototype bus were reduced 3.5 degrees Celsius by the installation of the roof — presumably reducing the fuel needed to run the air conditioning. There is, however, a major drawback in terms of fuel consumption.

Gardens are heavy, and they are not exactly aerodynamic.

Significant additional weight
So far, digging through the specs on PhytoKinetic's website, I've been unable to find stats on fuel consumption. But I did confirm that at maximum water capacity, the system adds 60kg/m2, or roughly the weight of one person every square meter. This added weight sparked discussion on a post about the project over at Urban Gardens, with one commenter voicing similar concerns to my own.

Here's the response posted from Granen: 

"Of course — as is the case with the buses, which have a gas engine on the roof, an air conditioning system, or electric batteries. BUT, the more weight you add on the top of a public bus, the fewer people can travel inside, so the weight remains more or less the same. Here in Europe, the law says that when something is installed on the roof, the bus has to accept fewer standing passengers. For each 68Kg you add, one standing passenger place is lost. If you were to look for it, you would see that buses with items on their roofs, have fewer passengers. PhytoKinetic works exactly the same way.
Yet here too, I see a major drawback.

Number of passengers impacts efficiency
Given that buses are designed to carry as many people as possible from A to B, their efficiency should be judged not per vehicle, but per passenger. And reduced passenger capacity means more fuel used per passenger mile. 

Aesthetics and delight do matter
That said, aesthetics and comfort are important considerations too. And these gardens do look delightful. (As evidence, check out the time-lapse video from Urban Gardens below.) Reading through comments from around the web, it seems many people are enamored with these moving parks. And igniting passion for, and interest in, our mass transit systems should be openly encouraged.

I just can't help feeling these should go on bus stops first. We've got plenty of other surfaces to green before we start weighing down our buses. 

Related on MNN:

Does a green roof on a bus really make sense?
Greening the urban environment is important. Sticking a roof garden on a moving bus is an odd way to do it.