From his newsletter:
Sustainability. We call this our rooftop salsa garden. Last year we had 45 earthboxes planted with tomatoes and chiles which we used in the restaurant. This year we're adding another 35 boxes. Earthboxes are a self contained gardening system, they're self-watering and self-fertilizing with the water reservoir in the bottom. The water wicks up through the soil into the roots, which means that it uses significantly less water than conventional gardening. Each box is 2 1/2 feet long, 15 inches wide and 1 foot tall, so that they can fit almost anywhere.
Earthboxes are used by the Growing Connection, which is a grassroots project developed by the Food and Agriculture Organization of the United Nations (FAO) supported by a progressive coalition of private and public sector partners. How does it work? School gardening programs and community gardens around the world grow vegetables in an EarthBox system that becomes a common growing platform for all participants. Students grow food, conduct horticultural experiments and share their lessons and experiences with each other using IT connectivity. If you'd like to contribute to this worthwhile program, you can make a donation on-line at The Growing Connection.org.

it may be also possible to use simple drainage composites by just cutting the sheets and place at the bottom
http://www.geosyntheticsworld.com/search/label/Roof%20garden
Posted by: hkki | April 13, 2009 at 04:43 AM