Cloud 9 Rooftop Farm Uses Biochar in Growing Media

Cloud 9 Rooftop Farm brought food-growing into Philadelphia’s urban landscape. Wakefield’s earlier account describes the project using biochar in its rooftop growing system. It is a useful application example, though that account does not provide a mix recipe or measured before-and-after results.

The organization is now known as Cloud 9 Community Farms. Its history records a 2020 merger with Urbanstead and a broader focus on community-led food access and youth gardening. The rooftop story belongs to that history; it should not be read as a description of every current Cloud 9 garden.

The Challenge of Growing on a Rooftop

A rooftop growing mix has to provide room for roots, store useful water and let excess water drain. Its weight also matters, especially after rain or irrigation. A light ingredient is helpful only if the finished growing system meets both plant needs and the roof’s design requirements.

Research by Bollman and colleagues evaluates green-roof media through wet weight, water storage and the movement of water through the mix. The researchers found that drainage behavior was harder to predict from individual ingredients than some other properties. Testing the finished blend matters.

Where Biochar Can Fit in the Mix

Biochar is one possible ingredient in a designed rooftop growing medium. Its value depends on what it replaces and how it changes the finished mix, rather than on a general promise that adding biochar makes any roof garden lighter or more productive.

In a controlled study, Cao and colleagues added green-waste biochar to two scoria-based green-roof substrates. The amended mixes had lower bulk density and greater water-holding capacity. Some treatments also increased water available to plants and delayed wilting, but biochar did not increase plant growth in that experiment.

These findings support evaluating biochar as a growing-media ingredient. They do not establish Cloud 9’s results or a recommended amendment rate for another rooftop.

Water Storage and Drainage Must Work Together

Holding more water is not the only goal. Roots also need air, and water must move through the growing medium during heavy rain. In Werdin and colleagues’ study, finer biochar improved water retention but reduced infiltration and air-filled pore space. Coarser particles preserved those drainage and aeration properties while moderately improving plant-available water.

That tradeoff makes particle size and amendment rate part of the design decision. A mix selected for water storage needs to be checked for drainage as well. Research on specific green-roof substrates is a starting point for trials, not a universal recipe for food-producing rooftop beds.

Planning a Biochar-Amended Rooftop System

Use the Cloud 9 story as an example of biochar use, then evaluate the requirements of your own site. A qualified rooftop design team should establish structural capacity and assess the complete system, including the growing medium at its design wet weight, drainage and irrigation.

For the growing mix, compare candidate materials and test the finished formulation before scaling up. Premium Biochar is an ingredient option for soil blends; its selection here does not identify the product historically used at Cloud 9 or certify a rooftop mix. Keep crop nutrition and irrigation in the plan, and judge performance from the complete growing system.