Urban greenhouses are considered to be a great alternative to conventional farming in regions where climate is more unpredictable, such as in the Northeast of the United States, and in Montreal. Because greenhouses are enclosed spaces, growing conditions can be closely monitored and kept consistent to allow for year round growing and harvesting.
Greenhouses complement urban settings well; the most solar exposure in cities is found on rooftops, which is perfect for greenhouses that require solar energy to heat the system. Additionally, the most ideal spaces for rooftop greenhouses are buildings that generate lots of heat, like manufacturing and kitchens. This is because
the residual heat can be recaptured and used to heat the greenhouse in winter months. Another option is to use a photovoltaic system that uses solar radiation to supply electricity.
In terms of water, some greenhouses use desalination methods, where a sea water source is transferred to fresh water and used for irrigation. This might not be feasible in cities like Montreal, where salt water is not as accessible throughout the city. Recovered water from rain and snow are more likely candidates for reclaimed irrigation sources.
The size of greenhouses depends on the rooftops being used, and the building codes of the city. However, once space and resources are sorted, greenhouses can support both traditional farming as well as alternatives like hydroponics and aquaponics, making them versatile.
There is some pushback to these alternative agricultural systems. In conversation with Chris Grallert of Green City Growers, the whole picture of greenhouses and hydroponics was looked at. Grallert said, “consumers are looking for local… [but] most of the fertilizers – 98% or more – of the nitrogen fertilizer anyway, used in greenhouse agriculture is petroleum derived. It’s coming from the Haber-Bosch process and it’s coming from oil reserves so it’s great to do liquid fertilizer organically but it’s more expensive and it adds other issues making it a standardized material to use.”
There is seemingly still a long way to go to optimize urban rooftop agriculture systems. Perhaps the solution is to incorporate more methods from conventional farming (with soil, tilling, and crop rotation), or perhaps there is a need for research to better the nutrient mixes being used in hydroponic and greenhouse systems. It’s important to consider all aspects, both the good and bad, when trying to approach just sustainability and form systems that are equitable for all inhabitants of a city.