A green roof project that wants to be a rooftop farm. That sounds cool right? Well in researching this I soon found out that rooftop farming is a very young concept in the green roof industry . Although green roofs began to be designed into buildings over 40 years ago, the incorporation of rooftop gardens for food production have really only taken root in the last decade. Farms like the Brooklyn Grange in NYC being one of the best examples of a large scale food production in a city and on a roof.
In doing such a project there a number of concerns that immediately come to mind. First on is weight and type of "soil" medium to use. Then comes irrigation, drainage, water use and all the other supportive infrastructure needed. To address these issues I began to research the companies and manufactures of green roof products as well as talk to the farmers currently farming the roof. I interviewed a number of farmers and acquired samples from over a dozen companies. I soon decided that it was good to start experimenting with some prototype mock ups to test it all out on a small scale before a 11,000 square foot farm was to be built.
We designed and built a few prototypes. The client paired me up with Daniel Miller of Spiral Gardens who runs an amazing community food security project and Nursery. We started with a few onsite, yet I moved on to my home where I could observe more readily and test the growing medium. I continued to tweak the mixes we were blending and the build up with some of the materials we were planning to use to measure performance, water retention, fertility and overall performance. Side plexiglass viewing was installed to observe root growth and water drainage and wicking capacity.
We started with a depth of 20" as the original building design was to allow for this. I quickly noticed how these boxes would be great installed systems on their own as they fit on a pallet and could ship. Also reminded me of my good friend and mentor, Michael Abelmen's project in Vancover Sole Food Farms.
After a first planting it was clear that the growing medium was super fertile and we had a good thing going. I did a number of mixes with the same base ingredients and worked with a local supplier and soil blender with the products they offered. Lava Rock, Organic compost, biochar, pumice, clay balls, expanded shale, rock powders.... I worked with over a dozen ingredients and made 9 different mixes that we would test. Came down to two main mixes that we trailed in the first prototype
For the next 10 months our project lagged in the red tape of Berkeley permitting process, which allowed us to keep designing and making adjustments. I observed that the profile was too deep as the mix readily drained, which was good, yet I had to water much more often and the water being held in the drain cups didn't wick as I had hoped as it was too far away from the main root zone. Also our mix was and system was just a bit too heavy at 70 lb. per cubic foot saturated we needed to lighten it to 55. The box did perform amazingly well with 3 rotations in 10 months, producing over $10 per square foot. A year later now I am sending the soil mix back to the lab to compare it's nutrient value as well as it's density.
With the change in design we lowered our profile to 12" and added a lot more aggregate. So far all seems to be going well. Brought the weight down to 52 psf and have good drainage with good uptake, the rockier soil will be the interesting thing to observe.
In a few months we will be moving from the coast up to Berkeley to set up shop, at that point I will dig these up and see how the roots were to the filter fabric and conditions of overall soil density. A lot of other additions will be made to the RFS 3.0 including air and water circulation at root zone and new irrigation emitters to dial in flow rates. The system on the roof is being installed in August - November with test operations happening a bit for the new year. The Garden Village farm in on schedule for full operations for the 2016 growing season. Very excited about the potential of all this!
Benjamin Fahrer is the owner and operator of Top Leaf Farms and has over 18 years of organic farming experience, building, design work and teaching in natural resource management.