"Green Roofs" are very different from "roof gardens". Without irrigation or hundreds of tons of soil or both, "planted" roofs can only sustain the most drought-tolerant species--generally sedums. Even the MEC roof, which is "xeriscaped" using somewhat dry-loving prarie species, needs irrigating daily.

Hydroponics, not having any soil, are by far the most weight efficient solution out there, obviously an important consideration for a roof. Many existing buildings couldn't support even a light sedum carpet, so hydro then becomes a useful retrofitting option.

However green roofs don't just function by shading, but also evaporation, by holding a layer of water next to the building fabric. Maybe the ideal solution is to suspend a hydroponic system above a sedum carpet...

A great site for rooftop/balcony/fire escape hydro is:
http://www.rooftopgardens.ca/
I don't think Treehugger has done them yet. You definitely should!

I am guessing this was posted because it is amusing, because otherwise it is rather ineffective if environmental benefit is the reason to use a green roof.

First of all, few green roofs use a lawn for vegetation, which is what the sweet potatoes were compared to get the "1.5 times as effective" headline.

Instead, the many of the more eco friendly green roofs try to use a diverse number of native species to create an urban ecosystem. Most native prarie plants are naturally nitrogen fixing so fertilizer isn't required. These are very low maintenance and generally perenials so plants are growing at all times. (Obviously, sweet potatoes would die seasonally and need replanting, so much of the year the roof wouldn't be covered).

Creating a diversely planted environment encourages insects, birds and other additions to a healthy environment. A prime example would be the well known City of Chicago City Hall green roof, which has 120 species of native plants, as well as a producing bee hive, all in the middle of downtown Chicago.

The modular planting systems used these days are very light weight and (in Chicago at least) never need to be watered.

It is clear that the sweet potatoes are covering (in this example) only a small portion of the roof, which makes the heat island reduction questionable.

In addition to preventing heat island effects, green roofs can be useful and effective at adding insulation to the roof for all season efficiency. In addition to shading/evaporation, the plants and media insulate. Constructed of lightweight foam the growing modules and lightweight growing media add 12r to the roof insulation rating. This can be further improved through ballasted insulation which is common in green roof systems.

A big reason to use urban green roofs is to control rain water run off. This also requires substantial amounts of planted coverage, and year round plants, in order to be effective.

With all the serious, well thought out research being done on green roofs, I have to hope that this post was included because of some humor angle, because using sweet potatoes as a green roof designed for environmental benefit misses some pretty nice benefits that other well known green roof approaches offer.

Really, the only benefit I can see to using sweet potato plants on a green roof is to grow sweet potatoes. Using a sweet potato green roof is otherwise difficult to justify using environmental criteria.