The Possibilities of Season Extension

7 Jul

Within Minneapolis these has been a lot of talk recently about high tunnels, hoop houses, and greenhouses. My sense is that there is some confusion over specific definitions and the implications of each building type. Today I am going to define each of the categories and how each is useful for the urban farmer.

The simplest method of season extension is side-dressing a crop with hot compost. As compost ages, it goes through stages; at different stages the compost lets off a different amount of heat. By putting warm compost on the sides of the rows, cool weather crops can be kept in the ground an extra couple of weeks.  This method of season extension needs no policy changes by the city and few people would object to farmers using this method.

Photo from

High tunnels and hoop houses can more or less be used interchangeably. They are made out of metal tubes bent in an arch and covered in plastic. They look something like this if it is in my back yard:

Photo from

For a commercial farm in the city it would most likely look like this:

The rather recent innovation in hoop houses (last 20 years), is changing the possibilities for small farmers and those in cold climates. Eliot Coleman was one of the pioneers of this method at his farm in Maine. We are both at roughly the 45th parallel. (

Growing Lots farm in Minneapolis is building a hoop house right now and will be finished soon. Within their hoop house they are planting 5 rows of heat loving plants like peppers and tomatoes along with some basil interplanted in the rows. This high tunnel will allow them to grow more and better tomatoes in the summer and cool weather greens late into the fall. In Minneapolis 9-10 months of production in a high tunnel wouldn’t be unusual.

In a business like farming where the margins are thin, this season extension tool can be very helpful. As a “temporary structure” the city regulations only allow a high tunnel to operate for 6 months per year. This doesn’t match up with the needs of the farmers and this city policy needs to change. The city should allow for year-round use of these structures. There is some push back to the loosening of regulations because of concerns over visual appeal. Some argue that they do not want to see 10 ft tall plastic structures in residential neighborhoods. Though I would say that the tunnels look better than an abandoned lot, I get the point, however the policy should still change.

One good solution is the construction of greenhouses. This will become a more viable option as urban farms mature and have the money to build. They will also need secure land access to warrant such an investment. But land access is next week’s topic.

Greenhouses fit the style of a residential neighborhood better by being built out of more traditional housing material. They provide many of the same benefits as hoop houses with a few new ones. One added benefit is the ability to build inside of it. At Growing Power some of their greenhouses have large fish tanks in the middle. These large tanks filled with thousands of fish produce an additional year round source of food and income. These fish are part of a brilliant system called aquaponics. This is the fusion of hydroponics, the practice of growing plants in water, and aquaculture, the practice of farmed fish. The nitrogen wastes from the fish cycles back into layers of plants growing above the fish. These plants filter the excess  nitrogen, providing filtration for the fish and extra nutrients for the plants.

GP greenhouseThese fish tanks have the added benefit of helping regulate temperature. The large tanks absorb heat in the summer, keeping the greenhouse from overheating. In the winter the tanks radiate heat keeping the greenhouse above freezing without any additional heat source. This seems like a practical evolution of urban farming that will start to become more common in the next 10 years.

Though urban farming will never feed 100% of a city’s need, it can do quite a bit. Right now urban farming doesn’t have yields that are particularly higher than rural farms. These solutions and more will help to make future urban agriculture grow more healthy food per acre. It will take ingenuity and time to find the best solutions for each place. Please support those in Minneapolis and across the country who are doing just that. Spending years researching and practicing different models of urban farms looking for a new way forward.


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