Ask Mr. Smarty Plants

Parthenocissus quinquefolia (Virginia creeper) isn't the best ground cover in a green roof application. A good rule of thumb would be to stay away from any woody plant. Plants which age into a woody stem you can count on preferring deeper soil and although Virginia creeper will grow as a ground cover through loose leaf mulch the tap root of this plant will dive down as deep as it can go. It might work for a bit, but long term, it would be unhappy. 

Depending on your construction, you would typically be looking for no less than four inches of soil but rarely would you be able to do much more than that so you need to think about plants that have a shallow root system and you also typically need to focus on plants that can take full sun and full exposure to the elements. You also may be dealing with a slope of some kind so keep that in mind as well and look for plants that can hold what little soil is available to them, in place. This is why bunch grasses and sedges are used so frequently. If grasses seems too plain, don't despair, many smaller plants both perennial and reseeding annuals will liven the space up with texture and color. 

Green roofs provide many benefits but are tricky to install and maintain. Take plenty of time researching both the construction as well as the plants and soil you will be using. As the popularity of these systems rise so has the availability of information. We at the Lady Bird Johnson Wildflower Center have been working on green roof systems for many years under the guidance and direction of Dr Mark Simmons. Take a look at this information from the green roof page of our web site which tells you a little bit about the work we have done at the Center. It also has some links included that might give you some ideas. 

As you are in New York also take a look at the City of New York Parks and Recreation web site, it has a nice list of plants that they recommend for green roof applications with native plant recommendations for your area.  Some of the plants included in this list are: Tridens flavus (Purpletop tridens), Andropogon virginicus (Broomsedge bluestem), Geum canadense (White avens), Pycnanthemum virginianum (Virginia mountain mint), Symphyotrichum laeve (Smooth blue aster)